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

From: Vikas Shivappa
Date: Mon Mar 21 2016 - 15:49:31 EST

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)


+ /*
+ * 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.