Re: [PATCH 1/3] perf/core: Flush PMU internal buffers for per-CPU events

[Peter Zijlstra]

On Mon, Nov 09, 2020 at 09:49:31AM -0500, Liang, Kan wrote:
> > Maybe we can frob x86_pmu_enable()...
> 
> Could you please elaborate?

Something horrible like this. It will detect the first time we enable
the PMU on a new task (IOW we did a context switch) and wipe the
counters when user RDPMC is on...

diff --git a/arch/x86/events/core.c b/arch/x86/events/core.c
index 77b963e5e70a..d862927baaef 100644
--- a/arch/x86/events/core.c
+++ b/arch/x86/events/core.c
@@ -1289,6 +1289,15 @@ static void x86_pmu_enable(struct pmu *pmu)
                perf_events_lapic_init();
        }
 
+       if (cpuc->current != current) {
+               struct mm_struct *mm = current->mm;
+
+               cpuc->current = current;
+
+               if (mm && atomic_read(&mm->context.perf_rdpmc_allowed))
+                       wipe_dirty_counters();
+       }
+
        cpuc->enabled = 1;
        barrier();
 
diff --git a/arch/x86/events/perf_event.h b/arch/x86/events/perf_event.h
index 7895cf4c59a7..d16118cb3bd0 100644
--- a/arch/x86/events/perf_event.h
+++ b/arch/x86/events/perf_event.h
@@ -248,6 +248,8 @@ struct cpu_hw_events {
        unsigned int            txn_flags;
        int                     is_fake;
 
+       void                    *current;
+
        /*
         * Intel DebugStore bits
         */