lkml.org 
[lkml]   [2013]   [Nov]   [14]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
/
Date
From
SubjectRe: [PATCH] Perf: Correct Assumptions about Sample Timestamps in Passes
On 11/14/13, 3:05 AM, Ingo Molnar wrote:

> What am I missing?

I have spent quite a bit of time on this problem on this well. I think
the flush time needs to be based on the start time of each round, not
the minimum time observed across mmaps. I have tried the minimum time
stamp route and it still fails often enough to be annoying.

See builtin-kvm.c, perf_kvm__mmap_read(). The problem is that it takes
time to move from mmap to mmap and sample can come in behind you - an
mmap that has already be scanned with a timestamp less than what is
determined to be the lowest minimum for the samples actually read. 'perf
kvm stat live' in a nested environment is a stress test for the problem.

I believe reading perf_clock at the start of each round and using that
as the flush time will fix the problem (to the degree that perf_clock is
monotonic across all cpus and sockets).

But, right now we have no means of reading the perf_clock timestamp in
userspace.

David


\
 
 \ /
  Last update: 2013-11-14 16:41    [W:0.101 / U:0.188 seconds]
©2003-2020 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site