perf: Store active software events in a hashlist

Each time a software event triggers, we need to walk through
the entire list of events from the current cpu and task contexts
to retrieve a running perf event that matches.
We also need to check a matching perf event is actually counting.

This walk is wasteful and makes the event fast path scaling
down with a growing number of events running on the same
contexts.

To solve this, we store the running perf events in a hashlist to
get an immediate access to them against their type:event_id when
they trigger.

v2: - Fix SWEVENT_HLIST_SIZE definition (and re-learn some basic
      maths along the way)
    - Only allocate hlist for online cpus, but keep track of the
      refcount on offline possible cpus too, so that we allocate it
      if needed when it becomes online.
    - Drop the kref use as it's not adapted to our tricks anymore.

v3: - Fix bad refcount check (address instead of value). Thanks to
      Eric Dumazet who spotted this.
    - While exiting cpu, move the hlist release out of the IPI path
      to lock the hlist mutex sanely.

Signed-off-by: Frederic Weisbecker <fweisbec@gmail.com>
Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
Cc: Arnaldo Carvalho de Melo <acme@redhat.com>
Cc: Paul Mackerras <paulus@samba.org>
Cc: Ingo Molnar <mingo@elte.hu>

1 files changed, 12 insertions, 0 deletions

diff --git a/include/linux/perf_event.h b/include/linux/perf_event.h
index 6e96cc8225d..bf896d0b2e9 100644
--- a/include/linux/perf_event.h
+++ b/include/linux/perf_event.h
@@ -589,6 +589,14 @@ enum perf_group_flag {
 	PERF_GROUP_SOFTWARE = 0x1,
 };
 
+#define SWEVENT_HLIST_BITS	8
+#define SWEVENT_HLIST_SIZE	(1 << SWEVENT_HLIST_BITS)
+
+struct swevent_hlist {
+	struct hlist_head	heads[SWEVENT_HLIST_SIZE];
+	struct rcu_head		rcu_head;
+};
+
 /**
  * struct perf_event - performance event kernel representation:
  */
@@ -597,6 +605,7 @@ struct perf_event {
 	struct list_head		group_entry;
 	struct list_head		event_entry;
 	struct list_head		sibling_list;
+	struct hlist_node		hlist_entry;
 	int				nr_siblings;
 	int				group_flags;
 	struct perf_event		*group_leader;
@@ -744,6 +753,9 @@ struct perf_cpu_context {
 	int				active_oncpu;
 	int				max_pertask;
 	int				exclusive;
+	struct swevent_hlist		*swevent_hlist;
+	struct mutex			hlist_mutex;
+	int				hlist_refcount;
 
 	/*
 	 * Recursion avoidance: