Patch "perf/core: Fix pmu::filter_match for SW-led groups" has been added to the 4.4-stable tree

This is a note to let you know that I've just added the patch titled

perf/core: Fix pmu::filter_match for SW-led groups

to the 4.4-stable tree which can be found at:
http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary

The filename of the patch is:
perf-core-fix-pmu-filter_match-for-sw-led-groups.patch
and it can be found in the queue-4.4 subdirectory.

If you, or anyone else, feels it should not be added to the stable tree, please let <[email protected]> know about it.

From: Mark Rutland <[email protected]>
Date: Tue, 14 Jun 2016 16:10:41 +0100
Subject: perf/core: Fix pmu::filter_match for SW-led groups

From: Mark Rutland <[email protected]>

commit 2c81a6477081966fe80b8c6daa68459bca896774 upstream.

The following commit:

66eb579e66ec ("perf: allow for PMU-specific event filtering")

added the pmu::filter_match() callback. This was intended to avoid HW constraints on events from resulting in extremely pessimistic scheduling.

However, pmu::filter_match() is only called for the leader of each event group. When the leader is a SW event, we do not filter the groups, and may fail at pmu::add() time, and when this happens we'll give up on scheduling any event groups later in the list until they are rotated ahead of the failing group.

This can result in extremely sub-optimal event scheduling behaviour, e.g. if running the following on a big.LITTLE platform:

$ taskset -c 0 ./perf stat \
 -e 'a57{context-switches,armv8_cortex_a57/config=0x11/}' \
 -e 'a53{context-switches,armv8_cortex_a53/config=0x11/}' \
 ls

     <not counted>      context-switches                                              (0.00%)
     <not counted>      armv8_cortex_a57/config=0x11/                                 (0.00%)
                24      context-switches                                              (37.36%)
          57589154      armv8_cortex_a53/config=0x11/                                 (37.36%)

Here the 'a53' event group was always eligible to be scheduled, but the 'a57' group never eligible to be scheduled, as the task was always affine to a Cortex-A53 CPU. The SW (group leader) event in the 'a57' group was eligible, but the HW event failed at pmu::add() time, resulting in ctx_flexible_sched_in giving up on scheduling further groups with HW events.

One way of avoiding this is to check pmu::filter_match() on siblings as well as the group leader. If any of these fail their pmu::filter_match() call, we must skip the entire group before attempting to add any events.

---
 kernel/events/core.c |   23 ++++++++++++++++++++++-
 1 file changed, 22 insertions(+), 1 deletion(-)

@@ -1539,12 +1539,33 @@ static int __init perf_workqueue_init(vo
 
 core_initcall(perf_workqueue_init);
 
-static inline int pmu_filter_match(struct perf_event *event)
+static inline int __pmu_filter_match(struct perf_event *event)
 {
 	struct pmu *pmu = event->pmu;
 	return pmu->filter_match ? pmu->filter_match(event) : 1;
 }
 
+/*
+ * Check whether we should attempt to schedule an event group based on
+ * PMU-specific filtering. An event group can consist of HW and SW events,
+ * potentially with a SW leader, so we must check all the filters, to
+ * determine whether a group is schedulable:
+ */
+static inline int pmu_filter_match(struct perf_event *event)
+{
+	struct perf_event *child;
+
+	if (!__pmu_filter_match(event))
+		return 0;
+
+	list_for_each_entry(child, &event->sibling_list, group_entry) {
+		if (!__pmu_filter_match(child))
+			return 0;
+	}
+
+	return 1;
+}
+
 static inline int
 event_filter_match(struct perf_event *event)
 {

Patches currently in stable-queue which might be from [email protected] are

queue-4.4/perf-core-fix-pmu-filter_match-for-sw-led-groups.patch