linux-kernel - Re: [PATCH v2] perf/core: Fix installing cgroup event into cpu

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Date:   Wed, 24 Jan 2018 16:32:38 +0800
From:   Lin Xiulei <linxiulei@...il.com>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Jiri Olsa <jolsa@...hat.com>, mingo@...hat.com, acme@...nel.org,
        alexander.shishkin@...ux.intel.com, linux-kernel@...r.kernel.org,
        tglx@...utronix.de, Stephane Eranian <eranian@...il.com>,
        torvalds@...ux-foundation.org, linux-perf-users@...r.kernel.org,
        Brendan Gregg <brendan.d.gregg@...il.com>,
        yang_oliver@...mail.com, jinli.zjl@...baba-inc.com,
        "leilei.lin" <leilei.lin@...baba-inc.com>
Subject: Re: [PATCH v2] perf/core: Fix installing cgroup event into cpu

2018-01-24 16:20 GMT+08:00 Peter Zijlstra <peterz@...radead.org>:
> On Wed, Jan 24, 2018 at 03:50:10PM +0800, linxiulei@...il.com wrote:
>> From: "leilei.lin" <leilei.lin@...baba-inc.com>
>>
>> Do not install cgroup event into the CPU context if the cgroup
>> is not running on this CPU
>>
>> While there is no task of cgroup running specified CPU, current
>> kernel still install cgroup event into CPU context, that causes
>> another cgroup event can't be installed into this CPU.
>
> This changelog doesn't really cover the extend of the changes done.
>

See below please

>
>> Signed-off-by: leilei.lin <leilei.lin@...baba-inc.com>
>> ---
>>  kernel/events/core.c | 44 +++++++++++++++++++++++++++++++-------------
>>  1 file changed, 31 insertions(+), 13 deletions(-)
>>
>> diff --git a/kernel/events/core.c b/kernel/events/core.c
>> index 4df5b69..f766b60 100644
>> --- a/kernel/events/core.c
>> +++ b/kernel/events/core.c
>> @@ -933,31 +933,36 @@ list_update_cgroup_event(struct perf_event *event,
>>  {
>>       struct perf_cpu_context *cpuctx;
>>       struct list_head *cpuctx_entry;
>> +     struct perf_cgroup *cgrp;
>>
>>       if (!is_cgroup_event(event))
>>               return;
>>
>>       /*
>>        * Because cgroup events are always per-cpu events,
>>        * this will always be called from the right CPU.
>>        */
>>       cpuctx = __get_cpu_context(ctx);
>> +     cgrp = perf_cgroup_from_task(current, ctx);
>>
>> +     /* cpuctx->cgrp is NULL unless a cgroup event is running in this CPU .*/
>> +     if (cgroup_is_descendant(cgrp->css.cgroup, event->cgrp->css.cgroup)) {
>> +             if (add)
>>                       cpuctx->cgrp = cgrp;
>> +             else
>> +                     cpuctx->cgrp = NULL;
>>       }
>> +
>> +     if (add && ctx->nr_cgroups++)
>> +             return;
>> +     else if (!add && --ctx->nr_cgroups)
>> +             return;
>> +
>> +     cpuctx_entry = &cpuctx->cgrp_cpuctx_entry;
>> +     if (add)
>> +             list_add(cpuctx_entry, this_cpu_ptr(&cgrp_cpuctx_list));
>> +     else
>> +             list_del(cpuctx_entry);
>>  }
>
> I'm a little confused; you unconditionally set cpuctx->cgrp for every
> add/delete.
>
> So if we have >1 cgroup events on, and we remove one, you still clear
> cpuctx->cgrp, that seems wrong.
>
> Why did you change that? The Changelog doesn't include enough clues for
> me to know what you were trying to do.

if we have > 1 cgroup events on, whenever a cgroup was really to be
deleted, only if
this cgroup is the same as the cgroup running on this cpu, I would
clear cpuctx->cgrp.
Reverse is the same.

Here is the problem, previous version didn't set cpuctx->cgrp anymore
if ctx->nr_cgroups > 1,
which cases a second event would not be activated immediately because
cpuctx->cgrp isn't equal
to event->cgrp at event_filter_match()