[PATCH] perf: Allocate context task_ctx_data for child event

[Jiri Olsa]

Currently we use perf_event_context::task_ctx_data to save
and restore the LBR status when the task is scheduled out
and in.

We don't allocate it for child contexts, which results in
shorter task's LBR stack, because we don't save the history
from previous run and start over every time we schedule the
task in.

I made a test to generate samples with LBR call stack
and got higher numbers on bigger chain depths:

                            before:     after:
  LBR call chain: nr: 1       60561     498127
  LBR call chain: nr: 2           0          0
  LBR call chain: nr: 3      107030       2172
  LBR call chain: nr: 4      466685      62758
  LBR call chain: nr: 5     2307319     878046
  LBR call chain: nr: 6       48713     495218
  LBR call chain: nr: 7        1040       4551
  LBR call chain: nr: 8         481        172
  LBR call chain: nr: 9         878        120
  LBR call chain: nr: 10       2377       6698
  LBR call chain: nr: 11      28830     151487
  LBR call chain: nr: 12      29347     339867
  LBR call chain: nr: 13          4         22
  LBR call chain: nr: 14          3         53

Cc: Yan Zheng <zheng.z.yan@xxxxxxxxx>
Cc: Andi Kleen <ak@xxxxxxxxxxxxxxx>
Signed-off-by: Jiri Olsa <jolsa@xxxxxxxxxx>
---
 kernel/events/core.c | 13 +++++++++++++
 1 file changed, 13 insertions(+)

diff --git a/kernel/events/core.c b/kernel/events/core.c
index 5961ef6dfd64..5389b75bf981 100644
--- a/kernel/events/core.c
+++ b/kernel/events/core.c
@@ -10703,6 +10703,18 @@ inherit_event(struct perf_event *parent_event,
 	if (IS_ERR(child_event))
 		return child_event;
 
+
+	if ((child_event->attach_state & PERF_ATTACH_TASK_DATA) &&
+	    !child_ctx->task_ctx_data) {
+		struct pmu *pmu = child_event->pmu;
+
+		child_ctx->task_ctx_data = kzalloc(pmu->task_ctx_size, GFP_KERNEL);
+		if (!child_ctx->task_ctx_data) {
+			free_event(child_event);
+			return NULL;
+		}
+	}
+
 	/*
 	 * is_orphaned_event() and list_add_tail(&parent_event->child_list)
 	 * must be under the same lock in order to serialize against
@@ -10713,6 +10725,7 @@ inherit_event(struct perf_event *parent_event,
 	if (is_orphaned_event(parent_event) ||
 	    !atomic_long_inc_not_zero(&parent_event->refcount)) {
 		mutex_unlock(&parent_event->child_mutex);
+		/* task_ctx_data is freed with child_ctx */
 		free_event(child_event);
 		return NULL;
 	}
-- 
2.13.6