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From:   Dmitry Vyukov <dvyukov@google.com>
Date:   Fri, 10 Jun 2022 11:04:33 +0200
Message-ID: <CACT4Y+ZyrWuZxqpO_fKBjdXbTY-GJu6M7GARVk7YQnyv790mFw@mail.gmail.com>
Subject: Re: [PATCH 1/8] perf/hw_breakpoint: Optimize list of per-task breakpoints
To:     Marco Elver <elver@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 20:37, Marco Elver <elver@google.com> wrote:
> > /On Thu, 9 Jun 2022 at 16:56, 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. :-)
> >
> > Let's wait for Peter to shout then :)
> > A significant part of this change is having per-task data w/o having
> > per-task data.
> >
> > The current perf-related data in task_struct is already multiple words
> > and it's also not used in lots of production cases.
> > Maybe we could have something like:
> >
> >   struct perf_task_data* lazily_allocated_perf_data;
> >
> > that's lazily allocated on first use instead of the current
> > perf_event_ctxp/perf_event_mutex/perf_event_list.
> > This way we could both reduce task_size when perf is not used and have
> > more perf-related data (incl breakpoints) when it's used.
>
> I don't mind either option, so keeping task_struct bloat in mind, we have:
>
>   1. rhashtable option, no changes to task_struct.
>
>   2. add the breakpoint mutex + list to task_struct.
>
>   3. add something like hw_breakpoint_task_data* and allocate lazily.
>
>   4. (your proposal) move all of perf data into a new struct (+add
> hw_breakpoint things in there) that is lazily allocated.
>
> I don't think perf is that infrequently used, and I can't estimate
> performance impact, so I don't like #4 too much personally. My
> preferred compromise would be #3, but at the same time I'd rather not
> bloat task_struct even with 8 extra infrequently used bytes. Am I too
> paranoid?
>
> Preferences?


There is also this "could eventually get its own" comment:

static struct pmu perf_breakpoint = {
  .task_ctx_nr = perf_sw_context, /* could eventually get its own */
https://elixir.bootlin.com/linux/v5.19-rc1/source/kernel/events/hw_breakpoint.c#L669

If it gets its own, then it also gets a perf_event_context pointer in
task_struct:
https://elixir.bootlin.com/linux/v5.19-rc1/source/include/linux/sched.h#L1229
And perf_event_context has its own mutex and lots of other stuff.
But I don't know what other implications it has.