perf: remove sw events from multiplexing rotation list

Software events (sw,tracepoints,...) do not need to be multiplexed. However, when using a SW event, the SW PMU (TYPE_SOFTWARE, TYPE_TRACEPOINT) was added to the per-cpu rotation_list. That list is used by perf_rotate_context() to iterate over all the PMUs to rotate event lists and multiplex.

This is not needed for SW events. Doing this adds some unnecessary overhead, so drop it by rejecting SW PMUs in perf_pmu_rotate_start().

---

index 7b9df35..40f42b8 100644
@@ -651,6 +651,10 @@ static void perf_pmu_rotate_start(struct pmu *pmu)
 	struct perf_cpu_context *cpuctx = this_cpu_ptr(pmu->pmu_cpu_context);
 	struct list_head *head = &__get_cpu_var(rotation_list);
 
+	/* no need to mutliplex software events */
+	if (pmu->task_ctx_nr == perf_sw_context)
+		return;
+
 	WARN_ON(!irqs_disabled());
 
 	if (list_empty(&cpuctx->rotation_list))

Thanks!