Date: Thu, 10 Nov 2016 11:05:17 +0000
From: Mark Rutland <mark.rutland@arm.com>
To: Peter Zijlstra <peterz@infradead.org>
Cc: kan.liang@intel.com, mingo@redhat.com, linux-kernel@vger.kernel.org,
        acme@kernel.org, tglx@linutronix.de,
        alexander.shishkin@linux.intel.com, vince@deater.net,
        eranian@google.com, andi@firstfloor.org
Subject: Re: [PATCH] perf/core: introduce context per CPU event list
Message-ID: <20161110110516.GA4418@leverpostej>
References: <1478718286-12824-1-git-send-email-kan.liang@intel.com>
 <20161110083355.GO3568@worktop.programming.kicks-ass.net>
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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?

Thanks,
Mark.