From: David Ahern <dsahern@gmail.com>
To: Ingo Molnar <mingo@kernel.org>,
Joseph Schuchart <joseph.schuchart@tu-dresden.de>,
Peter Zijlstra <a.p.zijlstra@chello.nl>,
Fr??d??ric Weisbecker <fweisbec@gmail.com>
Cc: Paul Mackerras <paulus@samba.org>, Ingo Molnar <mingo@redhat.com>,
Arnaldo Carvalho de Melo <acme@ghostprotocols.net>,
thomas.ilsche@tu-dresden.de, linux-kernel@vger.kernel.org
Subject: Re: [PATCH] Perf: Correct Assumptions about Sample Timestamps in Passes
Date: Thu, 14 Nov 2013 07:26:06 -0700 [thread overview]
Message-ID: <5284DD7E.30604@gmail.com> (raw)
In-Reply-To: <20131114100552.GA5064@gmail.com>
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
next prev parent reply other threads:[~2013-11-14 14:26 UTC|newest]
Thread overview: 24+ messages / expand[flat|nested] mbox.gz Atom feed top
2013-11-14 8:07 [PATCH] Perf: Correct Assumptions about Sample Timestamps in Passes Joseph Schuchart
2013-11-14 8:39 ` Ingo Molnar
2013-11-14 8:59 ` Joseph Schuchart
2013-11-14 10:05 ` Ingo Molnar
2013-11-14 14:26 ` David Ahern [this message]
2013-11-14 14:44 ` Peter Zijlstra
2013-11-14 15:02 ` David Ahern
2013-11-14 15:25 ` Peter Zijlstra
2013-11-21 14:55 ` Joseph Schuchart
2013-11-27 13:51 ` Ingo Molnar
2013-12-20 12:27 ` Joseph Schuchart
2013-12-20 17:09 ` David Ahern
2013-12-23 13:10 ` Frederic Weisbecker
2013-12-23 14:44 ` David Ahern
2013-12-26 15:14 ` Frederic Weisbecker
2013-12-26 15:24 ` David Ahern
2013-12-26 15:30 ` Frederic Weisbecker
2014-01-01 18:37 ` David Ahern
2014-01-03 22:07 ` Frederic Weisbecker
2014-01-03 22:45 ` David Ahern
2014-01-04 15:05 ` Frederic Weisbecker
2014-01-08 21:48 ` David Ahern
2014-01-09 15:19 ` Frederic Weisbecker
2014-01-12 15:46 ` David Ahern
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