ARM perf events spin locks

[Jamie Iles]

On Tue, Nov 30, 2010 at 01:49:32PM -0000, Will Deacon wrote:
> Jamie,
> 
> > On Tue, Nov 30, 2010 at 10:41:04AM -0000, Will Deacon wrote:
> > > > Hi Will,
> > >
> > > Hello,
> > >
> > > > I think we should convert the spinlocks in the ARM perf events code to raw
> > > > spinlocks for realtime. Should we wait for your split set to get merged first
> > > > before doing this?
> > >
> > > Since this is a logically separate change, I think we're better off waiting
> > > until the split stuff has been merged. As for the raw spinlocks, by realtime
> > > do you mean PREEMPT_RT? Also, do we actually *need* raw spinlocks in the perf
> > > code?
> > Yes, I meant PREEMPT_RT. It won't stop working without raw spinlocks but I'm
> > not convinced that we couldn't lose too much accuracy with normal spinlocks. I
> > am however willing to be convinced otherwise!
> 
> Well struct perf_event_ctx has a lock field which is of type raw_spinlock_t.
> I *think* this is always held by the core perf code before calling the backend,
> however IRQs may still be enabled so we probably do need to change our pmu_lock.
> 
> Is that a sane analysis?
That sounds perfectly reasonable to me! We don't hold these locks for any 
potentially long periods of time so it's probably better to be safe! Do you 
want me to make this change after perf-split is merged or is that something 
you're happy to do?

Jamie