[PATCH v2 3/4] perf/core: introduce PMU_EV_CAP_READ_ACTIVE_PKG

[David Carrillo-Cisneros]

Introduce the flag PMU_EV_CAP_READ_ACTIVE_PKG, useful for uncore events,
that allows a PMU to signal the generic perf code that an event is readable
in the current CPU if the event is active in a CPU in the same package as
the current CPU.

This is an optimization that avoids a unnecessary IPI for the common case
where uncore events are run and read in the same package but in
different CPUs.

As an example, the IPI removal speeds up perf_read in my Haswell system
as follows:
  - For event UNC_C_LLC_LOOKUP: From 260 us to 31 us.
  - For event RAPL's power/energy-cores/: From to 255 us to 27 us.

For the optimization to work, all events in the group must have it
(similarly to PERF_EV_CAP_SOFTWARE).

Signed-off-by: David Carrillo-Cisneros <davidcc@xxxxxxxxxx>
---
 include/linux/perf_event.h |  3 +++
 kernel/events/core.c       | 32 +++++++++++++++++++++++++-------
 2 files changed, 28 insertions(+), 7 deletions(-)

diff --git a/include/linux/perf_event.h b/include/linux/perf_event.h
index fa5617f..c8bb1b3 100644
--- a/include/linux/perf_event.h
+++ b/include/linux/perf_event.h
@@ -501,8 +501,11 @@ typedef void (*perf_overflow_handler_t)(struct perf_event *,
  * Event capabilities. For event_caps and groups caps.
  *
  * PERF_EV_CAP_SOFTWARE: Is a software event.
+ * PERF_EV_CAP_READ_ACTIVE_PKG: A CPU event (or cgroup event) that can be read
+ * from any CPU in the package where it is active.
  */
 #define PERF_EV_CAP_SOFTWARE		BIT(0)
+#define PERF_EV_CAP_READ_ACTIVE_PKG	BIT(1)
 
 #define SWEVENT_HLIST_BITS		8
 #define SWEVENT_HLIST_SIZE		(1 << SWEVENT_HLIST_BITS)
diff --git a/kernel/events/core.c b/kernel/events/core.c
index 34049cc..77f1bd3 100644
--- a/kernel/events/core.c
+++ b/kernel/events/core.c
@@ -3333,6 +3333,26 @@ struct perf_read_data {
 	int ret;
 };
 
+static int find_cpu_to_read(struct perf_event *event)
+{
+	bool active = event->state == PERF_EVENT_STATE_ACTIVE;
+	int event_cpu = event->oncpu, local_cpu = smp_processor_id();
+	u16 local_pkg, event_pkg;
+
+	if (!active)
+		return -1;
+
+	if (event->group_caps & PERF_EV_CAP_READ_ACTIVE_PKG) {
+		event_pkg =  topology_physical_package_id(event_cpu);
+		local_pkg =  topology_physical_package_id(local_cpu);
+
+		if (event_pkg == local_pkg)
+			return local_cpu;
+	}
+
+	return event_cpu;
+}
+
 /*
  * Cross CPU call to read the hardware event
  */
@@ -3454,19 +3474,17 @@ u64 perf_event_read_local(struct perf_event *event)
 
 static int perf_event_read(struct perf_event *event, bool group)
 {
-	int ret = 0;
+	int ret = 0, cpu_to_read;
 
-	/*
-	 * If event is enabled and currently active on a CPU, update the
-	 * value in the event structure:
-	 */
-	if (event->state == PERF_EVENT_STATE_ACTIVE) {
+	cpu_to_read = find_cpu_to_read(event);
+
+	if (cpu_to_read >= 0) {
 		struct perf_read_data data = {
 			.event = event,
 			.group = group,
 			.ret = 0,
 		};
-		ret = smp_call_function_single(event->oncpu,
+		ret = smp_call_function_single(cpu_to_read,
 					       __perf_event_read, &data, 1);
 		ret = ret ? : data.ret;
 	} else if (event->state == PERF_EVENT_STATE_INACTIVE) {
-- 
2.8.0.rc3.226.g39d4020