[PATCH v3 04/14] perf/hw_breakpoint: Optimize list of per-task breakpoints

[Marco Elver]

On Wed, 20 Jul 2022 at 17:29, Ian Rogers <irogers at google.com> wrote:
>
> On Mon, Jul 4, 2022 at 8:06 AM Marco Elver <elver at 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.
> >
> > Instead, make use of the "rhashtable" variant "rhltable" which stores
> > multiple items with the same key in a list. This results in average
> > runtime complexity of O(1) for task_bp_pinned().
> >
> > With the optimization, the benchmark shows:
> >
> >  | $> 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: 0.208 [sec]
> >  |
> >  |      108.422396 usecs/op
> >  |     6939.033333 usecs/op/cpu
> >
> > On this particular setup that's a speedup of ~1135x.
> >
> > While one option would be to make task_struct a breakpoint list node,
> > this would only further bloat task_struct for infrequently used data.
> > Furthermore, after all optimizations in this series, there's no evidence
> > it would result in better performance: later optimizations make the time
> > spent looking up entries in the hash table negligible (we'll reach the
> > theoretical ideal performance i.e. no constraints).
> >
> > Signed-off-by: Marco Elver <elver at google.com>
> > Reviewed-by: Dmitry Vyukov <dvyukov at google.com>
> > ---
> > v2:
> > * Commit message tweaks.
> > ---
> >  include/linux/perf_event.h    |  3 +-
> >  kernel/events/hw_breakpoint.c | 56 ++++++++++++++++++++++-------------
> >  2 files changed, 37 insertions(+), 22 deletions(-)
> >
> > diff --git a/include/linux/perf_event.h b/include/linux/perf_event.h
> > index 01231f1d976c..e27360436dc6 100644
> > --- a/include/linux/perf_event.h
> > +++ b/include/linux/perf_event.h
> > @@ -36,6 +36,7 @@ struct perf_guest_info_callbacks {
> >  };
> >
> >  #ifdef CONFIG_HAVE_HW_BREAKPOINT
> > +#include <linux/rhashtable-types.h>
> >  #include <asm/hw_breakpoint.h>
> >  #endif
> >
> > @@ -178,7 +179,7 @@ struct hw_perf_event {
> >                          * creation and event initalization.
> >                          */
> >                         struct arch_hw_breakpoint       info;
> > -                       struct list_head                bp_list;
> > +                       struct rhlist_head              bp_list;
>
> nit: perhaps it would be more intention revealing here to rename this
> to bp_hashtable?

The naming convention for uses of rhlist_head appears to be either
'list' or 'node' (also inside lib/rhashtable.c). I think this makes
sense because internally this struct is used to just append to the
bucket's list.

> Acked-by: Ian Rogers <irogers at google.com>

Thanks!
-- Marco