perf, x86: Don't reset the LBR as frequently

If we reset the LBR on each first counter, simple counter rotation which
first deschedules all counters and then reschedules the new ones will
lead to LBR reset, even though we're still in the same task context.

Reduce this by not flushing on the first counter but only flushing on
different task contexts.
Signed-off-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
Cc: Arnaldo Carvalho de Melo <acme@infradead.org>
Cc: paulus@samba.org
Cc: eranian@google.com
Cc: robert.richter@amd.com
Cc: fweisbec@gmail.com
LKML-Reference: <new-submission>
Signed-off-by: Ingo Molnar <mingo@elte.hu>

arch/x86/kernel/cpu/perf_event_intel_lbr.c

@@ -72,12 +72,11 @@ static void intel_pmu_lbr_enable(struct perf_event *event)
 	WARN_ON_ONCE(cpuc->enabled);
 
 	/*
-	 * Reset the LBR stack if this is the first LBR user or
-	 * we changed task context so as to avoid data leaks.
+	 * Reset the LBR stack if we changed task context to
+	 * avoid data leaks.
 	 */
 
-	if (!cpuc->lbr_users ||
-	    (event->ctx->task && cpuc->lbr_context != event->ctx)) {
+	if (event->ctx->task && cpuc->lbr_context != event->ctx) {
 		intel_pmu_lbr_reset();
 		cpuc->lbr_context = event->ctx;
 	}
@@ -93,7 +92,7 @@ static void intel_pmu_lbr_disable(struct perf_event *event)
 		return;
 
 	cpuc->lbr_users--;
-	BUG_ON(cpuc->lbr_users < 0);
+	WARN_ON_ONCE(cpuc->lbr_users < 0);
 
 	if (cpuc->enabled && !cpuc->lbr_users)
 		__intel_pmu_lbr_disable();