Re: Re: Re: Re: Re: Re: Re: Re: [RFC PATCH 0/2] kvm/vmx: Output TSC offset
Hi Marcelo, That is, you can't feed distinct instances of guest kernel trace. I'm not clear for distinct instances. Is this about SMP or multiple guests? Would you explain about this? Distinct boot instances. If the guest reboots TSC can be written to. OK, I understood. I'll resend a patch series for this feature. Thanks! -- Yoshihiro YUNOMAE Software Platform Research Dept. Linux Technology Center Hitachi, Ltd., Yokohama Research Laboratory E-mail: yoshihiro.yunomae...@hitachi.com -- To unsubscribe from this list: send the line unsubscribe kvm in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html
Re: Re: Re: Re: Re: Re: Re: [RFC PATCH 0/2] kvm/vmx: Output TSC offset
On Fri, Nov 30, 2012 at 10:36:43AM +0900, Yoshihiro YUNOMAE wrote: Hi Marcelo, Can you please write a succint but complete description of the method so it can be verified? Sure. - Prerequisite 1. the host TSC is synchronized and stable. 2. kvm_write_tsc_offset events include previous and next TSC offset values. 3. Every event's trace_clock is TSC. - Assumption for the sake of ease 1. One VCPU 2. The guest executes no write_tsc or write_tsc only once. - Configuration 1. On the host, kvm_exit/entry events are recorded in the buffer A and kvm_write_tsc_offset events are recorded in the buffer B. 2. Boot a guest - Sort method 1. Confirm which the kvm_write_tsc_offset events are recorded except for boot. Note that a vcpu thread writes TSC offset when boot as an initial operation. 1-1. If not recorded, it means that the guest did not execute write_tsc. So, we convert the guest TSC to the host TSC using TSC offset of boot. = END 1-2. If recorded, it means that the guest executed write_tsc. So, we use new kvm_write_tsc_offset event information. 2. We convert the host TSC(Th) of the kvm_write_tsc_offset event to the guest TSC(Tg) using previous_tsc_offset value: Tg = Th + previous_tsc_offset 3. To search the point where the guest executed write_tsc, we find n which satisfies the condition Tn Tg Tn+1 or Tn+1 Tn Tg from older events of the guest. The former condition means trace data of the guest were recorded monotonically. On the other hand, the latter condition means trace data of the guest moved backward. 4. We convert the guest TSC of trace data to the host TSC using previous_tsc_offset value before n and using next_tsc_offset value after n+1. = END - Note We assumed one vcpu and no write_tsc or write_tsc only once for the sake of ease. For other conditions, we will use similar method. Thanks, There is no certainty. Consider the following information available guest trace host trace 100: guest_tsc_write (tsc_offset=-100 = guest_tsc = 0) 104: guest_tsc_write (tsc_offset=-104 = guest_tsc = 0) 108: guest_tsc_write (tsc_offset=-108 = guest_tsc = 0) 1: eventA 2: eventB 3: eventC 1: eventD 2: eventE 3: eventF How can you tell which tsc_offset to use for eventD ? It could be either -104 or -108. The notion of next_tsc_offset is subject to such issue. I suppose its fine to restrict the interface as follows: the tool will accept one trace of events with monotonic timestamps and the user is responsible for correlating that to a host trace. OK, I'll add the restriction, which trace data must have monotonic timestamps to use this feature. I think guests seldom will execute write_tsc, so in many cases, timestamps will be monotonically recorded in trace data. That is, you can't feed distinct instances of guest kernel trace. I'm not clear for distinct instances. Is this about SMP or multiple guests? Would you explain about this? Distinct boot instances. If the guest reboots TSC can be written to. -- To unsubscribe from this list: send the line unsubscribe kvm in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html
Re: Re: Re: Re: Re: Re: [RFC PATCH 0/2] kvm/vmx: Output TSC offset
On Tue, Nov 27, 2012 at 07:53:47PM +0900, Yoshihiro YUNOMAE wrote: Hi Marcelo, (2012/11/27 8:16), Marcelo Tosatti wrote: On Mon, Nov 26, 2012 at 08:05:10PM +0900, Yoshihiro YUNOMAE wrote: 500h. event tsc_write tsc_offset=-3000 Then a guest trace containing events with a TSC timestamp. Which tsc_offset to use? (that is the problem, which unless i am mistaken can only be solved easily if the guest can convert RDTSC - TSC of host). There are three following cases of changing TSC offset: 1. Reset TSC at guest boot time 2. Adjust TSC offset due to some host's problems 3. Write TSC on guests The scenario which you mentioned is case 3, so we'll discuss this case. Here, we assume that a guest is allocated single CPU for the sake of ease. If a guest executes write_tsc, TSC values jumps to forward or backward. For the forward case, trace data are as follows: host guest cyclesevents cycles events 3000 tsc_offset=-2950 3001 kvm_enter 53 eventX 100 (write_tsc=+900) 3060 kvm_exit 3075 tsc_offset=-2050 3080 kvm_enter 1050 event1 1055 event2 ... This case is simple. The guest TSC of the first kvm_enter is calculated as follows: (host TSC of kvm_enter) + (current tsc_offset) = 3001 - 2950 = 51 Similarly, the guest TSC of the second kvm_enter is 130. So, the guest events between 51 and 130, that is, 53 eventX is inserted between the first pair of kvm_enter and kvm_exit. To insert events of the guests between 51 and 130, we convert the guest TSC to the host TSC using TSC offset 2950. For the backward case, trace data are as follows: host guest cyclesevents cycles events 3000 tsc_offset=-2950 3001 kvm_enter 53 eventX 100 (write_tsc=-50) 3060 kvm_exit 3075 tsc_offset=-2050 3080 kvm_enter 90 event1 95 event2 ... 3400 100(write_tsc=-50) 90event3 95event4 As you say, in this case, the previous method is invalid. When we calculate the guest TSC value for the tsc_offset=-3000 event, the value is 75 on the guest. This seems like prior event of write_tsc=-50 event. So, we need to consider more. In this case, it is important that we can understand where the guest executes write_tsc or the host rewrites the TSC offset. write_tsc on the guest equals wrmsr 0x0010, so this instruction induces vm_exit. This implies that the guest does not operate when the host changes TSC offset on the cpu. In other words, the guest cannot use new TSC before the host rewrites the new TSC offset. So, if timestamp on the guest is not monotonically increased, we can understand the guest executes write_tsc. Moreover, in the region where timestamp is decreasing, we can understand when the host rewrote the TSC offset in the guest trace data. Therefore, we can sort trace data in chronological order. This requires an entire trace of events. That is, to be able to reconstruct timeline you require the entire trace from the moment guest starts. So that you can correlate wrmsr-to-tsc on the guest with vmexit-due-to-tsc-write on the host. Which means that running out of space for trace buffer equals losing ability to order events. Is that desirable? It seems cumbersome to me. As you say, tracing events can overwrite important events like kvm_exit/entry or write_tsc_offset. So, Steven's multiple buffer is needed by this feature. Normal events which often hit record the buffer A, and important events which rarely hit record the buffer B. In our case, the important event is write_tsc_offset. Also the need to correlate each write_tsc event in the guest trace with a corresponding tsc_offset write in the host trace means that it is _necessary_ for the guest and host to enable tracing simultaneously. Correct? Also, there are WRMSR executions in the guest for which there is no event in the trace buffer. From SeaBIOS, during boot. In that case, there is no explicit event in the guest trace which you can correlate with tsc_offset changes in the host side. I understand that you want to say, but we don't correlate between write_tsc event and write_tsc_offset event directly. This is because the write_tsc tracepoint (also WRMSR instruction) is not prepared in the current kernel. So, in the previous mail (https://lkml.org/lkml/2012/11/22/53), I suggested the method which we don't need to prepare
Re: Re: Re: Re: Re: Re: Re: [RFC PATCH 0/2] kvm/vmx: Output TSC offset
Hi Marcelo, Can you please write a succint but complete description of the method so it can be verified? Sure. - Prerequisite 1. the host TSC is synchronized and stable. 2. kvm_write_tsc_offset events include previous and next TSC offset values. 3. Every event's trace_clock is TSC. - Assumption for the sake of ease 1. One VCPU 2. The guest executes no write_tsc or write_tsc only once. - Configuration 1. On the host, kvm_exit/entry events are recorded in the buffer A and kvm_write_tsc_offset events are recorded in the buffer B. 2. Boot a guest - Sort method 1. Confirm which the kvm_write_tsc_offset events are recorded except for boot. Note that a vcpu thread writes TSC offset when boot as an initial operation. 1-1. If not recorded, it means that the guest did not execute write_tsc. So, we convert the guest TSC to the host TSC using TSC offset of boot. = END 1-2. If recorded, it means that the guest executed write_tsc. So, we use new kvm_write_tsc_offset event information. 2. We convert the host TSC(Th) of the kvm_write_tsc_offset event to the guest TSC(Tg) using previous_tsc_offset value: Tg = Th + previous_tsc_offset 3. To search the point where the guest executed write_tsc, we find n which satisfies the condition Tn Tg Tn+1 or Tn+1 Tn Tg from older events of the guest. The former condition means trace data of the guest were recorded monotonically. On the other hand, the latter condition means trace data of the guest moved backward. 4. We convert the guest TSC of trace data to the host TSC using previous_tsc_offset value before n and using next_tsc_offset value after n+1. = END - Note We assumed one vcpu and no write_tsc or write_tsc only once for the sake of ease. For other conditions, we will use similar method. Thanks, There is no certainty. Consider the following information available guest trace host trace 100: guest_tsc_write (tsc_offset=-100 = guest_tsc = 0) 104: guest_tsc_write (tsc_offset=-104 = guest_tsc = 0) 108: guest_tsc_write (tsc_offset=-108 = guest_tsc = 0) 1: eventA 2: eventB 3: eventC 1: eventD 2: eventE 3: eventF How can you tell which tsc_offset to use for eventD ? It could be either -104 or -108. The notion of next_tsc_offset is subject to such issue. I suppose its fine to restrict the interface as follows: the tool will accept one trace of events with monotonic timestamps and the user is responsible for correlating that to a host trace. OK, I'll add the restriction, which trace data must have monotonic timestamps to use this feature. I think guests seldom will execute write_tsc, so in many cases, timestamps will be monotonically recorded in trace data. That is, you can't feed distinct instances of guest kernel trace. I'm not clear for distinct instances. Is this about SMP or multiple guests? Would you explain about this? Thanks, -- Yoshihiro YUNOMAE Software Platform Research Dept. Linux Technology Center Hitachi, Ltd., Yokohama Research Laboratory E-mail: yoshihiro.yunomae...@hitachi.com -- To unsubscribe from this list: send the line unsubscribe kvm in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html
Re: Re: Re: Re: Re: Re: [RFC PATCH 0/2] kvm/vmx: Output TSC offset
Hi Marcelo, (2012/11/27 8:16), Marcelo Tosatti wrote: On Mon, Nov 26, 2012 at 08:05:10PM +0900, Yoshihiro YUNOMAE wrote: 500h. event tsc_write tsc_offset=-3000 Then a guest trace containing events with a TSC timestamp. Which tsc_offset to use? (that is the problem, which unless i am mistaken can only be solved easily if the guest can convert RDTSC - TSC of host). There are three following cases of changing TSC offset: 1. Reset TSC at guest boot time 2. Adjust TSC offset due to some host's problems 3. Write TSC on guests The scenario which you mentioned is case 3, so we'll discuss this case. Here, we assume that a guest is allocated single CPU for the sake of ease. If a guest executes write_tsc, TSC values jumps to forward or backward. For the forward case, trace data are as follows: host guest cyclesevents cycles events 3000 tsc_offset=-2950 3001 kvm_enter 53 eventX 100 (write_tsc=+900) 3060 kvm_exit 3075 tsc_offset=-2050 3080 kvm_enter 1050 event1 1055 event2 ... This case is simple. The guest TSC of the first kvm_enter is calculated as follows: (host TSC of kvm_enter) + (current tsc_offset) = 3001 - 2950 = 51 Similarly, the guest TSC of the second kvm_enter is 130. So, the guest events between 51 and 130, that is, 53 eventX is inserted between the first pair of kvm_enter and kvm_exit. To insert events of the guests between 51 and 130, we convert the guest TSC to the host TSC using TSC offset 2950. For the backward case, trace data are as follows: host guest cyclesevents cycles events 3000 tsc_offset=-2950 3001 kvm_enter 53 eventX 100 (write_tsc=-50) 3060 kvm_exit 3075 tsc_offset=-2050 3080 kvm_enter 90 event1 95 event2 ... 3400 100(write_tsc=-50) 90event3 95event4 As you say, in this case, the previous method is invalid. When we calculate the guest TSC value for the tsc_offset=-3000 event, the value is 75 on the guest. This seems like prior event of write_tsc=-50 event. So, we need to consider more. In this case, it is important that we can understand where the guest executes write_tsc or the host rewrites the TSC offset. write_tsc on the guest equals wrmsr 0x0010, so this instruction induces vm_exit. This implies that the guest does not operate when the host changes TSC offset on the cpu. In other words, the guest cannot use new TSC before the host rewrites the new TSC offset. So, if timestamp on the guest is not monotonically increased, we can understand the guest executes write_tsc. Moreover, in the region where timestamp is decreasing, we can understand when the host rewrote the TSC offset in the guest trace data. Therefore, we can sort trace data in chronological order. This requires an entire trace of events. That is, to be able to reconstruct timeline you require the entire trace from the moment guest starts. So that you can correlate wrmsr-to-tsc on the guest with vmexit-due-to-tsc-write on the host. Which means that running out of space for trace buffer equals losing ability to order events. Is that desirable? It seems cumbersome to me. As you say, tracing events can overwrite important events like kvm_exit/entry or write_tsc_offset. So, Steven's multiple buffer is needed by this feature. Normal events which often hit record the buffer A, and important events which rarely hit record the buffer B. In our case, the important event is write_tsc_offset. Also the need to correlate each write_tsc event in the guest trace with a corresponding tsc_offset write in the host trace means that it is _necessary_ for the guest and host to enable tracing simultaneously. Correct? Also, there are WRMSR executions in the guest for which there is no event in the trace buffer. From SeaBIOS, during boot. In that case, there is no explicit event in the guest trace which you can correlate with tsc_offset changes in the host side. I understand that you want to say, but we don't correlate between write_tsc event and write_tsc_offset event directly. This is because the write_tsc tracepoint (also WRMSR instruction) is not prepared in the current kernel. So, in the previous mail (https://lkml.org/lkml/2012/11/22/53), I suggested the method which we don't need to prepare the write_tsc tracepoint. In the method, we enable ftrace before the guest boots, and we need to keep all write_tsc_offset events in the buffer. If we forgot
Re: Re: Re: Re: Re: [RFC PATCH 0/2] kvm/vmx: Output TSC offset
Hi Marcelo,
(2012/11/24 7:46), Marcelo Tosatti wrote:
On Thu, Nov 22, 2012 at 02:21:20PM +0900, Yoshihiro YUNOMAE wrote:
Hi Marcelo,
(2012/11/21 7:51), Marcelo Tosatti wrote:
On Tue, Nov 20, 2012 at 07:36:33PM +0900, Yoshihiro YUNOMAE wrote:
Hi Marcelo,
Sorry for the late reply.
(2012/11/17 4:15), Marcelo Tosatti wrote:
On Wed, Nov 14, 2012 at 05:26:10PM +0900, Yoshihiro YUNOMAE wrote:
Thank you for commenting on my patch set.
(2012/11/14 11:31), Steven Rostedt wrote:
On Tue, 2012-11-13 at 18:03 -0800, David Sharp wrote:
On Tue, Nov 13, 2012 at 6:00 PM, Steven Rostedt rost...@goodmis.org wrote:
On Wed, 2012-11-14 at 10:36 +0900, Yoshihiro YUNOMAE wrote:
To merge the data like previous pattern, we apply this patch set. Then, we can
get TSC offset of the guest as follows:
$ dmesg | grep kvm
[ 57.717180] kvm: (2687) write TSC offset 18446743360465545001, now clock ##
|
PID TSC offset |
HOST TSC value --+
Using printk to export something like this is IMO a nasty hack.
Can't we create a /sys or /proc file to export the same thing?
Since the value changes over the course of the trace, and seems to be
part of the context of the trace, I think I'd include it as a
tracepoint.
I'm fine with that too.
Using some tracepoint is a nice idea, but there is one problem. Here,
our discussion point is the event which TSC offset is changed does not
frequently occur, but the buffer must keep the event data.
There are two ideas for using tracepoint. First, we define new
tracepoint for changed TSC offset. This is simple and the overhead will
be low. However, this trace event stored in the buffer will be
overwritten by other trace events because this TSC offset event does
not frequently occur. Second, we add TSC offset information to the
tracepoint frequently occured. For example, we assume that TSC offset
information is added to arguments of trace_kvm_exit().
The TSC offset is in the host trace. So given a host trace with two TSC
offset updates, how do you know which events in the guest trace
(containing a number of events) refer to which tsc offset update?
Unless i am missing something, you can't solve this easily (well, except
exporting information to the guest that allows it to transform RDTSC -
host TSC value, which can be done via pvclock).
As you say, TSC offset events are in the host trace, but we don't need
to notify guests of updating TSC offset. The offset event will output
the next TSC offset value and the current TSC value, so we can
calculate the guest TSC (T1) for the event. Guest TSCs since T1 can be
converted to host TSC using the TSC offset, so we can integrate those
trace data.
Think of this scenario:
host trace
1h. event tsc write tsc_offset=-1000
3h. vmenter
4h. vmexit
... (event sequence)
99h. vmexit
100h. event tsc_write tsc_offset=-2000
101h. vmenter
... (event sequence).
500h. event tsc_write tsc_offset=-3000
Then a guest trace containing events with a TSC timestamp.
Which tsc_offset to use?
(that is the problem, which unless i am mistaken can only be solved
easily if the guest can convert RDTSC - TSC of host).
There are three following cases of changing TSC offset:
1. Reset TSC at guest boot time
2. Adjust TSC offset due to some host's problems
3. Write TSC on guests
The scenario which you mentioned is case 3, so we'll discuss this case.
Here, we assume that a guest is allocated single CPU for the sake of
ease.
If a guest executes write_tsc, TSC values jumps to forward or backward.
For the forward case, trace data are as follows:
host guest
cyclesevents cycles events
3000 tsc_offset=-2950
3001 kvm_enter
53 eventX
100 (write_tsc=+900)
3060 kvm_exit
3075 tsc_offset=-2050
3080 kvm_enter
1050 event1
1055 event2
...
This case is simple. The guest TSC of the first kvm_enter is calculated
as follows:
(host TSC of kvm_enter) + (current tsc_offset) = 3001 - 2950 = 51
Similarly, the guest TSC of the second kvm_enter is 130. So, the guest
events between 51 and 130, that is, 53 eventX is inserted between the
first pair of kvm_enter and kvm_exit. To insert events of the guests
between 51 and 130, we convert the guest TSC to the host TSC using TSC
offset 2950.
For the backward case, trace data are as follows:
host guest
cyclesevents cycles events
3000 tsc_offset=-2950
3001 kvm_enter
53 eventX
100 (write_tsc=-50)
3060 kvm_exit
Re: Re: Re: Re: Re: [RFC PATCH 0/2] kvm/vmx: Output TSC offset
On Mon, Nov 26, 2012 at 08:05:10PM +0900, Yoshihiro YUNOMAE wrote: 500h. event tsc_write tsc_offset=-3000 Then a guest trace containing events with a TSC timestamp. Which tsc_offset to use? (that is the problem, which unless i am mistaken can only be solved easily if the guest can convert RDTSC - TSC of host). There are three following cases of changing TSC offset: 1. Reset TSC at guest boot time 2. Adjust TSC offset due to some host's problems 3. Write TSC on guests The scenario which you mentioned is case 3, so we'll discuss this case. Here, we assume that a guest is allocated single CPU for the sake of ease. If a guest executes write_tsc, TSC values jumps to forward or backward. For the forward case, trace data are as follows: host guest cyclesevents cycles events 3000 tsc_offset=-2950 3001 kvm_enter 53 eventX 100 (write_tsc=+900) 3060 kvm_exit 3075 tsc_offset=-2050 3080 kvm_enter 1050 event1 1055 event2 ... This case is simple. The guest TSC of the first kvm_enter is calculated as follows: (host TSC of kvm_enter) + (current tsc_offset) = 3001 - 2950 = 51 Similarly, the guest TSC of the second kvm_enter is 130. So, the guest events between 51 and 130, that is, 53 eventX is inserted between the first pair of kvm_enter and kvm_exit. To insert events of the guests between 51 and 130, we convert the guest TSC to the host TSC using TSC offset 2950. For the backward case, trace data are as follows: host guest cyclesevents cycles events 3000 tsc_offset=-2950 3001 kvm_enter 53 eventX 100 (write_tsc=-50) 3060 kvm_exit 3075 tsc_offset=-2050 3080 kvm_enter 90 event1 95 event2 ... 3400100(write_tsc=-50) 90event3 95event4 As you say, in this case, the previous method is invalid. When we calculate the guest TSC value for the tsc_offset=-3000 event, the value is 75 on the guest. This seems like prior event of write_tsc=-50 event. So, we need to consider more. In this case, it is important that we can understand where the guest executes write_tsc or the host rewrites the TSC offset. write_tsc on the guest equals wrmsr 0x0010, so this instruction induces vm_exit. This implies that the guest does not operate when the host changes TSC offset on the cpu. In other words, the guest cannot use new TSC before the host rewrites the new TSC offset. So, if timestamp on the guest is not monotonically increased, we can understand the guest executes write_tsc. Moreover, in the region where timestamp is decreasing, we can understand when the host rewrote the TSC offset in the guest trace data. Therefore, we can sort trace data in chronological order. This requires an entire trace of events. That is, to be able to reconstruct timeline you require the entire trace from the moment guest starts. So that you can correlate wrmsr-to-tsc on the guest with vmexit-due-to-tsc-write on the host. Which means that running out of space for trace buffer equals losing ability to order events. Is that desirable? It seems cumbersome to me. As you say, tracing events can overwrite important events like kvm_exit/entry or write_tsc_offset. So, Steven's multiple buffer is needed by this feature. Normal events which often hit record the buffer A, and important events which rarely hit record the buffer B. In our case, the important event is write_tsc_offset. Also the need to correlate each write_tsc event in the guest trace with a corresponding tsc_offset write in the host trace means that it is _necessary_ for the guest and host to enable tracing simultaneously. Correct? Also, there are WRMSR executions in the guest for which there is no event in the trace buffer. From SeaBIOS, during boot. In that case, there is no explicit event in the guest trace which you can correlate with tsc_offset changes in the host side. I understand that you want to say, but we don't correlate between write_tsc event and write_tsc_offset event directly. This is because the write_tsc tracepoint (also WRMSR instruction) is not prepared in the current kernel. So, in the previous mail (https://lkml.org/lkml/2012/11/22/53), I suggested the method which we don't need to prepare the write_tsc tracepoint. In the method, we enable ftrace before the guest boots, and we need to keep all
Re: Re: Re: Re: [RFC PATCH 0/2] kvm/vmx: Output TSC offset
On Thu, Nov 22, 2012 at 02:21:20PM +0900, Yoshihiro YUNOMAE wrote:
Hi Marcelo,
(2012/11/21 7:51), Marcelo Tosatti wrote:
On Tue, Nov 20, 2012 at 07:36:33PM +0900, Yoshihiro YUNOMAE wrote:
Hi Marcelo,
Sorry for the late reply.
(2012/11/17 4:15), Marcelo Tosatti wrote:
On Wed, Nov 14, 2012 at 05:26:10PM +0900, Yoshihiro YUNOMAE wrote:
Thank you for commenting on my patch set.
(2012/11/14 11:31), Steven Rostedt wrote:
On Tue, 2012-11-13 at 18:03 -0800, David Sharp wrote:
On Tue, Nov 13, 2012 at 6:00 PM, Steven Rostedt rost...@goodmis.org
wrote:
On Wed, 2012-11-14 at 10:36 +0900, Yoshihiro YUNOMAE wrote:
To merge the data like previous pattern, we apply this patch set.
Then, we can
get TSC offset of the guest as follows:
$ dmesg | grep kvm
[ 57.717180] kvm: (2687) write TSC offset 18446743360465545001, now
clock ##
|
PID TSC offset
|
HOST TSC
value --+
Using printk to export something like this is IMO a nasty hack.
Can't we create a /sys or /proc file to export the same thing?
Since the value changes over the course of the trace, and seems to be
part of the context of the trace, I think I'd include it as a
tracepoint.
I'm fine with that too.
Using some tracepoint is a nice idea, but there is one problem. Here,
our discussion point is the event which TSC offset is changed does not
frequently occur, but the buffer must keep the event data.
There are two ideas for using tracepoint. First, we define new
tracepoint for changed TSC offset. This is simple and the overhead will
be low. However, this trace event stored in the buffer will be
overwritten by other trace events because this TSC offset event does
not frequently occur. Second, we add TSC offset information to the
tracepoint frequently occured. For example, we assume that TSC offset
information is added to arguments of trace_kvm_exit().
The TSC offset is in the host trace. So given a host trace with two TSC
offset updates, how do you know which events in the guest trace
(containing a number of events) refer to which tsc offset update?
Unless i am missing something, you can't solve this easily (well, except
exporting information to the guest that allows it to transform RDTSC -
host TSC value, which can be done via pvclock).
As you say, TSC offset events are in the host trace, but we don't need
to notify guests of updating TSC offset. The offset event will output
the next TSC offset value and the current TSC value, so we can
calculate the guest TSC (T1) for the event. Guest TSCs since T1 can be
converted to host TSC using the TSC offset, so we can integrate those
trace data.
Think of this scenario:
host trace
1h. event tsc write tsc_offset=-1000
3h. vmenter
4h. vmexit
... (event sequence)
99h. vmexit
100h. event tsc_write tsc_offset=-2000
101h. vmenter
... (event sequence).
500h. event tsc_write tsc_offset=-3000
Then a guest trace containing events with a TSC timestamp.
Which tsc_offset to use?
(that is the problem, which unless i am mistaken can only be solved
easily if the guest can convert RDTSC - TSC of host).
There are three following cases of changing TSC offset:
1. Reset TSC at guest boot time
2. Adjust TSC offset due to some host's problems
3. Write TSC on guests
The scenario which you mentioned is case 3, so we'll discuss this case.
Here, we assume that a guest is allocated single CPU for the sake of
ease.
If a guest executes write_tsc, TSC values jumps to forward or backward.
For the forward case, trace data are as follows:
host guest
cyclesevents cycles events
3000 tsc_offset=-2950
3001 kvm_enter
53 eventX
100 (write_tsc=+900)
3060 kvm_exit
3075 tsc_offset=-2050
3080 kvm_enter
1050 event1
1055 event2
...
This case is simple. The guest TSC of the first kvm_enter is calculated
as follows:
(host TSC of kvm_enter) + (current tsc_offset) = 3001 - 2950 = 51
Similarly, the guest TSC of the second kvm_enter is 130. So, the guest
events between 51 and 130, that is, 53 eventX is inserted between the
first pair of kvm_enter and kvm_exit. To insert events of the guests
between 51 and 130, we convert the guest TSC to the host TSC using TSC
offset 2950.
For the backward case, trace data are as follows:
host guest
cyclesevents cycles events
3000 tsc_offset=-2950
3001 kvm_enter
53 eventX
Re: Re: Re: Re: [RFC PATCH 0/2] kvm/vmx: Output TSC offset
Hi Marcelo,
(2012/11/21 7:51), Marcelo Tosatti wrote:
On Tue, Nov 20, 2012 at 07:36:33PM +0900, Yoshihiro YUNOMAE wrote:
Hi Marcelo,
Sorry for the late reply.
(2012/11/17 4:15), Marcelo Tosatti wrote:
On Wed, Nov 14, 2012 at 05:26:10PM +0900, Yoshihiro YUNOMAE wrote:
Thank you for commenting on my patch set.
(2012/11/14 11:31), Steven Rostedt wrote:
On Tue, 2012-11-13 at 18:03 -0800, David Sharp wrote:
On Tue, Nov 13, 2012 at 6:00 PM, Steven Rostedt rost...@goodmis.org wrote:
On Wed, 2012-11-14 at 10:36 +0900, Yoshihiro YUNOMAE wrote:
To merge the data like previous pattern, we apply this patch set. Then, we can
get TSC offset of the guest as follows:
$ dmesg | grep kvm
[ 57.717180] kvm: (2687) write TSC offset 18446743360465545001, now clock ##
|
PID TSC offset |
HOST TSC value --+
Using printk to export something like this is IMO a nasty hack.
Can't we create a /sys or /proc file to export the same thing?
Since the value changes over the course of the trace, and seems to be
part of the context of the trace, I think I'd include it as a
tracepoint.
I'm fine with that too.
Using some tracepoint is a nice idea, but there is one problem. Here,
our discussion point is the event which TSC offset is changed does not
frequently occur, but the buffer must keep the event data.
There are two ideas for using tracepoint. First, we define new
tracepoint for changed TSC offset. This is simple and the overhead will
be low. However, this trace event stored in the buffer will be
overwritten by other trace events because this TSC offset event does
not frequently occur. Second, we add TSC offset information to the
tracepoint frequently occured. For example, we assume that TSC offset
information is added to arguments of trace_kvm_exit().
The TSC offset is in the host trace. So given a host trace with two TSC
offset updates, how do you know which events in the guest trace
(containing a number of events) refer to which tsc offset update?
Unless i am missing something, you can't solve this easily (well, except
exporting information to the guest that allows it to transform RDTSC -
host TSC value, which can be done via pvclock).
As you say, TSC offset events are in the host trace, but we don't need
to notify guests of updating TSC offset. The offset event will output
the next TSC offset value and the current TSC value, so we can
calculate the guest TSC (T1) for the event. Guest TSCs since T1 can be
converted to host TSC using the TSC offset, so we can integrate those
trace data.
Think of this scenario:
host trace
1h. event tsc write tsc_offset=-1000
3h. vmenter
4h. vmexit
... (event sequence)
99h. vmexit
100h. event tsc_write tsc_offset=-2000
101h. vmenter
... (event sequence).
500h. event tsc_write tsc_offset=-3000
Then a guest trace containing events with a TSC timestamp.
Which tsc_offset to use?
(that is the problem, which unless i am mistaken can only be solved
easily if the guest can convert RDTSC - TSC of host).
There are three following cases of changing TSC offset:
1. Reset TSC at guest boot time
2. Adjust TSC offset due to some host's problems
3. Write TSC on guests
The scenario which you mentioned is case 3, so we'll discuss this case.
Here, we assume that a guest is allocated single CPU for the sake of
ease.
If a guest executes write_tsc, TSC values jumps to forward or backward.
For the forward case, trace data are as follows:
host guest
cyclesevents cycles events
3000 tsc_offset=-2950
3001 kvm_enter
53 eventX
100 (write_tsc=+900)
3060 kvm_exit
3075 tsc_offset=-2050
3080 kvm_enter
1050 event1
1055 event2
...
This case is simple. The guest TSC of the first kvm_enter is calculated
as follows:
(host TSC of kvm_enter) + (current tsc_offset) = 3001 - 2950 = 51
Similarly, the guest TSC of the second kvm_enter is 130. So, the guest
events between 51 and 130, that is, 53 eventX is inserted between the
first pair of kvm_enter and kvm_exit. To insert events of the guests
between 51 and 130, we convert the guest TSC to the host TSC using TSC
offset 2950.
For the backward case, trace data are as follows:
host guest
cyclesevents cycles events
3000 tsc_offset=-2950
3001 kvm_enter
53 eventX
100 (write_tsc=-50)
3060 kvm_exit
3075 tsc_offset=-2050
3080 kvm_enter
90 event1
95 event2
Re: Re: Re: [RFC PATCH 0/2] kvm/vmx: Output TSC offset
Hi Marcelo,
Sorry for the late reply.
(2012/11/17 4:15), Marcelo Tosatti wrote:
On Wed, Nov 14, 2012 at 05:26:10PM +0900, Yoshihiro YUNOMAE wrote:
Thank you for commenting on my patch set.
(2012/11/14 11:31), Steven Rostedt wrote:
On Tue, 2012-11-13 at 18:03 -0800, David Sharp wrote:
On Tue, Nov 13, 2012 at 6:00 PM, Steven Rostedt rost...@goodmis.org wrote:
On Wed, 2012-11-14 at 10:36 +0900, Yoshihiro YUNOMAE wrote:
To merge the data like previous pattern, we apply this patch set. Then, we can
get TSC offset of the guest as follows:
$ dmesg | grep kvm
[ 57.717180] kvm: (2687) write TSC offset 18446743360465545001, now clock ##
|
PID TSC offset |
HOST TSC value --+
Using printk to export something like this is IMO a nasty hack.
Can't we create a /sys or /proc file to export the same thing?
Since the value changes over the course of the trace, and seems to be
part of the context of the trace, I think I'd include it as a
tracepoint.
I'm fine with that too.
Using some tracepoint is a nice idea, but there is one problem. Here,
our discussion point is the event which TSC offset is changed does not
frequently occur, but the buffer must keep the event data.
There are two ideas for using tracepoint. First, we define new
tracepoint for changed TSC offset. This is simple and the overhead will
be low. However, this trace event stored in the buffer will be
overwritten by other trace events because this TSC offset event does
not frequently occur. Second, we add TSC offset information to the
tracepoint frequently occured. For example, we assume that TSC offset
information is added to arguments of trace_kvm_exit().
The TSC offset is in the host trace. So given a host trace with two TSC
offset updates, how do you know which events in the guest trace
(containing a number of events) refer to which tsc offset update?
Unless i am missing something, you can't solve this easily (well, except
exporting information to the guest that allows it to transform RDTSC -
host TSC value, which can be done via pvclock).
As you say, TSC offset events are in the host trace, but we don't need
to notify guests of updating TSC offset. The offset event will output
the next TSC offset value and the current TSC value, so we can
calculate the guest TSC (T1) for the event. Guest TSCs since T1 can be
converted to host TSC using the TSC offset, so we can integrate those
trace data.
Another issue as mentioned is lack of TSC synchronization in the host.
Should you provide such a feature without the possibility of proper
chronological order on systems with unsynchronized TSC?
I think, we cannot support this sorting feature using TSC on systems
with unsynchronized TSC. On systems with unsynchronized TSC, it is
difficult to sort not only trace data of guests and the host but trace
data of a guest or a host using TSC in chronological order. Actually,
if we want to output tracing data of ftrace in chronological order with
unsynchronized TSC, we will use the global mode as the timestamp. The
global mode uses wallclock added TSC correction, so the mode guarantees
to sort in chronological order for trace data of the guest or of
the host. If we use this mode to sort the trace data of guests and the
host in chronological order, we need to consider about the difference
between the guest and the host and timekeeping of guests and the host,
so it is difficult to solve these issues. At least, I haven't came up
with the good solution.
We cannot sort the trace data of guests and the host in chronological
order with unsynchronized TSC, but if we can set following
synchronization events for both guests and the host, we will know where
we should sort.
First, a guest and the host uses the global mode as the timestamp of
ftrace. Next, a user on the guest writes 1 to the synchronization I/F
as the ID, then the synchronization event 1 is recorded in a
ring-buffer of the guest. The synchronization operation induces
hypercall, so the host can handle the event. After the operation moves
to the host, the host records the event 1 in a ring-buffer of the
host. In the end, the operation returns to the host, and the
synchronization is finished.
When we integrate tracing data of the guest and the host, we
calculate difference of the timestamp between the synchronizing events
with the same ID. This value is a temporary offset. We will convert
the timestamp of the guests to the timestamp of the host before the
next synchronizing event. If the synchronizing event cycle is very
short, we will not need to consider the timekeeping. Then, we can sort
the trace data in chronological order.
Would you comment for this or do you have another idea?
Thanks,
--
Yoshihiro YUNOMAE
Software Platform Research Dept. Linux Technology Center
Hitachi, Ltd., Yokohama
Re: Re: Re: [RFC PATCH 0/2] kvm/vmx: Output TSC offset
Hi Steven,
Sorry for the late reply.
(2012/11/17 0:05), Steven Rostedt wrote:
On Wed, 2012-11-14 at 17:26 +0900, Yoshihiro YUNOMAE wrote:
Thank you for commenting on my patch set.
(2012/11/14 11:31), Steven Rostedt wrote:
On Tue, 2012-11-13 at 18:03 -0800, David Sharp wrote:
On Tue, Nov 13, 2012 at 6:00 PM, Steven Rostedt rost...@goodmis.org wrote:
On Wed, 2012-11-14 at 10:36 +0900, Yoshihiro YUNOMAE wrote:
To merge the data like previous pattern, we apply this patch set. Then, we can
get TSC offset of the guest as follows:
$ dmesg | grep kvm
[ 57.717180] kvm: (2687) write TSC offset 18446743360465545001, now clock ##
|
PID TSC offset |
HOST TSC value --+
Using printk to export something like this is IMO a nasty hack.
Can't we create a /sys or /proc file to export the same thing?
Since the value changes over the course of the trace, and seems to be
part of the context of the trace, I think I'd include it as a
tracepoint.
I'm fine with that too.
Using some tracepoint is a nice idea, but there is one problem. Here,
our discussion point is the event which TSC offset is changed does not
frequently occur, but the buffer must keep the event data.
If you can hold off a bit, for the 3.9 window, I plan on pushing
multiple buffers for ftrace. That is, you can create a separate buffer
just for the TSC offset events:
cd /sys/kernel/debug
echo tsc instances/new
echo 1 instances/tsc/events/tsc/offset/enable
Then the buffer will be used only for that event.
That's good. The tracepoint will output as follows:
qemu-kvm-12345 [000] 123456789: kvm_write_tsc_offset:
now_tsc=123456789 previous_offset=0 next_offset=123456780
Thanks,
--
Yoshihiro YUNOMAE
Software Platform Research Dept. Linux Technology Center
Hitachi, Ltd., Yokohama Research Laboratory
E-mail: yoshihiro.yunomae...@hitachi.com
--
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the body of a message to majord...@vger.kernel.org
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Re: Re: Re: [RFC PATCH 0/2] kvm/vmx: Output TSC offset
On Tue, Nov 20, 2012 at 07:36:33PM +0900, Yoshihiro YUNOMAE wrote:
Hi Marcelo,
Sorry for the late reply.
(2012/11/17 4:15), Marcelo Tosatti wrote:
On Wed, Nov 14, 2012 at 05:26:10PM +0900, Yoshihiro YUNOMAE wrote:
Thank you for commenting on my patch set.
(2012/11/14 11:31), Steven Rostedt wrote:
On Tue, 2012-11-13 at 18:03 -0800, David Sharp wrote:
On Tue, Nov 13, 2012 at 6:00 PM, Steven Rostedt rost...@goodmis.org
wrote:
On Wed, 2012-11-14 at 10:36 +0900, Yoshihiro YUNOMAE wrote:
To merge the data like previous pattern, we apply this patch set. Then,
we can
get TSC offset of the guest as follows:
$ dmesg | grep kvm
[ 57.717180] kvm: (2687) write TSC offset 18446743360465545001, now
clock ##
|
PID TSC offset
|
HOST TSC
value --+
Using printk to export something like this is IMO a nasty hack.
Can't we create a /sys or /proc file to export the same thing?
Since the value changes over the course of the trace, and seems to be
part of the context of the trace, I think I'd include it as a
tracepoint.
I'm fine with that too.
Using some tracepoint is a nice idea, but there is one problem. Here,
our discussion point is the event which TSC offset is changed does not
frequently occur, but the buffer must keep the event data.
There are two ideas for using tracepoint. First, we define new
tracepoint for changed TSC offset. This is simple and the overhead will
be low. However, this trace event stored in the buffer will be
overwritten by other trace events because this TSC offset event does
not frequently occur. Second, we add TSC offset information to the
tracepoint frequently occured. For example, we assume that TSC offset
information is added to arguments of trace_kvm_exit().
The TSC offset is in the host trace. So given a host trace with two TSC
offset updates, how do you know which events in the guest trace
(containing a number of events) refer to which tsc offset update?
Unless i am missing something, you can't solve this easily (well, except
exporting information to the guest that allows it to transform RDTSC -
host TSC value, which can be done via pvclock).
As you say, TSC offset events are in the host trace, but we don't need
to notify guests of updating TSC offset. The offset event will output
the next TSC offset value and the current TSC value, so we can
calculate the guest TSC (T1) for the event. Guest TSCs since T1 can be
converted to host TSC using the TSC offset, so we can integrate those
trace data.
Think of this scenario:
host trace
1h. event tsc write tsc_offset=-1000
3h. vmenter
4h. vmexit
... (event sequence)
99h. vmexit
100h. event tsc_write tsc_offset=-2000
101h. vmenter
... (event sequence).
500h. event tsc_write tsc_offset=-3000
Then a guest trace containing events with a TSC timestamp.
Which tsc_offset to use?
(that is the problem, which unless i am mistaken can only be solved
easily if the guest can convert RDTSC - TSC of host).
Another issue as mentioned is lack of TSC synchronization in the host.
Should you provide such a feature without the possibility of proper
chronological order on systems with unsynchronized TSC?
I think, we cannot support this sorting feature using TSC on systems
with unsynchronized TSC. On systems with unsynchronized TSC, it is
difficult to sort not only trace data of guests and the host but trace
data of a guest or a host using TSC in chronological order. Actually,
if we want to output tracing data of ftrace in chronological order with
unsynchronized TSC, we will use the global mode as the timestamp. The
global mode uses wallclock added TSC correction, so the mode guarantees
to sort in chronological order for trace data of the guest or of
the host. If we use this mode to sort the trace data of guests and the
host in chronological order, we need to consider about the difference
between the guest and the host and timekeeping of guests and the host,
so it is difficult to solve these issues. At least, I haven't came up
with the good solution.
I suppose the tradeoff is performance (RDTSC) versus reliability, when
using ftrace. But then, even ftrace on the host suffers from the
same problem, with unsynchronized TSCs.
We cannot sort the trace data of guests and the host in chronological
order with unsynchronized TSC, but if we can set following
synchronization events for both guests and the host, we will know where
we should sort.
First, a guest and the host uses the global mode as the timestamp of
ftrace. Next, a user on the guest writes 1 to the synchronization I/F
as the ID, then the synchronization event 1 is recorded in a
ring-buffer of the guest. The synchronization operation induces
hypercall, so the host can handle the event. After the
Re: Re: [RFC PATCH 0/2] kvm/vmx: Output TSC offset
Hi Marcelo, Thank you for commenting on my patch set. (2012/11/16 12:19), Marcelo Tosatti wrote: On Wed, Nov 14, 2012 at 10:36:21AM +0900, Yoshihiro YUNOMAE wrote: [...] In this summary, I suggest the patch which TSC offset for each guest can be output on the host. The guest TSC can change (for example if TSC scaling is used). Moreover TSC offset can change, and you'd have to monitor that. What Yes, that's true. Changing TSC offset is the key point to use TSC for merging trace data of guests and the host. about a module option so that tsc_offset is written as zero (to be used as debugging tool). Then the following restrictions apply: - TSC must be synchronized across CPUs/VCPUS. - TSC must be reliable. Would that suffice? (a module option to kvm.ko, say zero_tsc_offset). As you say, the guest TSC can change, so guest TSC needs to meet these two restrictions to merge the trace data in chronological order. However, the zero-TSC offset method is not enough, I think. I will use TSC values as the tracing timestamp not only for debugging but for failure analysis on actual operations. When we introduce the zero-TSC offset, normally it will be no problem. However, if the guest executes write_tsc or the guest works live migration, TSC offset will be changed. After all, we need to monitor the TSC offset value. Thank you, -- Yoshihiro YUNOMAE Software Platform Research Dept. Linux Technology Center Hitachi, Ltd., Yokohama Research Laboratory E-mail: yoshihiro.yunomae...@hitachi.com -- To unsubscribe from this list: send the line unsubscribe kvm in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html
Re: Re: [RFC PATCH 0/2] kvm/vmx: Output TSC offset
On Fri, Nov 16, 2012 at 05:09:40PM +0900, Yoshihiro YUNOMAE wrote: Hi Marcelo, Thank you for commenting on my patch set. (2012/11/16 12:19), Marcelo Tosatti wrote: On Wed, Nov 14, 2012 at 10:36:21AM +0900, Yoshihiro YUNOMAE wrote: [...] In this summary, I suggest the patch which TSC offset for each guest can be output on the host. The guest TSC can change (for example if TSC scaling is used). Moreover TSC offset can change, and you'd have to monitor that. What Yes, that's true. Changing TSC offset is the key point to use TSC for merging trace data of guests and the host. about a module option so that tsc_offset is written as zero (to be used as debugging tool). Then the following restrictions apply: - TSC must be synchronized across CPUs/VCPUS. - TSC must be reliable. Would that suffice? (a module option to kvm.ko, say zero_tsc_offset). As you say, the guest TSC can change, so guest TSC needs to meet these two restrictions to merge the trace data in chronological order. However, the zero-TSC offset method is not enough, I think. I will use TSC values as the tracing timestamp not only for debugging but for failure analysis on actual operations. When we introduce the zero-TSC offset, normally it will be no problem. However, if the guest executes write_tsc or the guest works live migration, TSC offset will be changed. After all, we need to monitor the TSC offset value. Thank you, What i wrote was not precise. With TSC scaling (of AMD's svm.c) or TSC trapping, the guest RDTSC is not monotonic with reference to the host TSC. That is, TSC scaling and/or trapping are fundamentally incompatible with the feature you propose. What you are saying is that is you'd like this feature for production, not debugging. Unfortunately this requires a synchronized TSC across CPUs, which is not widespread. Therefore the suggestion to introduce a debugging facility (you cannot rely on this for failure analysis on all systems). Even then, a more reliable method for reporting TSC offset must be used. Apart from these issues, it is very useful to correlate guest/host events in the way you propose. -- To unsubscribe from this list: send the line unsubscribe kvm in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html
Re: Re: [RFC PATCH 0/2] kvm/vmx: Output TSC offset
On Wed, 2012-11-14 at 17:26 +0900, Yoshihiro YUNOMAE wrote: Thank you for commenting on my patch set. (2012/11/14 11:31), Steven Rostedt wrote: On Tue, 2012-11-13 at 18:03 -0800, David Sharp wrote: On Tue, Nov 13, 2012 at 6:00 PM, Steven Rostedt rost...@goodmis.org wrote: On Wed, 2012-11-14 at 10:36 +0900, Yoshihiro YUNOMAE wrote: To merge the data like previous pattern, we apply this patch set. Then, we can get TSC offset of the guest as follows: $ dmesg | grep kvm [ 57.717180] kvm: (2687) write TSC offset 18446743360465545001, now clock ## | PID TSC offset | HOST TSC value --+ Using printk to export something like this is IMO a nasty hack. Can't we create a /sys or /proc file to export the same thing? Since the value changes over the course of the trace, and seems to be part of the context of the trace, I think I'd include it as a tracepoint. I'm fine with that too. Using some tracepoint is a nice idea, but there is one problem. Here, our discussion point is the event which TSC offset is changed does not frequently occur, but the buffer must keep the event data. If you can hold off a bit, for the 3.9 window, I plan on pushing multiple buffers for ftrace. That is, you can create a separate buffer just for the TSC offset events: cd /sys/kernel/debug echo tsc instances/new echo 1 instances/tsc/events/tsc/offset/enable Then the buffer will be used only for that event. The code is pretty much ready, but I want to test it more before the 3.8 window opens, which is why I'm waiting for 3.9. You can see the latest version here: git://git.kernel.org/pub/scm/linux/kernel/git/rostedt/linux-trace.git branch: rfc/multi-buffers-v7 -- Steve -- To unsubscribe from this list: send the line unsubscribe kvm in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html
Re: Re: [RFC PATCH 0/2] kvm/vmx: Output TSC offset
On Fri, Nov 16, 2012 at 10:05:54AM -0500, Steven Rostedt wrote:
On Wed, 2012-11-14 at 17:26 +0900, Yoshihiro YUNOMAE wrote:
Thank you for commenting on my patch set.
(2012/11/14 11:31), Steven Rostedt wrote:
On Tue, 2012-11-13 at 18:03 -0800, David Sharp wrote:
On Tue, Nov 13, 2012 at 6:00 PM, Steven Rostedt rost...@goodmis.org
wrote:
On Wed, 2012-11-14 at 10:36 +0900, Yoshihiro YUNOMAE wrote:
To merge the data like previous pattern, we apply this patch set.
Then, we can
get TSC offset of the guest as follows:
$ dmesg | grep kvm
[ 57.717180] kvm: (2687) write TSC offset 18446743360465545001, now
clock ##
|
PID TSC offset
|
HOST TSC
value --+
Using printk to export something like this is IMO a nasty hack.
Can't we create a /sys or /proc file to export the same thing?
Since the value changes over the course of the trace, and seems to be
part of the context of the trace, I think I'd include it as a
tracepoint.
I'm fine with that too.
Using some tracepoint is a nice idea, but there is one problem. Here,
our discussion point is the event which TSC offset is changed does not
frequently occur, but the buffer must keep the event data.
If you can hold off a bit, for the 3.9 window, I plan on pushing
multiple buffers for ftrace. That is, you can create a separate buffer
just for the TSC offset events:
cd /sys/kernel/debug
echo tsc instances/new
echo 1 instances/tsc/events/tsc/offset/enable
Then the buffer will be used only for that event.
The code is pretty much ready, but I want to test it more before the 3.8
window opens, which is why I'm waiting for 3.9.
You can see the latest version here:
git://git.kernel.org/pub/scm/linux/kernel/git/rostedt/linux-trace.git
branch: rfc/multi-buffers-v7
-- Steve
You need the tsc offset change event and the main events to be ordered,
so i can't see how this helps.
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Re: Re: [RFC PATCH 0/2] kvm/vmx: Output TSC offset
On Wed, Nov 14, 2012 at 05:26:10PM +0900, Yoshihiro YUNOMAE wrote:
Thank you for commenting on my patch set.
(2012/11/14 11:31), Steven Rostedt wrote:
On Tue, 2012-11-13 at 18:03 -0800, David Sharp wrote:
On Tue, Nov 13, 2012 at 6:00 PM, Steven Rostedt rost...@goodmis.org wrote:
On Wed, 2012-11-14 at 10:36 +0900, Yoshihiro YUNOMAE wrote:
To merge the data like previous pattern, we apply this patch set. Then,
we can
get TSC offset of the guest as follows:
$ dmesg | grep kvm
[ 57.717180] kvm: (2687) write TSC offset 18446743360465545001, now
clock ##
|
PID TSC offset
|
HOST TSC
value --+
Using printk to export something like this is IMO a nasty hack.
Can't we create a /sys or /proc file to export the same thing?
Since the value changes over the course of the trace, and seems to be
part of the context of the trace, I think I'd include it as a
tracepoint.
I'm fine with that too.
Using some tracepoint is a nice idea, but there is one problem. Here,
our discussion point is the event which TSC offset is changed does not
frequently occur, but the buffer must keep the event data.
There are two ideas for using tracepoint. First, we define new
tracepoint for changed TSC offset. This is simple and the overhead will
be low. However, this trace event stored in the buffer will be
overwritten by other trace events because this TSC offset event does
not frequently occur. Second, we add TSC offset information to the
tracepoint frequently occured. For example, we assume that TSC offset
information is added to arguments of trace_kvm_exit().
The TSC offset is in the host trace. So given a host trace with two TSC
offset updates, how do you know which events in the guest trace
(containing a number of events) refer to which tsc offset update?
Unless i am missing something, you can't solve this easily (well, except
exporting information to the guest that allows it to transform RDTSC -
host TSC value, which can be done via pvclock).
Another issue as mentioned is lack of TSC synchronization in the host.
Should you provide such a feature without the possibility of proper
chronological order on systems with unsynchronized TSC?
By adding the
information to the arguments, we can avoid the situation where the TSC
offset information is overwritten by other events. However, TSC offset
is not frequently changed and same information is output many times
because almost all data are waste. Therefore, only using tracepoint
is not good idea.
So, I suggest a hybrid method; record TSC offset change events and read
the last TSC offset from procfs when collecting the trace data.
In particular, the method is as follows:
1. Enable the tracepoint of TSC offset change and record the value
before and after changing
2. Start tracing
3. Stop tracing
4. Collect trace data and read /proc/pid/kvm/*
5. Check if any trace event recording the two TSC offsets exists
in the trace data
if(existing) = use trace event (flow 6)
else = use /proc/pid/kvm/* (flow 7)
6. Apply two TSC offsets of the trace event to the trace data and
sort the trace data
(Ex.)
* = tracepoint of changing TSC offset
. = another trace event
[START]*[END]
-- --
previous current
TSC offset TSC offset
7. Apply TSC offset of /proc/pid/kvm/* to the trace data and
sort the trace data
(Ex.)
. = another trace event(not tracepoint of changing TSC offset)
[START].[END]
---
current
TSC offset
Thanks,
--
Yoshihiro YUNOMAE
Software Platform Research Dept. Linux Technology Center
Hitachi, Ltd., Yokohama Research Laboratory
E-mail: yoshihiro.yunomae...@hitachi.com
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Re: [RFC PATCH 0/2] kvm/vmx: Output TSC offset
On Wed, Nov 14, 2012 at 10:36:21AM +0900, Yoshihiro YUNOMAE wrote:
Hi All,
The following patch set can make disordered trace data of a guest and a host
sorted in chronological order.
In a virtualization environment, it is difficult to analyze performance
problems, such as a delay of I/O request on a guest. This is because multiple
guests operate on the host. One of approaches for solving such kind of
problems
is to sort trace data of guests and the host in chronological order.
After we applied the patch set(https://lkml.org/lkml/2012/11/13/588), raw TSC
can be chosen as a timestamp of ftrace. TSC is useful for merging trace data
in chronological order by two reasons. One of the reasons is that guests can
directly read raw TSC from the CPU using rdtsc operation. This means that raw
TSC value is not software clock like sched_clock, so we don't need to consider
about how the timestamp is calculated. The other is that TSC of recent x86
CPUs
is constantly incremented. This means that we don't need to worry about pace
of
the timestamp. Therefore, choosing TSC as a timestamp for tracing is
reasonable
to integrate trace data of guests and a host.
Here, we need to consider about just one matter for using TSC on guests. TSC
value on a guest is always the host TSC plus the guest's TSC offset. In
other
words, to merge trace data using TSC as timestamp in chronological order, we
need to consider TSC offset of the guest.
However, only the host kernel can read the TSC offset from VMCS and TSC offset
is not output in anywhere now. In other words, tools in userland cannot get
the TSC offset value, so we cannot merge trace data of guest and the host in
chronological order. Therefore, the TSC offset should be exported for userland
tools.
In this patch set, TSC offset is exported by printk() on the host. I also
attached a tool for merging trace data of a guest and a host in chronological
order.
Example
We assume that wakeup-latency for a command is big on a guest. Normally
we will use ftrace's wakeup-latency tracer or event tracer on the guest, but
we
may not be able to solve this problem. This is because guests often exit to
the host for several reasons. In the next, we will use TSC as ftrace's
timestamp
and record the trace data on the guest and the host. Then, we get following
data:
/* guest data */
comm-3826 [000] d...49836825726903: sched_wakeup: [detail]
comm-3826 [000] d...49836832225344: sched_switch: [detail]
/* host data */
qemu-kvm-2687 [003] d...50550079203669: kvm_exit: [detail]
qemu-kvm-2687 [003] d...50550079206816: kvm_entry: [detail]
qemu-kvm-2687 [003] d...50550079240656: kvm_exit: [detail]
qemu-kvm-2687 [003] d...50550079243467: kvm_entry: [detail]
qemu-kvm-2687 [003] d...50550079256103: kvm_exit: [detail]
qemu-kvm-2687 [003] d...50550079268391: kvm_entry: [detail]
qemu-kvm-2687 [003] d...50550079280829: kvm_exit: [detail]
qemu-kvm-2687 [003] d...50550079286028: kvm_entry: [detail]
Since TSC offset is not considered, these data cannot be merged. If this trace
data is shown like as follows, we will be able to understand the reason:
qemu-kvm-2687 [003] d...50550079203669: kvm_exit: [detail]
qemu-kvm-2687 [003] d...50550079206816: kvm_entry: [detail]
comm-3826 [000] d.h.49836825726903: sched_wakeup: [detail] =
qemu-kvm-2687 [003] d...50550079240656: kvm_exit: [detail]
qemu-kvm-2687 [003] d...50550079243467: kvm_entry: [detail]
qemu-kvm-2687 [003] d...50550079256103: kvm_exit: [detail]
qemu-kvm-2687 [003] d...50550079268391: kvm_entry: [detail]
comm-3826 [000] d...49836832225344: sched_switch: [detail] =
qemu-kvm-2687 [003] d...50550079280829: kvm_exit: [detail]
qemu-kvm-2687 [003] d...50550079286028: kvm_entry: [detail]
In this case, we can understand wakeup-latency was big due to exit to host
twice. Getting this data sorted in chronological order is our goal.
To merge the data like previous pattern, we apply this patch set. Then, we can
get TSC offset of the guest as follows:
$ dmesg | grep kvm
[ 57.717180] kvm: (2687) write TSC offset 18446743360465545001, now clock ##
|
PID TSC offset |
HOST TSC value --+
We use this TSC offset value to a merge script and obtain the following data:
$ ./trace-merge.pl 18446743360465545001 host.data guest.data
hqemu-kvm-2687 [003] d...50550079203669: kvm_exit: [detail]
hqemu-kvm-2687 [003] d...50550079206816: kvm_entry: [detail]
gcomm-3826 [000] d.h.50550079226331: sched_wakeup: [detail] =
hqemu-kvm-2687 [003] d...50550079240656:
Re: Re: [RFC PATCH 0/2] kvm/vmx: Output TSC offset
Thank you for commenting on my patch set.
(2012/11/14 11:31), Steven Rostedt wrote:
On Tue, 2012-11-13 at 18:03 -0800, David Sharp wrote:
On Tue, Nov 13, 2012 at 6:00 PM, Steven Rostedt rost...@goodmis.org wrote:
On Wed, 2012-11-14 at 10:36 +0900, Yoshihiro YUNOMAE wrote:
To merge the data like previous pattern, we apply this patch set. Then, we can
get TSC offset of the guest as follows:
$ dmesg | grep kvm
[ 57.717180] kvm: (2687) write TSC offset 18446743360465545001, now clock ##
|
PID TSC offset |
HOST TSC value --+
Using printk to export something like this is IMO a nasty hack.
Can't we create a /sys or /proc file to export the same thing?
Since the value changes over the course of the trace, and seems to be
part of the context of the trace, I think I'd include it as a
tracepoint.
I'm fine with that too.
Using some tracepoint is a nice idea, but there is one problem. Here,
our discussion point is the event which TSC offset is changed does not
frequently occur, but the buffer must keep the event data.
There are two ideas for using tracepoint. First, we define new
tracepoint for changed TSC offset. This is simple and the overhead will
be low. However, this trace event stored in the buffer will be
overwritten by other trace events because this TSC offset event does
not frequently occur. Second, we add TSC offset information to the
tracepoint frequently occured. For example, we assume that TSC offset
information is added to arguments of trace_kvm_exit(). By adding the
information to the arguments, we can avoid the situation where the TSC
offset information is overwritten by other events. However, TSC offset
is not frequently changed and same information is output many times
because almost all data are waste. Therefore, only using tracepoint
is not good idea.
So, I suggest a hybrid method; record TSC offset change events and read
the last TSC offset from procfs when collecting the trace data.
In particular, the method is as follows:
1. Enable the tracepoint of TSC offset change and record the value
before and after changing
2. Start tracing
3. Stop tracing
4. Collect trace data and read /proc/pid/kvm/*
5. Check if any trace event recording the two TSC offsets exists
in the trace data
if(existing) = use trace event (flow 6)
else = use /proc/pid/kvm/* (flow 7)
6. Apply two TSC offsets of the trace event to the trace data and
sort the trace data
(Ex.)
* = tracepoint of changing TSC offset
. = another trace event
[START]*[END]
-- --
previous current
TSC offset TSC offset
7. Apply TSC offset of /proc/pid/kvm/* to the trace data and
sort the trace data
(Ex.)
. = another trace event(not tracepoint of changing TSC offset)
[START].[END]
---
current
TSC offset
Thanks,
--
Yoshihiro YUNOMAE
Software Platform Research Dept. Linux Technology Center
Hitachi, Ltd., Yokohama Research Laboratory
E-mail: yoshihiro.yunomae...@hitachi.com
--
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the body of a message to majord...@vger.kernel.org
More majordomo info at http://vger.kernel.org/majordomo-info.html
[RFC PATCH 0/2] kvm/vmx: Output TSC offset
Hi All,
The following patch set can make disordered trace data of a guest and a host
sorted in chronological order.
In a virtualization environment, it is difficult to analyze performance
problems, such as a delay of I/O request on a guest. This is because multiple
guests operate on the host. One of approaches for solving such kind of problems
is to sort trace data of guests and the host in chronological order.
After we applied the patch set(https://lkml.org/lkml/2012/11/13/588), raw TSC
can be chosen as a timestamp of ftrace. TSC is useful for merging trace data
in chronological order by two reasons. One of the reasons is that guests can
directly read raw TSC from the CPU using rdtsc operation. This means that raw
TSC value is not software clock like sched_clock, so we don't need to consider
about how the timestamp is calculated. The other is that TSC of recent x86 CPUs
is constantly incremented. This means that we don't need to worry about pace of
the timestamp. Therefore, choosing TSC as a timestamp for tracing is reasonable
to integrate trace data of guests and a host.
Here, we need to consider about just one matter for using TSC on guests. TSC
value on a guest is always the host TSC plus the guest's TSC offset. In other
words, to merge trace data using TSC as timestamp in chronological order, we
need to consider TSC offset of the guest.
However, only the host kernel can read the TSC offset from VMCS and TSC offset
is not output in anywhere now. In other words, tools in userland cannot get
the TSC offset value, so we cannot merge trace data of guest and the host in
chronological order. Therefore, the TSC offset should be exported for userland
tools.
In this patch set, TSC offset is exported by printk() on the host. I also
attached a tool for merging trace data of a guest and a host in chronological
order.
Example
We assume that wakeup-latency for a command is big on a guest. Normally
we will use ftrace's wakeup-latency tracer or event tracer on the guest, but we
may not be able to solve this problem. This is because guests often exit to
the host for several reasons. In the next, we will use TSC as ftrace's timestamp
and record the trace data on the guest and the host. Then, we get following
data:
/* guest data */
comm-3826 [000] d...49836825726903: sched_wakeup: [detail]
comm-3826 [000] d...49836832225344: sched_switch: [detail]
/* host data */
qemu-kvm-2687 [003] d...50550079203669: kvm_exit: [detail]
qemu-kvm-2687 [003] d...50550079206816: kvm_entry: [detail]
qemu-kvm-2687 [003] d...50550079240656: kvm_exit: [detail]
qemu-kvm-2687 [003] d...50550079243467: kvm_entry: [detail]
qemu-kvm-2687 [003] d...50550079256103: kvm_exit: [detail]
qemu-kvm-2687 [003] d...50550079268391: kvm_entry: [detail]
qemu-kvm-2687 [003] d...50550079280829: kvm_exit: [detail]
qemu-kvm-2687 [003] d...50550079286028: kvm_entry: [detail]
Since TSC offset is not considered, these data cannot be merged. If this trace
data is shown like as follows, we will be able to understand the reason:
qemu-kvm-2687 [003] d...50550079203669: kvm_exit: [detail]
qemu-kvm-2687 [003] d...50550079206816: kvm_entry: [detail]
comm-3826 [000] d.h.49836825726903: sched_wakeup: [detail] =
qemu-kvm-2687 [003] d...50550079240656: kvm_exit: [detail]
qemu-kvm-2687 [003] d...50550079243467: kvm_entry: [detail]
qemu-kvm-2687 [003] d...50550079256103: kvm_exit: [detail]
qemu-kvm-2687 [003] d...50550079268391: kvm_entry: [detail]
comm-3826 [000] d...49836832225344: sched_switch: [detail] =
qemu-kvm-2687 [003] d...50550079280829: kvm_exit: [detail]
qemu-kvm-2687 [003] d...50550079286028: kvm_entry: [detail]
In this case, we can understand wakeup-latency was big due to exit to host
twice. Getting this data sorted in chronological order is our goal.
To merge the data like previous pattern, we apply this patch set. Then, we can
get TSC offset of the guest as follows:
$ dmesg | grep kvm
[ 57.717180] kvm: (2687) write TSC offset 18446743360465545001, now clock ##
|
PID TSC offset |
HOST TSC value --+
We use this TSC offset value to a merge script and obtain the following data:
$ ./trace-merge.pl 18446743360465545001 host.data guest.data
hqemu-kvm-2687 [003] d...50550079203669: kvm_exit: [detail]
hqemu-kvm-2687 [003] d...50550079206816: kvm_entry: [detail]
gcomm-3826 [000] d.h.50550079226331: sched_wakeup: [detail] =
hqemu-kvm-2687 [003] d...50550079240656: kvm_exit: [detail]
hqemu-kvm-2687 [003] d...50550079243467: kvm_entry: [detail]
hqemu-kvm-2687 [003] d...50550079256103: kvm_exit: [detail]
h
Re: [RFC PATCH 0/2] kvm/vmx: Output TSC offset
On Wed, 2012-11-14 at 10:36 +0900, Yoshihiro YUNOMAE wrote:
To merge the data like previous pattern, we apply this patch set. Then, we can
get TSC offset of the guest as follows:
$ dmesg | grep kvm
[ 57.717180] kvm: (2687) write TSC offset 18446743360465545001, now clock ##
|
PID TSC offset |
HOST TSC value --+
Using printk to export something like this is IMO a nasty hack.
Can't we create a /sys or /proc file to export the same thing?
-- Steve
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Re: [RFC PATCH 0/2] kvm/vmx: Output TSC offset
On 11/13/2012 06:00 PM, Steven Rostedt wrote: Using printk to export something like this is IMO a nasty hack. Can't we create a /sys or /proc file to export the same thing? Maybe we need a /proc/pid/kvm/* directory? -hpa -- To unsubscribe from this list: send the line unsubscribe kvm in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html
Re: [RFC PATCH 0/2] kvm/vmx: Output TSC offset
On Tue, Nov 13, 2012 at 6:00 PM, Steven Rostedt rost...@goodmis.org wrote:
On Wed, 2012-11-14 at 10:36 +0900, Yoshihiro YUNOMAE wrote:
To merge the data like previous pattern, we apply this patch set. Then, we
can
get TSC offset of the guest as follows:
$ dmesg | grep kvm
[ 57.717180] kvm: (2687) write TSC offset 18446743360465545001, now clock
##
|
PID TSC offset |
HOST TSC value --+
Using printk to export something like this is IMO a nasty hack.
Can't we create a /sys or /proc file to export the same thing?
Since the value changes over the course of the trace, and seems to be
part of the context of the trace, I think I'd include it as a
tracepoint.
-- Steve
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Re: [RFC PATCH 0/2] kvm/vmx: Output TSC offset
On Tue, 2012-11-13 at 18:03 -0800, David Sharp wrote:
On Tue, Nov 13, 2012 at 6:00 PM, Steven Rostedt rost...@goodmis.org wrote:
On Wed, 2012-11-14 at 10:36 +0900, Yoshihiro YUNOMAE wrote:
To merge the data like previous pattern, we apply this patch set. Then, we
can
get TSC offset of the guest as follows:
$ dmesg | grep kvm
[ 57.717180] kvm: (2687) write TSC offset 18446743360465545001, now
clock ##
|
PID TSC offset
|
HOST TSC value
--+
Using printk to export something like this is IMO a nasty hack.
Can't we create a /sys or /proc file to export the same thing?
Since the value changes over the course of the trace, and seems to be
part of the context of the trace, I think I'd include it as a
tracepoint.
I'm fine with that too.
-- Steve
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