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

[Paul Mackerras <paulus@samba.org>]

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.