On Wed, May 24, 2017 at 08:47:00PM +0200, Thomas Gleixner wrote:
> On Wed, 24 May 2017, Paul E. McKenney wrote:
> > > @@ -8920,7 +8912,7 @@ perf_event_mux_interval_ms_store(struct
> > > pmu->hrtimer_interval_ms = timer;
> > >
> > > /* update all cpuctx for this PMU */
> > > - get_online_cpus();
> > > + cpus_read_lock();
> >
> > OK, I'll bite...
> >
> > Why is this piece using cpus_read_lock() instead of pmus_lock?
> >
> > My guess is for the benefit of the cpu_function_call() below, but if
> > the code instead cycled through the perf_online_mask, wouldn't any
> > CPU selected be guaranteed to be online?
>
> Indeed.
>
> > Or is there some reason that it would be necessary to specially handle
> > CPUs that perf does not consider to be active, but that are still at
> > least partway online?
>
> I have to delegate that question to Peter :)
Another reason might be a desire to avoid contention on pmus_lock,
if this function is called often. If that is the case, I cannot
resist suggesting percpu_rw_sem. ;-)
Thanx, Paul
