[PATCH v2 2/4] perf/core: generalize event->group_flags

[David Carrillo-Cisneros]

Currently, PERF_GROUP_SOFTWARE is used in the group_flags field of a
group's leader to indicate that is_software_event(event) is true for all
events in a group. This is the only usage of event->group_flags.

This pattern of setting a group level flags when all events in the group
share a property is useful for the flag introduced in the next patch and
for future CQM/CMT flags. So this patches generalizes group_flags to work
as an aggregate of event level flags.

PERF_GROUP_SOFTWARE denotes an inmutable event's property. All other flags
that I intend to add are also determinable at event initialization.
To better convey the above, this patch renames event's group_flags to
group_caps and PERF_GROUP_SOFTWARE to PERF_EV_CAP_SOFTWARE.

Individual event flags are stored in the new event->event_caps. Since the
cap flags do not change after event initialization, there is no need to
serialize event_caps. This new field is used when events are added to a
context, similarly to how PERF_GROUP_SOFTWARE and is_software_event()
worked.

Lastly, for consistency, updates is_software_event() to rely in event_cap
instead of the context index.

Signed-off-by: David Carrillo-Cisneros <davidcc@xxxxxxxxxx>
---
 include/linux/perf_event.h | 18 +++++++++++++-----
 kernel/events/core.c       | 16 ++++++++--------
 2 files changed, 21 insertions(+), 13 deletions(-)

diff --git a/include/linux/perf_event.h b/include/linux/perf_event.h
index 7921f4f..fa5617f 100644
--- a/include/linux/perf_event.h
+++ b/include/linux/perf_event.h
@@ -497,9 +497,12 @@ typedef void (*perf_overflow_handler_t)(struct perf_event *,
 					struct perf_sample_data *,
 					struct pt_regs *regs);
 
-enum perf_group_flag {
-	PERF_GROUP_SOFTWARE		= 0x1,
-};
+/*
+ * Event capabilities. For event_caps and groups caps.
+ *
+ * PERF_EV_CAP_SOFTWARE: Is a software event.
+ */
+#define PERF_EV_CAP_SOFTWARE		BIT(0)
 
 #define SWEVENT_HLIST_BITS		8
 #define SWEVENT_HLIST_SIZE		(1 << SWEVENT_HLIST_BITS)
@@ -555,7 +558,12 @@ struct perf_event {
 	struct hlist_node		hlist_entry;
 	struct list_head		active_entry;
 	int				nr_siblings;
-	int				group_flags;
+
+	/* Not serialized. Only written during event initialization. */
+	int				event_caps;
+	/* The cumulative AND of all event_caps for events in this group. */
+	int				group_caps;
+
 	struct perf_event		*group_leader;
 	struct pmu			*pmu;
 	void				*pmu_private;
@@ -968,7 +976,7 @@ static inline bool is_sampling_event(struct perf_event *event)
  */
 static inline int is_software_event(struct perf_event *event)
 {
-	return event->pmu->task_ctx_nr == perf_sw_context;
+	return event->event_caps & PERF_EV_CAP_SOFTWARE;
 }
 
 extern struct static_key perf_swevent_enabled[PERF_COUNT_SW_MAX];
diff --git a/kernel/events/core.c b/kernel/events/core.c
index 96523fd..34049cc 100644
--- a/kernel/events/core.c
+++ b/kernel/events/core.c
@@ -1405,8 +1405,7 @@ list_add_event(struct perf_event *event, struct perf_event_context *ctx)
 	if (event->group_leader == event) {
 		struct list_head *list;
 
-		if (is_software_event(event))
-			event->group_flags |= PERF_GROUP_SOFTWARE;
+		event->group_caps = event->event_caps;
 
 		list = ctx_group_list(event, ctx);
 		list_add_tail(&event->group_entry, list);
@@ -1561,9 +1560,7 @@ static void perf_group_attach(struct perf_event *event)
 
 	WARN_ON_ONCE(group_leader->ctx != event->ctx);
 
-	if (group_leader->group_flags & PERF_GROUP_SOFTWARE &&
-			!is_software_event(event))
-		group_leader->group_flags &= ~PERF_GROUP_SOFTWARE;
+	group_leader->group_caps &= event->event_caps;
 
 	list_add_tail(&event->group_entry, &group_leader->sibling_list);
 	group_leader->nr_siblings++;
@@ -1669,7 +1666,7 @@ static void perf_group_detach(struct perf_event *event)
 		sibling->group_leader = sibling;
 
 		/* Inherit group flags from the previous leader */
-		sibling->group_flags = event->group_flags;
+		sibling->group_caps = event->group_caps;
 
 		WARN_ON_ONCE(sibling->ctx != event->ctx);
 	}
@@ -2070,7 +2067,7 @@ static int group_can_go_on(struct perf_event *event,
 	/*
 	 * Groups consisting entirely of software events can always go on.
 	 */
-	if (event->group_flags & PERF_GROUP_SOFTWARE)
+	if (event->group_caps & PERF_EV_CAP_SOFTWARE)
 		return 1;
 	/*
 	 * If an exclusive group is already on, no other hardware
@@ -9359,6 +9356,9 @@ SYSCALL_DEFINE5(perf_event_open,
 			goto err_alloc;
 	}
 
+	if (pmu->task_ctx_nr == perf_sw_context)
+		event->event_caps |= PERF_EV_CAP_SOFTWARE;
+
 	if (group_leader &&
 	    (is_software_event(event) != is_software_event(group_leader))) {
 		if (is_software_event(event)) {
@@ -9372,7 +9372,7 @@ SYSCALL_DEFINE5(perf_event_open,
 			 */
 			pmu = group_leader->pmu;
 		} else if (is_software_event(group_leader) &&
-			   (group_leader->group_flags & PERF_GROUP_SOFTWARE)) {
+			   (group_leader->group_caps & PERF_EV_CAP_SOFTWARE)) {
 			/*
 			 * In case the group is a pure software group, and we
 			 * try to add a hardware event, move the whole group to
-- 
2.8.0.rc3.226.g39d4020