Messages in this thread | | | Date | Thu, 10 Nov 2016 12:04:23 +0000 | From | Mark Rutland <> | Subject | Re: [PATCH] perf/core: introduce context per CPU event list |
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On Thu, Nov 10, 2016 at 12:37:05PM +0100, Peter Zijlstra wrote: > On Thu, Nov 10, 2016 at 11:05:17AM +0000, Mark Rutland wrote: > > On Thu, Nov 10, 2016 at 09:33:55AM +0100, Peter Zijlstra wrote: > > > Yes this is a problem, but no this cannot be done. We can't have per-cpu > > > storage per task. That rapidly explodes. > > > > > > Mark is looking at replacing this stuff with an rb-tree for big-little, > > > that would also allow improving this I think. > > > > Unfortunately I've not had the chance to look at that since returning > > from Plumbers. Also, I was leaning towards the alternative approach we > > discussed, with a perf_event_task_contexts container, as that also > > solved some other issues with the way we used perf_event_context::pmu in > > big.LITTLE systems. > > > > Looking at the way perf_iterate_ctx is used, it seems that we're just > > trying to iterate over the active events for a context (i.e. those > > programmed into the HW at this point in time). Though I'm missing some > > subtlety, since we check event->state < PERF_EVENT_STATE_INACTIVE. > > > > We have a similar issue with perf_event_task_tick() needing to know the > > relevant contexts, and for that we have the active_ctx_list. Can't we do > > something similar and add an active_events_list to perf_event_context? > > So the problem is finding which events are active when.
Sure.
If we only care about PERF_EVENT_STATE_ACTIVE, then I think we can fairly easily maintain a perf_event_context::active_event_list at event_sched_{in,out}() time (or somewhere close to that).
If we need PERF_EVENT_STATE_INACTIVE events, then that doesn't work, since we can give up early and not schedule some eligible events.
> If we stick all events in an RB-tree sorted on: {pmu,cpu,runtime} we > can, fairly easily, find the relevant subtree and limit the iteration. > Esp. if we use a threaded tree.
That would cater for big.LITTLE, certainly, but I'm not sure I follow how that helps to find active events -- you'll still have to iterate through the whole PMU subtree to find which are active, no?
Thanks, Mark.
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