[PATCH v2 5/6] perf/core: Always set cpuctx cgrp when enable cgroup event

[Chengming Zhou <zhouchengming@bytedance.com>]

When enable a cgroup event, cpuctx->cgrp setting is conditional
on the current task cgrp matching the event's cgroup, so have to
do it for every new event. It brings complexity but no advantage.

To keep it simple, this patch would always set cpuctx->cgrp
when enable the first cgroup event, and reset to NULL when disable
the last cgroup event.

In this way, perf_cgroup_match() won't see cpuctx->cgrp == NULL if
it's a cgroup event, so add a WARN_ON_ONCE(!cpuctx->cgrp) there.

Signed-off-by: Chengming Zhou <zhouchengming@bytedance.com>
---
 kernel/events/core.c | 20 +++-----------------
 1 file changed, 3 insertions(+), 17 deletions(-)

diff --git a/kernel/events/core.c b/kernel/events/core.c
index 849a81299906..4c8657b08301 100644
--- a/kernel/events/core.c
+++ b/kernel/events/core.c
@@ -703,7 +703,7 @@ perf_cgroup_match(struct perf_event *event)
 		return true;
 
 	/* wants specific cgroup scope but @cpuctx isn't associated with any */
-	if (!cpuctx->cgrp)
+	if (WARN_ON_ONCE(!cpuctx->cgrp))
 		return false;
 
 	/*
@@ -975,22 +975,10 @@ perf_cgroup_event_enable(struct perf_event *event, struct perf_event_context *ct
 	 */
 	cpuctx = container_of(ctx, struct perf_cpu_context, ctx);
 
-	/*
-	 * Since setting cpuctx->cgrp is conditional on the current @cgrp
-	 * matching the event's cgroup, we must do this for every new event,
-	 * because if the first would mismatch, the second would not try again
-	 * and we would leave cpuctx->cgrp unset.
-	 */
-	if (ctx->is_active && !cpuctx->cgrp) {
-		struct perf_cgroup *cgrp = perf_cgroup_from_task(current, ctx);
-
-		if (cgroup_is_descendant(cgrp->css.cgroup, event->cgrp->css.cgroup))
-			cpuctx->cgrp = cgrp;
-	}
-
 	if (ctx->nr_cgroups++)
 		return;
 
+	cpuctx->cgrp = perf_cgroup_from_task(current, ctx);
 	list_add(&cpuctx->cgrp_cpuctx_entry,
 			per_cpu_ptr(&cgrp_cpuctx_list, event->cpu));
 }
@@ -1012,9 +1000,7 @@ perf_cgroup_event_disable(struct perf_event *event, struct perf_event_context *c
 	if (--ctx->nr_cgroups)
 		return;
 
-	if (ctx->is_active && cpuctx->cgrp)
-		cpuctx->cgrp = NULL;
-
+	cpuctx->cgrp = NULL;
 	list_del(&cpuctx->cgrp_cpuctx_entry);
 
 	if (list_empty(per_cpu_ptr(&cgrp_cpuctx_list, event->cpu)))
-- 
2.20.1