Messages in this thread | | | Date | Tue, 25 May 2010 12:06:49 +0200 | From | Frederic Weisbecker <> | Subject | Re: [PATCH 2/4] perf: Add exclude_task perf event attribute |
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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.
Ingo proposed me another trick while discussing other details: having a per context count instead of a single whole one.
So instead of having event->count, we can have event->task_count/softirq_count and hardirq_count. Each time we enter irq_enter() (non-nested), we read the count register and we compute the difference on irq_exit() and add the result on event->hardirq_count. (similar kind of tricks for task and softirq counts).
So when we want to get the total, we just need to compute the sum, wrt the exclude_* options we have.
Now that still requires to keep the samples proxy. And the samples will stay a bit async as the interrupt period won't be paused when we enter a filtered context, something that would only be solved with a round of ->stop(). But as you said, I really suspect this is not viable.
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