[PATCH 3/3] perf: Optimize perf_init_event() for TYPE_SOFTWARE

[Peter Zijlstra <peterz@infradead.org>]

From: "Liang, Kan" <kan.liang@linux.intel.com>

Andi reported that he was hitting the linear search in
perf_init_event() a lot. Now that all !TYPE_SOFTWARE events should hit
the IDR, make sure the TYPE_SOFTWARE events are at the head of the
list such that we'll quickly find the right PMU (provided a valid
event was given).

Reported-by: Andi Kleen <andi@firstfloor.org>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
---
 kernel/events/core.c |   11 ++++++++++-
 1 file changed, 10 insertions(+), 1 deletion(-)

--- a/kernel/events/core.c
+++ b/kernel/events/core.c
@@ -10167,7 +10167,16 @@ int perf_pmu_register(struct pmu *pmu, c
 	if (!pmu->event_idx)
 		pmu->event_idx = perf_event_idx_default;
 
-	list_add_rcu(&pmu->entry, &pmus);
+	/*
+	 * Ensure the TYPE_SOFTWARE PMUs are at the head of the list,
+	 * since these cannot be in the IDR. This way the linear search
+	 * is fast, provided a valid software event is provided.
+	 */
+	if (type == PERF_TYPE_SOFTWARE || !name)
+		list_add_rcu(&pmu->entry, &pmus);
+	else
+		list_add_tail_rcu(&pmu->entry, &pmus);
+
 	atomic_set(&pmu->exclusive_cnt, 0);
 	ret = 0;
 unlock: