Re: [tip:perf/urgent] perf/x86/mbm: Implement RMID recycling

[Vikas Shivappa]

On Mon, 21 Mar 2016, Matt Fleming wrote:

> On Mon, 21 Mar, at 02:53:04AM, tip-bot for Vikas Shivappa wrote:
> > @@ -489,6 +496,22 @@ static u32 intel_cqm_xchg_rmid(struct perf_event *group, u32 rmid)
> >
> >  	raw_spin_unlock_irq(&cache_lock);
> >
> > +	/*
> > +	 * If the allocation is for mbm, init the mbm stats.
> > +	 * Need to check if each event in the group is mbm event
> > +	 * because there could be multiple type of events in the same group.
> > +	 */
> > +	if (__rmid_valid(rmid)) {
> > +		event = group;
> > +		if (is_mbm_event(event->attr.config))
> > +			init_mbm_sample(rmid, event->attr.config);
> > +
> > +		list_for_each_entry(event, head, hw.cqm_group_entry) {
> > +			if (is_mbm_event(event->attr.config))
> > +				init_mbm_sample(rmid, event->attr.config);
> > +		}
> > +	}
> > +
> >  	return old_rmid;
> >  }
>
> You're calling init_mbm_sample() without holding cache_lock. Won't
> this potentially trash the existing value in MSR_IA32_QM_EVTSEL, if
> say, we're reading the counter at the same time as the recycling
> worker is running?

The init_mbm_sample calls the update_sample to read the MSR in IPI .. Since the 
count is also in IPI , they should not trash each other ?

Basically all the MSR read/writes are in high irql , except for the mbm overflow 
timer and read calls which holds an irqsave spinlock.

Thanks,
Vikas


>