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SubjectRe: [PATCH v3 04/14] perf/hw_breakpoint: Optimize list of per-task breakpoints
On Wed, 20 Jul 2022 at 17:29, Ian Rogers <irogers@google.com> wrote:
>
> On Mon, Jul 4, 2022 at 8:06 AM 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.
> >
> > 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@google.com>
> > Reviewed-by: Dmitry Vyukov <dvyukov@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@google.com>

Thanks!
-- Marco

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