perf/cgroup: Fix perf cgroup hierarchy support

perf/cgroup: Fix perf cgroup hierarchy support

BugLink: http://bugs.launchpad.net/bugs/1731971
commit be96b316deff35e119760982c43af74e606fa143 upstream.

The following commit:

  864c2357ca89 ("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 <tj@kernel.org>
Acked-by: Thomas Gleixner <tglx@linutronix.de>
Cc: Arnaldo Carvalho de Melo <acme@kernel.org>
Cc: David Carrillo-Cisneros <davidcc@google.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: kernel-team@fb.com
Fixes: 864c2357ca89 ("perf/core: Do not set cpuctx->cgrp for unscheduled cgroups")
Link: http://lkml.kernel.org/r/20171028164237.GA972780@devbig577.frc2.facebook.com
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Stefan Bader <stefan.bader@canonical.com>
Signed-off-by: Thadeu Lima de Souza Cascardo <cascardo@canonical.com>