Re: [PATCH 2/4] perf: Add exclude_task perf event attribute

[Peter Zijlstra <peterz@infradead.org>]

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.