perf/cgroup: Fix perf cgroup hierarchy support

The following commit:

  864c235 ("perf/core: Do not set cpuctx->cgrp for unscheduled cgroups")

made list_update_cgroup_event() skip setting cpuctx->cgrp if no cgroup event
targets %current's cgroup.

This breaks perf_event's hierarchical support because events which target one
of the ancestors get ignored.

Fix it by using cgroup_is_descendant() test instead of equality.

Signed-off-by: Tejun Heo <[email protected]>
Acked-by: Thomas Gleixner <[email protected]>
Cc: Arnaldo Carvalho de Melo <[email protected]>
Cc: David Carrillo-Cisneros <[email protected]>
Cc: Linus Torvalds <[email protected]>
Cc: Peter Zijlstra <[email protected]>
Cc: [email protected]
Cc: [email protected] # v4.9+
Fixes: 864c235 ("perf/core: Do not set cpuctx->cgrp for unscheduled cgroups")
Link: http://lkml.kernel.org/r/[email protected]
Signed-off-by: Ingo Molnar <[email protected]>

Author: htejun

kernel/events/core.c

@@ -901,9 +901,11 @@ list_update_cgroup_event(struct perf_event *event,
 	cpuctx_entry = &cpuctx->cgrp_cpuctx_entry;
 	/* cpuctx->cgrp is NULL unless a cgroup event is active in this CPU .*/
 	if (add) {
+		struct perf_cgroup *cgrp = perf_cgroup_from_task(current, ctx);
+
 		list_add(cpuctx_entry, this_cpu_ptr(&cgrp_cpuctx_list));
-		if (perf_cgroup_from_task(current, ctx) == event->cgrp)
-			cpuctx->cgrp = event->cgrp;
+		if (cgroup_is_descendant(cgrp->css.cgroup, event->cgrp->css.cgroup))
+			cpuctx->cgrp = cgrp;
 	} else {
 		list_del(cpuctx_entry);
 		cpuctx->cgrp = NULL;