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

Dmitry Vyukov
2022-06-09T16:53:38Z

.
/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.