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Date: Tue, 25 May 2010 11:43:23 +1000 From: Paul Mackerras <paulus@...ba.org> To: Frederic Weisbecker <fweisbec@...il.com> Cc: Ingo Molnar <mingo@...e.hu>, LKML <linux-kernel@...r.kernel.org>, Peter Zijlstra <a.p.zijlstra@...llo.nl>, Arnaldo Carvalho de Melo <acme@...hat.com> Subject: Re: [PATCH 2/4] perf: Add exclude_task perf event attribute On Fri, May 21, 2010 at 04:05:13PM +0200, Frederic Weisbecker wrote: > Excluding is useful when you want to trace only hard and softirqs. > > For this we use a new generic perf_exclude_event() (the previous > one beeing turned into perf_exclude_swevent) to which you can pass > the preemption offset to which your events trigger. > > Computing preempt_count() - offset gives us the preempt_count() of > the context that the event has interrupted, on top of which we > can filter the non-irq contexts. How does this work for hardware events when we are sampling and getting an interrupt every N events? It seems like the hardware is still counting all events and interrupting every N events, but we are only recording a sample if the interrupt occurred in the context we want. In other words the context of the Nth event is considered to be the context for the N-1 events preceding that, which seems a pretty poor approximation. Also, for hardware events, if we are counting rather than sampling, the exclude_task bit will have no effect. So perhaps in that case the perf_event_open should fail rather than appear to succeed but give wrong data. Paul. -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@...r.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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