linux-kernel - Re: [PATCH v3 10/10] perf/cgroup: Do not switch system-wide events in cgroup switch

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Date:   Thu, 14 Nov 2019 08:46:51 -0500
From:   "Liang, Kan" <kan.liang@...ux.intel.com>
To:     Peter Zijlstra <peterz@...radead.org>,
        Ian Rogers <irogers@...gle.com>
Cc:     Ingo Molnar <mingo@...hat.com>,
        Arnaldo Carvalho de Melo <acme@...nel.org>,
        Mark Rutland <mark.rutland@....com>,
        Alexander Shishkin <alexander.shishkin@...ux.intel.com>,
        Jiri Olsa <jolsa@...hat.com>,
        Namhyung Kim <namhyung@...nel.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Masahiro Yamada <yamada.masahiro@...ionext.com>,
        Kees Cook <keescook@...omium.org>,
        Catalin Marinas <catalin.marinas@....com>,
        Petr Mladek <pmladek@...e.com>,
        Mauro Carvalho Chehab <mchehab+samsung@...nel.org>,
        Qian Cai <cai@....pw>, Joe Lawrence <joe.lawrence@...hat.com>,
        Tetsuo Handa <penguin-kernel@...ove.sakura.ne.jp>,
        Sri Krishna chowdary <schowdary@...dia.com>,
        "Uladzislau Rezki (Sony)" <urezki@...il.com>,
        Andy Shevchenko <andriy.shevchenko@...ux.intel.com>,
        Changbin Du <changbin.du@...el.com>,
        Ard Biesheuvel <ardb@...nel.org>,
        "David S. Miller" <davem@...emloft.net>,
        Kent Overstreet <kent.overstreet@...il.com>,
        Gary Hook <Gary.Hook@....com>, Arnd Bergmann <arnd@...db.de>,
        linux-kernel@...r.kernel.org,
        Stephane Eranian <eranian@...gle.com>,
        Andi Kleen <ak@...ux.intel.com>
Subject: Re: [PATCH v3 10/10] perf/cgroup: Do not switch system-wide events in
 cgroup switch



On 11/14/2019 5:43 AM, Peter Zijlstra wrote:
> On Wed, Nov 13, 2019 at 04:30:42PM -0800, Ian Rogers wrote:
>> From: Kan Liang <kan.liang@...ux.intel.com>
>>
>> When counting system-wide events and cgroup events simultaneously, the
>> system-wide events are always scheduled out then back in during cgroup
>> switches, bringing extra overhead and possibly missing events. Switching
>> out system wide flexible events may be necessary if the scheduled in
>> task's cgroups have pinned events that need to be scheduled in at a higher
>> priority than the system wide flexible events.
> 
> I'm thinking this patch is actively broken. groups->index 'group' wide
> and therefore across cpu/cgroup boundaries.
> 
> There is no !cgroup to cgroup hierarchy as this patch seems to assume,
> specifically look at how the merge sort in visit_groups_merge() allows
> cgroup events to be picked before !cgroup events.


No, the patch intends to avoid switch !cgroup during cgroup context 
switch.

In perf_cgroup_switch(), when the cgroup is scheduled out, current 
implementation schedule out everything including !cgroup. I think it 
definitely breaks the semantics of !cgroup aka system-wide event.

The patch itself doesn't touch the merge sort in visit_groups_merge().
The perf_cgroup_skip_switch() just skips the !cgroup in schedule_in().
Because !cgroup wasn't scheduled out. We don't want to schedule !cgroup 
in again.
The cgroup events must be after !cgroup events, since !cgroup never be 
switched.

Thanks,
Kan