Re: [RFC PATCH 1/4] perf: Starter and stopper events

[Frederic Weisbecker]

On Tue, Mar 15, 2011 at 10:36:18PM +0800, Lin Ming wrote:
> On Tue, Mar 15, 2011 at 3:18 AM, Frederic Weisbecker <fweisbec@xxxxxxxxx> wrote:
> 
> >
> > +static void perf_event_pause_resume(struct perf_event *event, int nmi,
> > +                                   struct perf_sample_data *data,
> > +                                   struct pt_regs *regs)
> > +{
> > +       struct perf_event *iter;
> > +       unsigned long flags;
> > +
> > +       /*
> > +        * Ensure the targets can't be sched in/out concurrently.
> > +        * Disabling irqs is sufficient for that because starters/stoppers
> > +        * are on the same cpu/task.
> > +        */
> > +       local_irq_save(flags);
> 
> Could you explain this more detail?

Yeah, I should have detailed that more.

So, I put a constraint in starters and stoppers: those must be attached
to the same task and cpu than the target. That allows us to do this
pause/resume lockless if we can ensure that:

- target sched in/out can't interrupt perf_event_pause_resume()
- perf_event_pause_resume() can interrupt the target in the middle of
event_sched_in()

So that both are strictly serialized.

We need to ensure that the target event can not be concurrently scheduled
in (->add()) or scheduled out (->del() ), so that when we check
PERF_EVENT_STATE_ACTIVE, we know that the event is currently running
and is not going to move while we do our checks and we call start() and
stop().

So the rationale is that the target can not be in the middle of
event_sched_in() or event_sched_out() when the starter/stopper
trigger. We have no guarantee of that currently, especially because
of events that trigger in NMIs, but also for other corner cases may
be, so I'll need to think about it later. Why not by using pmu_disable_all()
on the starter/stopper when the target is about to schedule in/out, until
we know the event->state really reflects the hardware and logical states.

Now event_sched_in() and event_sched_out() can still be called from an
IPI to enable/disable an event. Hence the interrupts disabled to prevent
from that.
 
> > +
> > +
> > +       /* Prevent the targets from being removed under us. */
> > +       rcu_read_lock();
> > +
> > +       list_for_each_entry_rcu(iter, &event->starter_list, starter_entry) {
> > +               /*
> > +                * There is a small race window here, between the state
> > +                * gets set to active and the event is actually ->add().
> > +                * We need to find a way to ensure the starter/stopper
> > +                * can't trigger in between.
> 
> Can we set the state to active after the event is actually ->add()?

If we do so, the event may trigger before its state gets updated and that state
can be checked from event fast paths. As is the case from hrtimer for example.

Or perf_event_update_userpage().

Not sure how we deal with that with del() called after we update the state.
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