perf/core: Introduce PMU_EV_CAP_READ_ACTIVE_PKG

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@google.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Cc: Alexander Shishkin <alexander.shishkin@linux.intel.com>
Cc: Arnaldo Carvalho de Melo <acme@redhat.com>
Cc: David Carrillo-Cisneros <davidcc@google.com>
Cc: Jiri Olsa <jolsa@redhat.com>
Cc: Kan Liang <kan.liang@intel.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Paul Turner <pjt@google.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Stephane Eranian <eranian@google.com>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Vegard Nossum <vegard.nossum@gmail.com>
Cc: Vince Weaver <vincent.weaver@maine.edu>
Link: http://lkml.kernel.org/r/1471467307-61171-4-git-send-email-davidcc@google.com
Signed-off-by: Ingo Molnar <mingo@kernel.org>

2 files changed, 26 insertions, 2 deletions

diff --git a/include/linux/perf_event.h b/include/linux/perf_event.h
index 6f7459f72dfd..5c5362584aba 100644
--- a/include/linux/perf_event.h
+++ b/include/linux/perf_event.h
@@ -514,8 +514,11 @@ 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 8c42a5ae9030..3f07e6cfc1b6 100644
--- a/kernel/events/core.c
+++ b/kernel/events/core.c
@@ -3424,6 +3424,22 @@ struct perf_read_data {
         int ret;
 };
 
+static int find_cpu_to_read(struct perf_event *event, int local_cpu)
+{
+        int event_cpu = event->oncpu;
+        u16 local_pkg, event_pkg;
+
+        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
  */
@@ -3545,7 +3561,7 @@ 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, local_cpu;
 
         /*
          * If event is enabled and currently active on a CPU, update the
@@ -3557,7 +3573,12 @@ static int perf_event_read(struct perf_event *event, bool group)
                         .group = group,
                         .ret = 0,
                 };
-                ret = smp_call_function_single(event->oncpu, __perf_event_read, &data, 1);
+
+                local_cpu = get_cpu();
+                cpu_to_read = find_cpu_to_read(event, local_cpu);
+                put_cpu();
+
+                ret = smp_call_function_single(cpu_to_read, __perf_event_read, &data, 1);
                 /* The event must have been read from an online CPU: */
                 WARN_ON_ONCE(ret);
                 ret = ret ? : data.ret;