On Tue, Oct 22, 2019 at 11:20:18AM +0200, Peter Zijlstra wrote: > Andi reported that when creating a lot of events, a lot of time is > spend in IPIs and asked if it would be possible to elide some of that. > > Now when, as for example the perf-tool always does, events are created > disabled, then these events will not need to be scheduled when added > to the context (they're still disable) and therefore the IPI is not > required -- except for the very first event, that will need to set > ctx->is_active. > > ( it might be possible to set ctx->is_active remotely for cpu_ctx, but > we really need the IPI for task_ctx, so lets not make that > distinction ) > > Also use __perf_effective_state() since group events depend on the > state of the leader, if the leader is OFF, the whole group is OFF. > > So when sibling events are created enabled (XXX check tool) then we > only need a single IPI to create and enable the whole group (+ that > initial IPI to initialize the context).
Arguably, we could possible do something like so as well, but I'm not sure it makes sense as it will not help if IOC_ENABLE is called in creation order. Because in that case we'll enable the group leader before the siblings and we'll schedule them one at a time, instead of the whole group at once. --- diff --git a/kernel/events/core.c b/kernel/events/core.c index f9a5d4356562..2a5e6d9654e1 100644 --- a/kernel/events/core.c +++ b/kernel/events/core.c @@ -2791,6 +2791,7 @@ static void __perf_event_enable(struct perf_event *event, static void _perf_event_enable(struct perf_event *event) { struct perf_event_context *ctx = event->ctx; + struct perf_event *leader; raw_spin_lock_irq(&ctx->lock); if (event->state >= PERF_EVENT_STATE_INACTIVE || @@ -2808,6 +2809,17 @@ static void _perf_event_enable(struct perf_event *event) */ if (event->state == PERF_EVENT_STATE_ERROR) event->state = PERF_EVENT_STATE_OFF; + + /* + * If this is a sibling event and the group leader is still OFF + * then there is no point in trying to schedule this event. + */ + leader = event->group_leader; + if (leader != event && leader->state == PERF_EVENT_STATE_OFF) { + event->state = PERF_EVENT_STATE_INACTIVE; + raw_spin_unlock_irq(&ctx->lock); + return; + } raw_spin_unlock_irq(&ctx->lock); event_function_call(event, __perf_event_enable, NULL);