linux-kernel - Re: [PATCH] perf/core: introduce context per CPU event list

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Date:   Thu, 10 Nov 2016 12:37:05 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Mark Rutland <mark.rutland@....com>
Cc:     kan.liang@...el.com, mingo@...hat.com,
        linux-kernel@...r.kernel.org, acme@...nel.org, tglx@...utronix.de,
        alexander.shishkin@...ux.intel.com, vince@...ter.net,
        eranian@...gle.com, andi@...stfloor.org
Subject: Re: [PATCH] perf/core: introduce context per CPU event list

On Thu, Nov 10, 2016 at 11:05:17AM +0000, Mark Rutland wrote:
> Hi,
> 
> 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.

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.