Re: [PATCH 1/8] perf/hw_breakpoint: Optimize list of per-task breakpoints

Marco Elver
2022-06-09T18:37:47Z

On Thu, 9 Jun 2022 at 18:53, Dmitry Vyukov <dvyukov@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?