Re: [PATCH v4 2/9] arm64: perf: Enable pmu counter direct access for perf event on armv8

[Will Deacon]

On Fri, Nov 13, 2020 at 06:06:33PM +0000, Mark Rutland wrote:
> On Thu, Oct 01, 2020 at 09:01:09AM -0500, Rob Herring wrote:
> > +static void armv8pmu_event_unmapped(struct perf_event *event, struct mm_struct *mm)
> > +{
> > +	if (!(event->hw.flags & ARMPMU_EL0_RD_CNTR))
> > +		return;
> > +
> > +	if (atomic_dec_and_test(&mm->context.pmu_direct_access))
> > +		on_each_cpu_mask(mm_cpumask(mm), refresh_pmuserenr, NULL, 1);
> > +}
> 
> I didn't think we kept our mm_cpumask() up-to-date in all cases on
> arm64, so I'm not sure we can use it like this.
> 
> Will, can you confirm either way?

We don't update mm_cpumask() as the cost of the atomic showed up in some
benchmarks I did years ago and we've never had any need for the thing anyway
because out TLB invalidation is one or all.

Will