Re: [PATCH] perf_events: improve x86 event scheduling (v5)

[Peter Zijlstra <peterz@infradead.org>]

On Mon, 2010-01-18 at 15:12 +0100, Stephane Eranian wrote:
> > That said you do have a point, maybe we can express this particular
> > thing differently.. maybe a pre and post group call like:
> >
> >  void hw_perf_group_sched_in_begin(struct pmu *pmu)
> >  int  hw_perf_group_sched_in_end(struct pmu *pmu)
> >
> The issue with hw_perf_group_sched_in() is that because we do not know
> when we are done scheduling, we have to defer actual activation until
> hw_perf_enable(). But  we have to still mark the events as ACTIVE,
> otherwise things go wrong in the generic layer and for non-PMU events.
> That leads to partial duplication of event_sched_in()/event_sched_out()
> in the PMU specific layer.
> 
> As Frederic pointed out, the more natural way would be to simply rely
> on event_sched_in()/event_sched_out() and the rollback logic and just
> drop hw_perf_group_sched_in() which is there as an optimization and
> not for correctness. Scheduling can be done incrementally from the
> event_sched_in() function.
> 
> > That way we know we need to track more state for rollback and can give
> > the pmu implementation leeway to delay scheduling/availablility tests.
> >
> Rollback would still be handled by the generic code, wouldn't it?

I'm not sure I understand your reply. Sure dropping
hw_perf_group_sched_in() is still correct, but its also less optimal,
since we have to determine schedulability for each incremental event.