linux-kernel - Re: [PATCH v0 3/5] perf: Introduce instruction trace filtering

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Date:	Fri, 11 Dec 2015 17:48:03 +0200
From:	Alexander Shishkin <alexander.shishkin@...ux.intel.com>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	Ingo Molnar <mingo@...hat.com>, linux-kernel@...r.kernel.org,
	vince@...ter.net, eranian@...gle.com,
	Arnaldo Carvalho de Melo <acme@...radead.org>,
	Mathieu Poirier <mathieu.poirier@...aro.org>
Subject: Re: [PATCH v0 3/5] perf: Introduce instruction trace filtering

Peter Zijlstra <peterz@...radead.org> writes:

> On Fri, Dec 11, 2015 at 05:27:22PM +0200, Alexander Shishkin wrote:
>> Peter Zijlstra <peterz@...radead.org> writes:
>> 
>> > On Fri, Dec 11, 2015 at 03:36:36PM +0200, Alexander Shishkin wrote:
>> >> +static int perf_event_itrace_filters_setup(struct perf_event *event)
>> >> +{
>> >> +	int ret;
>> >> +
>> >> +	/*
>> >> +	 * We can't use event_function_call() here, because that would
>> >> +	 * require ctx::mutex, but one of our callers is called with
>> >> +	 * mm::mmap_sem down, which would cause an inversion, see bullet
>> >> +	 * (2) in put_event().
>> >> +	 */
>> >> +	do {
>> >> +		if (READ_ONCE(event->state) != PERF_EVENT_STATE_ACTIVE) {
>> >> +			ret = event->pmu->itrace_filter_setup(event);
>> >> +			break;
>> >
>> > So this is tricky, if its not active it can be any moment, there is
>> > nothing serializing against that.
>> 
>> Indeed. But we should be able to call pmu::itrace_filter_setup()
>> multiple times, so if after this we re-check that the event is still
>> inactive, we can return, otherwise proceed with the cross-call. Does
>> this make sense?
>
> Dunno, I worry :-)
>
> What if:
>
> 	if (READ_ONCE(event->state) != PERF_EVENT_STATE_ACTIVE) {
> 		// we were INACTIVE, but now the event gets scheduled in
> 		// on _another_ CPU
> 		event->pmu->itrace_filter_setup() := {
> 			if (event->state == PERF_EVENT_STATE_ACTIVE) {
> 				/* muck with hardware */
> 			}
> 		}
> 	}
>
> Here too I feel a strict validation vs programming split would make sense.
>
> We can always call the validation thing, we must not call the program
> thing !ACTIVE is a clear and simple rule.

Ah, but pmu::itrace_filter_setup() does not touch the hardware,
pmu::start() does. The former keeps an array of, say, MSR values ready
for programming in event::hw and the latter actually writes the MSRs. So
the above example should not be a problem.

So in a way validation and programming are split already. And PT, for
example, won't have it any other way, you can only program stuff into
the registers while tracing is disabled.

Regards,
--
Alex
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