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

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Date:	Fri, 11 Dec 2015 16:20:37 +0100
From:	Peter Zijlstra <peterz@...radead.org>
To:	Alexander Shishkin <alexander.shishkin@...ux.intel.com>
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

On Fri, Dec 11, 2015 at 04:02:36PM +0100, Peter Zijlstra wrote:
> 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.

One solution is to keep a filter generation counter and check that on
pmu::add(), and if mis-matched recompute the hardware setup at that
point.

> > +		}
> > +
> > +		/* matches smp_wmb() in event_sched_in() */
> > +		smp_rmb();
> > +
> > +		ret = cpu_function_call(READ_ONCE(event->oncpu),
> > +					__perf_event_itrace_filters_setup, event);
> 
> This otoh, running with IRQs disabled on the CPU the thing is active on
> guarantees it will not become inactive -- nothing can come in and switch
> it off.
> 
> > +	} while (ret == -EAGAIN);
> > +
> > +	return ret;
> > +}
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