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Date:   Mon, 9 Nov 2020 18:33:00 +0100
From:   Peter Zijlstra <peterz@infradead.org>
To:     "Liang, Kan" <kan.liang@linux.intel.com>
Cc:     mingo@kernel.org, linux-kernel@vger.kernel.org,
        namhyung@kernel.org, eranian@google.com, irogers@google.com,
        gmx@google.com, acme@kernel.org, jolsa@redhat.com,
        ak@linux.intel.com
Subject: Re: [PATCH 1/3] perf/core: Flush PMU internal buffers for per-CPU
 events
Message-ID: <20201109173300.GM2611@hirez.programming.kicks-ass.net>
References: <20201106212935.28943-1-kan.liang@linux.intel.com>
 <20201109095235.GC2594@hirez.programming.kicks-ass.net>
 <20201109110405.GN2651@hirez.programming.kicks-ass.net>
 <0a1db246-c34a-22a3-160c-3e0c0a38119d@linux.intel.com>
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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
 	 */