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From:   Marco Elver <elver@google.com>
Date:   Thu, 9 Jun 2022 16:55:46 +0200
Message-ID: <CANpmjNNtV_6kgoLv=VX3z_oM6ZEvWJNAOj9z4ADcymqmhc+crw@mail.gmail.com>
Subject: Re: [PATCH 1/8] perf/hw_breakpoint: Optimize list of per-task breakpoints
To:     Dmitry Vyukov <dvyukov@google.com>
Cc:     Peter Zijlstra <peterz@infradead.org>,
        Frederic Weisbecker <frederic@kernel.org>,
        Ingo Molnar <mingo@kernel.org>,
        Thomas Gleixner <tglx@linutronix.de>,
        Arnaldo Carvalho de Melo <acme@kernel.org>,
        Mark Rutland <mark.rutland@arm.com>,
        Alexander Shishkin <alexander.shishkin@linux.intel.com>,
        Jiri Olsa <jolsa@redhat.com>,
        Namhyung Kim <namhyung@kernel.org>,
        linux-perf-users@vger.kernel.org, x86@kernel.org,
        linux-sh@vger.kernel.org, kasan-dev@googlegroups.com,
        linux-kernel@vger.kernel.org
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On Thu, 9 Jun 2022 at 16:29, Dmitry Vyukov <dvyukov@google.com> wrote:
>
> On Thu, 9 Jun 2022 at 13:31, Marco Elver <elver@google.com> wrote:
> >
> > On a machine with 256 CPUs, running the recently added perf breakpoint
> > benchmark results in:
> >
> >  | $> perf bench -r 30 breakpoint thread -b 4 -p 64 -t 64
> >  | # Running 'breakpoint/thread' benchmark:
> >  | # Created/joined 30 threads with 4 breakpoints and 64 parallelism
> >  |      Total time: 236.418 [sec]
> >  |
> >  |   123134.794271 usecs/op
> >  |  7880626.833333 usecs/op/cpu
> >
> > The benchmark tests inherited breakpoint perf events across many
> > threads.
> >
> > Looking at a perf profile, we can see that the majority of the time is
> > spent in various hw_breakpoint.c functions, which execute within the
> > 'nr_bp_mutex' critical sections which then results in contention on that
> > mutex as well:
> >
> >     37.27%  [kernel]       [k] osq_lock
> >     34.92%  [kernel]       [k] mutex_spin_on_owner
> >     12.15%  [kernel]       [k] toggle_bp_slot
> >     11.90%  [kernel]       [k] __reserve_bp_slot
> >
> > The culprit here is task_bp_pinned(), which has a runtime complexity of
> > O(#tasks) due to storing all task breakpoints in the same list and
> > iterating through that list looking for a matching task. Clearly, this
> > does not scale to thousands of tasks.
> >
> > While one option would be to make task_struct a breakpoint list node,
> > this would only further bloat task_struct for infrequently used data.
>
> task_struct already has:
>
> #ifdef CONFIG_PERF_EVENTS
>   struct perf_event_context *perf_event_ctxp[perf_nr_task_contexts];
>   struct mutex perf_event_mutex;
>   struct list_head perf_event_list;
> #endif
>
> Wonder if it's possible to use perf_event_mutex instead of the task_sharded_mtx?
> And possibly perf_event_list instead of task_bps_ht? It will contain
> other perf_event types, so we will need to test type as well, but on
> the positive side, we don't need any management of the separate
> container.

Hmm, yes, I looked at that but then decided against messing the
perf/core internals. The main issue I have with using perf_event_mutex
is that we might interfere with perf/core's locking rules as well as
interfere with other concurrent perf event additions. Using
perf_event_list is very likely a no-go because it requires reworking
perf/core as well.

I can already hear Peter shouting, but maybe I'm wrong. :-)