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    SubjectRe: [PATCH v2] perf/core: set cgroup in cpu contexts for new cgroup events
    Hi Vegard,

    I don't think this patch fixes your bug, but it touches some code that
    may be related.

    David

    On Wed, Aug 3, 2016 at 8:46 AM, Vegard Nossum <vegard.nossum@gmail.com> wrote:
    > Hi,
    >
    > On 2 August 2016 at 09:48, David Carrillo-Cisneros <davidcc@google.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@google.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@oracle.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

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