On 2016/10/22 18:05, Michael Kerrisk (man-pages) wrote:
On 10/21/2016 11:25 PM, Vince Weaver wrote:
On Fri, 21 Oct 2016, Wang Nan wrote:
context_switch : 1, /* context switch data */
-
- __reserved_1 : 37;
+ write_backward : 1, /* Write ring buffer from end to beginning */
+ __reserved_1 : 36;
This removes a blank line, not sure if intentional or not.
Maybe it would be better to keep it. I don't feel too strongly about
this though.
+.IR "write_backward" " (since Linux 4.6)"
It didn't committed until Linux 4.7 from what I can tell?
Yes, that's my recollection too.
+This makes the resuling event use a backward ring-buffer, which
resulting
+writes samples from the end of the ring-buffer.
+
+It is not allowed to connect events with backward and forward
+ring-buffer settings together using
+.B PERF_EVENT_IOC_SET_OUTPUT.
+
+Backward ring-buffer is useful when the ring-buffer is overwritable
+(created by readonly
+.BR mmap (2)
+).
A ring buffer is over-writable when it is mmapped readonly?
Is this a hard requirement?
I'd like to explain over-writable ring buffer in patch 1/1 like this:
diff --git a/man2/perf_event_open.2 b/man2/perf_event_open.2
index fade28c..561331c 100644
--- a/man2/perf_event_open.2
+++ b/man2/perf_event_open.2
@@ -1687,6 +1687,15 @@ the
.I data_tail
value should be written by user space to reflect the last read data.
In this case, the kernel will not overwrite unread data.
+
+When the mapping is read only (without
+.BR PROT_WRITE ),
+setting .I data_tail is not allowed.
+In this case, the kernel will overwrite data when sample coming, unless
+the ring buffer is paused by a
+.BR PERF_EVENT_IOC_PAUSE_OUTPUT
+.BR ioctl (2)
+system call before reading.
.TP
.IR data_offset " (since Linux 4.1)"
.\" commit e8c6deac69629c0cb97c3d3272f8631ef17f8f0f
The ring buffer become over-writable because there's no way to tell kernel
the positioin of the last read data when mmaped read only.
Can you set the read-backwards bit if not mapped readonly?
I don't understand why we need read-backwards.
Mapped with PROT_WRITE is the *default* setting. In this case user program
like perf is able to tell the reading position to kernel through writing to
'data_tail'. In this case kernel won't overwrite unread data, it reads
forwardly.
Or do you think the naming is confusing? The name of 'write_backward' is
kernel-centric, means adjust kernel behavior. kernel *write* data, so I
call it 'write_backward'. The name 'read-backwards' is user-centric,
because user 'read' data.
Thank you.
