[PATCH 2/2] perf: Fix mixed hw/sw event group initialization

[Namhyung Kim]

From: Namhyung Kim <namhyung.kim@xxxxxxx>

There's a problem with mixed hw/sw group when the leader is a software
event.  For instance:

  $ perf stat -e '{task-clock,cycles,faults}' sleep 1

   Performance counter stats for 'sleep 1':

            0.273436 task-clock                #    0.000 CPUs utilized
             962,965 cycles                    #    3.522 GHz
     <not supported> faults

         1.000804279 seconds time elapsed

Jiri's patch 0231bb533675 ("perf: Fix event group context move") fixed
a part of problem but there's a devil still..

The problem arose when a sw event is added to already moved (to hw
context) group whose leader also is a sw event.  In the above example

 1. task-clock (sw event) is a group leader (has PERF_GROUP_SOFTWARE)
 2. cycles (hw event) is added, so the leader moved to the hw context
 3. faults (sw event) is added but the leader also is a sw event
 4. after find_get_context(), ctx is not same as leader->ctx since the
    leader had moved to the hw context (-EINVAL)

Fix it by adding new PERF_GROUP_MIXED flag and use leader's ctx->pmu
if it's set.

  $ perf -state -e '{task-clock,cycles,faults}' sleep 1

   Performance counter stats for 'sleep 1':

            0.670405 task-clock                #    0.001 CPUs utilized
             933,264 cycles                    #    1.392 GHz
                 176 faults                    #    0.263 M/sec

         1.001506178 seconds time elapsed

Reported-by: Andreas Hollmann <hollmann@xxxxxxxxx>
Cc: Jiri Olsa <jolsa@xxxxxxxxxx>
Cc: Vince Weaver <vince@xxxxxxxxxx>
Cc: Frederic Weisbecker <fweisbec@xxxxxxxxx>
Signed-off-by: Namhyung Kim <namhyung@xxxxxxxxxx>
---
 include/linux/perf_event.h |  1 +
 kernel/events/core.c       | 37 ++++++++++++++++++++++---------------
 2 files changed, 23 insertions(+), 15 deletions(-)

diff --git a/include/linux/perf_event.h b/include/linux/perf_event.h
index e47ee462c2f2..001a3b64fe61 100644
--- a/include/linux/perf_event.h
+++ b/include/linux/perf_event.h
@@ -285,6 +285,7 @@ typedef void (*perf_overflow_handler_t)(struct perf_event *,
 
 enum perf_group_flag {
 	PERF_GROUP_SOFTWARE		= 0x1,
+	PERF_GROUP_MIXED		= 0x2,
 };
 
 #define SWEVENT_HLIST_BITS		8
diff --git a/kernel/events/core.c b/kernel/events/core.c
index 007dfe846d4d..06266d5ed500 100644
--- a/kernel/events/core.c
+++ b/kernel/events/core.c
@@ -6441,6 +6441,8 @@ out:
  * @pid:		target pid
  * @cpu:		target cpu
  * @group_fd:		group leader event fd
+ * @flags:		flags which controls the meaning of arguments.
+ * 			see PERF_FLAG_*
  */
 SYSCALL_DEFINE5(perf_event_open,
 		struct perf_event_attr __user *, attr_uptr,
@@ -6536,26 +6538,30 @@ SYSCALL_DEFINE5(perf_event_open,
 	 */
 	pmu = event->pmu;
 
-	if (group_leader &&
-	    (is_software_event(event) != is_software_event(group_leader))) {
-		if (is_software_event(event)) {
-			/*
-			 * If event and group_leader are not both a software
-			 * event, and event is, then group leader is not.
-			 *
-			 * Allow the addition of software events to !software
-			 * groups, this is safe because software events never
-			 * fail to schedule.
-			 */
-			pmu = group_leader->pmu;
-		} else if (is_software_event(group_leader) &&
-			   (group_leader->group_flags & PERF_GROUP_SOFTWARE)) {
+	if (group_leader) {
+		if (group_leader->group_flags & PERF_GROUP_SOFTWARE) {
 			/*
 			 * In case the group is a pure software group, and we
 			 * try to add a hardware event, move the whole group to
 			 * the hardware context.
 			 */
-			move_group = 1;
+			if (!is_software_event(event))
+				move_group = 1;
+		} else if (group_leader->group_flags & PERF_GROUP_MIXED) {
+			/*
+			 * The group leader was moved on to a hardware context,
+			 * so move this event also.
+			 */
+			if (is_software_event(event))
+				pmu = group_leader->ctx->pmu;
+		} else if (!is_software_event(group_leader)) {
+			/*
+			 * Allow the addition of software events to !software
+			 * groups, this is safe because software events never
+			 * fail to schedule.
+			 */
+			if (is_software_event(event))
+				pmu = group_leader->pmu;
 		}
 	}
 
@@ -6650,6 +6656,7 @@ SYSCALL_DEFINE5(perf_event_open,
 			perf_install_in_context(ctx, sibling, event->cpu);
 			get_ctx(ctx);
 		}
+		group_leader->group_flags = PERF_GROUP_MIXED;
 	}
 
 	perf_install_in_context(ctx, event, event->cpu);
-- 
1.7.11.7

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