Messages in this thread | | | From | Song Liu <> | Subject | Re: [PATCH v6] perf: Sharing PMU counters across compatible events | Date | Tue, 5 Nov 2019 23:51:42 +0000 |
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More details on where I am heading...
> On Sep 18, 2019, at 10:23 PM, Song Liu <songliubraving@fb.com> wrote: > > This patch tries to enable PMU sharing. To make perf event scheduling > fast, we use special data structures. > > An array of "struct perf_event_dup" is added to the perf_event_context, > to remember all the duplicated events under this ctx. All the events > under this ctx has a "dup_id" pointing to its perf_event_dup. Compatible > events under the same ctx share the same perf_event_dup. The following > figure shows a simplified version of the data structure. > > ctx -> perf_event_dup -> master > ^ > | > perf_event /| > | > perf_event / > > Connection among perf_event and perf_event_dup are built when events are > added or removed from the ctx. So these are not on the critical path of > schedule or perf_rotate_context(). > > On the critical paths (add, del read), sharing PMU counters doesn't > increase the complexity. Helper functions event_pmu_[add|del|read]() are > introduced to cover these cases. All these functions have O(1) time > complexity. > > We allocate a separate perf_event for perf_event_dup->master. This needs > extra attention, because perf_event_alloc() may sleep. To allocate the > master event properly, a new pointer, tmp_master, is added to perf_event. > tmp_master carries a separate perf_event into list_[add|del]_event(). > The master event has valid ->ctx and holds ctx->refcount.
If we do GFP_ATOMIC in perf_event_alloc(), maybe with an extra option, we don't need the tmp_master hack. So we only allocate master when we will use it.
> > Details about the handling of the master event is added to > include/linux/perf_event.h, before struct perf_event_dup. > > Cc: Peter Zijlstra <peterz@infradead.org> > Cc: Arnaldo Carvalho de Melo <acme@redhat.com> > Cc: Jiri Olsa <jolsa@kernel.org> > Cc: Alexey Budankov <alexey.budankov@linux.intel.com> > Cc: Namhyung Kim <namhyung@kernel.org> > Cc: Tejun Heo <tj@kernel.org> > Signed-off-by: Song Liu <songliubraving@fb.com> > --- > include/linux/perf_event.h | 61 ++++++++ > kernel/events/core.c | 294 ++++++++++++++++++++++++++++++++++--- > 2 files changed, 332 insertions(+), 23 deletions(-) > > diff --git a/include/linux/perf_event.h b/include/linux/perf_event.h > index 61448c19a132..a694e5eee80a 100644 > --- a/include/linux/perf_event.h > +++ b/include/linux/perf_event.h > @@ -722,6 +722,12 @@ struct perf_event { > #endif > > struct list_head sb_list; > + > + /* for PMU sharing */ > + struct perf_event *tmp_master; > + int dup_id;
I guess we can get rid of dup_id here, and just have struct perf_event *dup_master.
> + u64 dup_base_count; > + u64 dup_base_child_count; > #endif /* CONFIG_PERF_EVENTS */ > }; > > @@ -731,6 +737,58 @@ struct perf_event_groups { > u64 index; > }; > > +/* > + * Sharing PMU across compatible events > + * > + * If two perf_events in the same perf_event_context are counting same > + * hardware events (instructions, cycles, etc.), they could share the > + * hardware PMU counter. > + * > + * When a perf_event is added to the ctx (list_add_event), it is compared > + * against other events in the ctx. If they can share the PMU counter, > + * a perf_event_dup is allocated to represent the sharing. > + * > + * Each perf_event_dup has a virtual master event, which is called by > + * pmu->add() and pmu->del(). We cannot call perf_event_alloc() in > + * list_add_event(), so it is allocated and carried by event->tmp_master > + * into list_add_event(). > + * > + * Virtual master in different cases/paths: > + * > + * < I > perf_event_open() -> close() path: > + * > + * 1. Allocated by perf_event_alloc() in sys_perf_event_open(); > + * 2. event->tmp_master->ctx assigned in perf_install_in_context(); > + * 3.a. if used by ctx->dup_events, freed in perf_event_release_kernel(); > + * 3.b. if not used by ctx->dup_events, freed in perf_event_open(). > + * > + * < II > inherit_event() path: > + * > + * 1. Allocated by perf_event_alloc() in inherit_event(); > + * 2. tmp_master->ctx assigned in inherit_event(); > + * 3.a. if used by ctx->dup_events, freed in perf_event_release_kernel(); > + * 3.b. if not used by ctx->dup_events, freed in inherit_event(). > + * > + * < III > perf_pmu_migrate_context() path: > + * all dup_events removed during migration (no sharing after the move). > + * > + * < IV > perf_event_create_kernel_counter() path: > + * not supported yet. > + */ > +struct perf_event_dup { > + /* > + * master event being called by pmu->add() and pmu->del(). > + * This event is allocated with perf_event_alloc(). When > + * attached to a ctx, this event should hold ctx->refcount. > + */ > + struct perf_event *master; > + /* number of events in the ctx that shares the master */ > + int total_event_count; > + /* number of active events of the master */ > + int active_event_count; > +};
And hopefully get rid of this.
Please let me know if this doesn't work.
Thanks, Song
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