Messages in this thread | | | Date | Tue, 8 Jun 2010 21:02:27 +0200 | From | Frederic Weisbecker <> | Subject | Re: [PATCH 2/4] perf: Add exclude_task perf event attribute |
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On Tue, Jun 08, 2010 at 08:59:17PM +0200, Ingo Molnar wrote: > > * Frederic Weisbecker <fweisbec@gmail.com> wrote: > > > On Tue, May 25, 2010 at 08:58:08AM +0200, Peter Zijlstra wrote: > > > On Tue, 2010-05-25 at 11:43 +1000, Paul Mackerras wrote: > > > > 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. > > > > > > Right, so for hardware event we'd need to go with those irq_{enter,exit} > > > hooks and either fully disable the call, or do as Ingo suggested, read > > > the count delta and add that to period_left, so that we'll delay the > > > sample (and subtract from ->count, which is I think the trickiest bit as > > > it'll generate a non-monotonic ->count). > > > > > > So I prefer the disable/enable from irq_enter/exit, however I also > > > suspect that that is by far the most expensive option. > > > > > > Playing with that, it's easy to contain the counting on the filtered > > contexts: I can just flush (event->read()) when we enter/exit a context > > but filter the update of event->count depending on exclude_* things. > > > > There are several problems with that though: > > > > - overflow interrupts continue, we can block them, but still... > > - periods become randomly async as the interrupts happen. We > > could save the period_left on context enter to solve this > > > > > > It would be certainly easier and clearer to use stop/start things on context > > enter/exit. > > > > And the only thing that seem to happen in these paths is a write > > to the event config register. > > Is it what is going to be too slow? > > If you compare that to all the reads on the counter, > > the interrupts that still need to be serviced and filtered with the > > other solution, may be the stop/start solution is eventually better > > in contrast. > > > > How much time approximately does it take to write in this config register? > > it should be fast enough. I think we should first go for a good, high-quality > implementation that has a correct model for collecting information - and then, > if in practice there's any significant slowdown, we could perhaps add a > speedup that cuts corners. > > If we first cut corners we'll never be able to fully trust the info, and we'll > never know how it would all have played out via the disable/enable method. > > Thanks, > > Ingo
All agreed, I'm taking that direction then.
Thanks.
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