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From:   Namhyung Kim <namhyung@kernel.org>
Date:   Wed, 24 Mar 2021 10:06:13 +0900
Message-ID: <CAM9d7cjrrUkgdsrAOmWDJ0rTQpC5npnOYZzHx-3Pe-rfDB7uRg@mail.gmail.com>
Subject: Re: [PATCH 1/2] perf/core: Share an event with multiple cgroups
To:     Song Liu <songliubraving@fb.com>
Cc:     Peter Zijlstra <peterz@infradead.org>,
        Ingo Molnar <mingo@kernel.org>,
        Arnaldo Carvalho de Melo <acme@kernel.org>,
        Jiri Olsa <jolsa@redhat.com>,
        Mark Rutland <mark.rutland@arm.com>,
        Alexander Shishkin <alexander.shishkin@linux.intel.com>,
        LKML <linux-kernel@vger.kernel.org>,
        Stephane Eranian <eranian@google.com>,
        Andi Kleen <ak@linux.intel.com>,
        Ian Rogers <irogers@google.com>, Tejun Heo <tj@kernel.org>
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Hi Song,

On Wed, Mar 24, 2021 at 9:30 AM Song Liu <songliubraving@fb.com> wrote:
>
>
>
> > On Mar 23, 2021, at 9:21 AM, Namhyung Kim <namhyung@kernel.org> wrote:
> >
> > As we can run many jobs (in container) on a big machine, we want to
> > measure each job's performance during the run.  To do that, the
> > perf_event can be associated to a cgroup to measure it only.
> >
> > However such cgroup events need to be opened separately and it causes
> > significant overhead in event multiplexing during the context switch
> > as well as resource consumption like in file descriptors and memory
> > footprint.
> >
> > As a cgroup event is basically a cpu event, we can share a single cpu
> > event for multiple cgroups.  All we need is a separate counter (and
> > two timing variables) for each cgroup.  I added a hash table to map
> > from cgroup id to the attached cgroups.
> >
> > With this change, the cpu event needs to calculate a delta of event
> > counter values when the cgroups of current and the next task are
> > different.  And it attributes the delta to the current task's cgroup.
> >
> > This patch adds two new ioctl commands to perf_event for light-weight
> > cgroup event counting (i.e. perf stat).
> >
> > * PERF_EVENT_IOC_ATTACH_CGROUP - it takes a buffer consists of a
> >     64-bit array to attach given cgroups.  The first element is a
> >     number of cgroups in the buffer, and the rest is a list of cgroup
> >     ids to add a cgroup info to the given event.
> >
> > * PERF_EVENT_IOC_READ_CGROUP - it takes a buffer consists of a 64-bit
> >     array to get the event counter values.  The first element is size
> >     of the array in byte, and the second element is a cgroup id to
> >     read.  The rest is to save the counter value and timings.
> >
> > This attaches all cgroups in a single syscall and I didn't add the
> > DETACH command deliberately to make the implementation simple.  The
> > attached cgroup nodes would be deleted when the file descriptor of the
> > perf_event is closed.
>
> This is very interesting idea!

Thanks!

>
> Could you please add some description of the relationship among
> perf_event and contexts? The code is a little confusing. For example,
> why do we need cgroup_ctx_list?

Sure, a perf_event belongs to an event context (hw or sw, mostly) which
takes care of multiplexing, timing, locking and so on.  So many of the
fields in the perf_event are protected by the context lock.  A context has
a list of perf_events and there are per-cpu contexts and per-task contexts.

The cgroup_ctx_list is to traverse contexts (in that cpu) only having
perf_events with attached cgroups.

Hope this makes it clear.  Please let me know if you need more. :)

Thanks,
Namhyung