linux-kernel - Re: [PATCH v2] perf/core: set cgroup in cpu contexts for new cgroup events

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Date:	Wed, 3 Aug 2016 17:46:08 +0200
From:	Vegard Nossum <vegard.nossum@...il.com>
To:	David Carrillo-Cisneros <davidcc@...gle.com>
Cc:	LKML <linux-kernel@...r.kernel.org>,
	"x86@...nel.org" <x86@...nel.org>, Ingo Molnar <mingo@...hat.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Andi Kleen <ak@...ux.intel.com>,
	Kan Liang <kan.liang@...el.com>,
	Peter Zijlstra <peterz@...radead.org>,
	Paul Turner <pjt@...gle.com>,
	Stephane Eranian <eranian@...gle.com>
Subject: Re: [PATCH v2] perf/core: set cgroup in cpu contexts for new cgroup events

Hi,

On 2 August 2016 at 09:48, David Carrillo-Cisneros <davidcc@...gle.com> wrote:
> There is an optimization in perf_cgroup_sched_{in,out} that skips the
> switch of cgroup events if the old and new cgroups in a task switch are
> the same. This optimization interacts with the current code in two ways
> that cause a cpu context's cgroup (cpuctx->cgrp) to be NULL even if a
> cgroup event matches the current task. These are:
>
>   1. On creation of the first cgroup event in a CPU: In current code,
>   cpuctx->cpu is only set in perf_cgroup_sched_in, but due to the
>   aforesaid optimization, perf_cgroup_sched_in will run until the next
>   cgroup switches in that cpu. This may happen late or never happen,
>   depending on system's number of cgroups, cpu load, etc.
>
>   2. On deletion of the last cgroup event in a cpuctx: In list_del_event,
>   cpuctx->cgrp is set NULL. Any new cgroup event will not be sched in
>   because cpuctx->cgrp == NULL until a cgroup switch occurs and
>   perf_cgroup_sched_in is executed (updating cpuctx->cgrp).
>
> This patch fixes both problems by setting cpuctx->cgrp in list_add_event,
> mirroring what list_del_event does when removing a cgroup event from CPU
> context, as introduced in:
> commit 68cacd29167b ("perf_events: Fix stale ->cgrp pointer in
> update_cgrp_time_from_cpuctx()")
>
> With this patch, cpuctx->cgrp is always set/clear when installing/removing
> the first/last cgroup event in/from the cpu context. With cpuctx->cgrp
> correctly set, event_filter_match works as intended when events are
> sched in/out.
>
> The problem is easy to observe in a machine with only one cgroup:
>
>   $ perf stat -e cycles -I 1000 -C 0 -G /
>   #          time             counts unit events
>       1.000161699      <not counted>      cycles                    /
>       2.000355591      <not counted>      cycles                    /
>       3.000565154      <not counted>      cycles                    /
>       4.000951350      <not counted>      cycles                    /
>
> After the fix, the output is as expected:
>
>   $ perf stat -e cycles -I 1000 -a -G /
>   #         time             counts unit events
>      1.004699159          627342882      cycles                    /
>      2.007397156          615272690      cycles                    /
>      3.010019057          616726074      cycles                    /
>
> Rebased at peterz/queue/perf/core.
>
> Changes in v2:
>   - Fix build error when no CONFIG_CGROUP_PERF.
>   - Unify add and del cases into list_update_cgroup_event.
>   - Remove cgroup exclusive variables from builds
>   without CONFIG_CGROUP_PERF.
>
> Signed-off-by: David Carrillo-Cisneros <davidcc@...gle.com>

Is this supposed to fix the bug I reported here?

https://www.mail-archive.com/linux-kernel@vger.kernel.org/msg1197757.html

If so, you may want to add:

1) Fixes: f2fb6bef92514 ("perf/core: Optimize side-band event delivery")
2) Reported-by: Vegard Nossum <vegard.nossum@...cle.com>
3) a link to the thread

and I can give it a test to see if it fixes the problem I was running into.

If not, please ignore :-)

Thanks,


Vegard