Messages in this thread | | | From | Marco Elver <> | Date | Wed, 20 Jul 2022 17:39:06 +0200 | Subject | Re: [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
| |