Re: [PATCH 1/3] arm/pmu: Reject groups spanning multiple hardware PMUs

[Peter Zijlstra <peterz@infradead.org>]

On Tue, Mar 10, 2015 at 12:05:21PM +0000, Mark Rutland wrote:
> On Tue, Mar 10, 2015 at 11:27:23AM +0000, Peter Zijlstra wrote:
> > On Mon, Mar 09, 2015 at 12:46:30PM +0000, Suzuki K. Poulose wrote:
> > > From: "Suzuki K. Poulose" <suzuki.poulose@arm.com>
> > > 
> > > Don't allow grouping hardware events from different PMUs
> > >  (eg. CCI + CPU).
> > 
> > Uhm, how does this work? If we have multiple hardware PMUs we'll stop
> > scheduling events after the first failed event schedule. This can leave
> > one of the PMUs severely under utilized.
> 
> The problem is here group validation at pmu::event_init() time, not
> scheduling.

Maybe make that a little more explicit.

> We don't allow grouping across disparate HW PMUs because we can't
> provide group semantics anyway. Scheduling is not a problem in this case
> (unlike the big.LITTLE case I have a patch for [1]).

Right, I remember that; I was wondering if this was related.

> We have a CPU PMU and an "uncore" CCI PMU. You can't create task-bound
> events for the CCI, but you can create CPU-bound events for the CCI on
> the nominal CPU the CCI is monitored from.

Indeed, ok.

> The context check you added in c3c87e770458aa00 "perf: Tighten (and fix)
> the grouping condition" implicitly rejects groups that have CPU and CCI
> events (each event::ctx will be the relevant pmu::pmu_cpu_context and
> will differ), and this is sane -- you can't provide group semantics
> across disparate HW PMUs.

Agreed.

> Unfortunately that happens after we've done the
> event->pmu->event_init(event) dance on each event, and in our event_init
> function we try to verify the group is sane. In our verification we
> ignore SW events, but assume that all !SW events are for the CPU PMU.
> If you add a CPU event to a CCI group, that's not the case, and we use
> container_of on an unsuitable object, derefence garbage, invoke the
> eschaton and so on.

Indeed, on x86 we explicitly ignore everything not an x86_pmu event.

> It would be nicer if we could prevent this in the core so we're not
> reliant on every PMU driver doing the same verification. My initial
> thought was that seemed like unnecessary duplication of the ctx checking
> above, but if we're going to end up shoving it into several drivers
> anyway perhaps it's the lesser evil.

Again, agreed, that would be better and less error prone. But I'm not
entirely sure how to go about doing it :/ I'll have to go think about
that; and conferences are not the best place for that.

Suggestions on that are welcome of course ;)