Am 10.09.2015 um 03:55 schrieb Wanpeng Li:
> On 9/9/15 9:39 PM, Christian Borntraeger wrote:
>> Am 03.09.2015 um 16:07 schrieb Wanpeng Li:
>>> v6 -> v7:
>>> * explicit signal (set a bool)
>>> * fix the tracepoint
>>>
>>> v5 -> v6:
>>> * fix wait_ns and poll_ns
>>>
>>> v4 -> v5:
>>> * set base case 10us and max poll time 500us
>>> * handle short/long halt, idea from David, many thanks David
>>>
>>> v3 -> v4:
>>> * bring back grow vcpu->halt_poll_ns when interrupt arrives and shrinks
>>> when idle VCPU is detected
>>>
>>> v2 -> v3:
>>> * grow/shrink vcpu->halt_poll_ns by *halt_poll_ns_grow or
>>> /halt_poll_ns_shrink
>>> * drop the macros and hard coding the numbers in the param definitions
>>> * update the comments "5-7 us"
>>> * remove halt_poll_ns_max and use halt_poll_ns as the max halt_poll_ns
>>> time,
>>> vcpu->halt_poll_ns start at zero
>>> * drop the wrappers
>>> * move the grow/shrink logic before "out:" w/ "if (waited)"
>>>
>>> v1 -> v2:
>>> * change kvm_vcpu_block to read halt_poll_ns from the vcpu instead of
>>> the module parameter
>>> * use the shrink/grow matrix which is suggested by David
>>> * set halt_poll_ns_max to 2ms
>>>
>>> There is a downside of always-poll since poll is still happened for idle
>>> vCPUs which can waste cpu usage. This patchset add the ability to adjust
>>> halt_poll_ns dynamically, to grow halt_poll_ns when shot halt is detected,
>>> and to shrink halt_poll_ns when long halt is detected.
>>>
>>> There are two new kernel parameters for changing the halt_poll_ns:
>>> halt_poll_ns_grow and halt_poll_ns_shrink.
>>>
>>> no-poll always-poll dynamic-poll
>>> -----------------------------------------------------------------------
>>> Idle (nohz) vCPU %c0 0.15% 0.3% 0.2%
>>> Idle (250HZ) vCPU %c0 1.1% 4.6%~14% 1.2%
>>> TCP_RR latency 34us 27us 26.7us
>>>
>>> "Idle (X) vCPU %c0" is the percent of time the physical cpu spent in
>>> c0 over 60 seconds (each vCPU is pinned to a pCPU). (nohz) means the
>>> guest was tickless. (250HZ) means the guest was ticking at 250HZ.
>>>
>>> The big win is with ticking operating systems. Running the linux guest
>>> with nohz=off (and HZ=250), we save 3.4%~12.8% CPUs/second and get close
>>> to no-polling overhead levels by using the dynamic-poll. The savings
>>> should be even higher for higher frequency ticks.
>>>
>>> Wanpeng Li (3):
>>> KVM: make halt_poll_ns per-vCPU
>>> KVM: dynamic halt-polling
>>> KVM: trace kvm_halt_poll_ns grow/shrink
>>>
>>> include/linux/kvm_host.h | 1 +
>>> include/trace/events/kvm.h | 30 +++++++++++++++++++
>>> virt/kvm/kvm_main.c | 72
>>> ++++++++++++++++++++++++++++++++++++++++++----
>>> 3 files changed, 97 insertions(+), 6 deletions(-)
>>>
>> I get some nice improvements for uperf between 2 guests,
>
> Good to hear that.
>
>> but there is one "bug":
>> If there is already some polling ongoing, its impossible to disable the
>> polling,
>
> The polling will stop if long halt is detected, and there is no need to
> manual tuning. Just like dynamise PLE window can detect false positive and
> handle ple window suitably.
Yes, but as soon as somebody sets halt_poll_ns to 0, polling will never stop,
as grow and shrink are only handled if halt_poll_ns is !=0.
[...]
if (halt_poll_ns) {
if (block_ns <= vcpu->halt_poll_ns)
;
/* we had a long block, shrink polling */
else if (vcpu->halt_poll_ns && block_ns > halt_poll_ns)
shrink_halt_poll_ns(vcpu);
/* we had a short halt and our poll time is too small */
else if (vcpu->halt_poll_ns < halt_poll_ns &&
block_ns < halt_poll_ns)
grow_halt_poll_ns(vcpu);
}
[...]
so maybe just do something like
diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c
index 4662a88..48828d6 100644
--- a/virt/kvm/kvm_main.c
+++ b/virt/kvm/kvm_main.c
@@ -2012,6 +2012,8 @@ out:
else if (vcpu->halt_poll_ns < halt_poll_ns &&
block_ns < halt_poll_ns)
grow_halt_poll_ns(vcpu);
+ } else {
+ vcpu->halt_poll_ns = 0;
}
trace_kvm_vcpu_wakeup(block_ns, waited);
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