cgroup: Make !percpu threadgroup_rwsem operations optional

3942a9bd7b58 ("locking, rcu, cgroup: Avoid synchronize_sched() in
__cgroup_procs_write()") disabled percpu operations on threadgroup_rwsem
because the impiled synchronize_rcu() on write locking was pushing up the
latencies too much for android which constantly moves processes between
cgroups.

This makes the hotter paths - fork and exit - slower as they're always
forced into the slow path. There is no reason to force this on everyone
especially given that more common static usage pattern can now completely
avoid write-locking the rwsem. Write-locking is elided when turning on and
off controllers on empty sub-trees and CLONE_INTO_CGROUP enables seeding a
cgroup without grabbing the rwsem.

Restore the default percpu operations and introduce the mount option
"favordynmods" and config option CGROUP_FAVOR_DYNMODS for users who need
lower latencies for the dynamic operations.

Signed-off-by: Tejun Heo <tj@kernel.org>
Cc: Christian Brauner <brauner@kernel.org>
Cc: Michal Koutn� <mkoutny@suse.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: John Stultz <john.stultz@linaro.org>
Cc: Dmitry Shmidt <dimitrysh@google.com>
Cc: Oleg Nesterov <oleg@redhat.com>

1 files changed, 16 insertions, 3 deletions

diff --git a/include/linux/cgroup-defs.h b/include/linux/cgroup-defs.h
index 672de25e3ec8..63bf43c7ca3b 100644
--- a/include/linux/cgroup-defs.h
+++ b/include/linux/cgroup-defs.h
@@ -89,19 +89,32 @@ enum {
 	CGRP_ROOT_NS_DELEGATE	= (1 << 3),
 
 	/*
+	 * Reduce latencies on dynamic cgroup modifications such as task
+	 * migrations and controller on/offs by disabling percpu operation on
+	 * cgroup_threadgroup_rwsem. This makes hot path operations such as
+	 * forks and exits into the slow path and more expensive.
+	 *
+	 * The static usage pattern of creating a cgroup, enabling controllers,
+	 * and then seeding it with CLONE_INTO_CGROUP doesn't require write
+	 * locking cgroup_threadgroup_rwsem and thus doesn't benefit from
+	 * favordynmod.
+	 */
+	CGRP_ROOT_FAVOR_DYNMODS = (1 << 4),
+
+	/*
 	 * Enable cpuset controller in v1 cgroup to use v2 behavior.
 	 */
-	CGRP_ROOT_CPUSET_V2_MODE = (1 << 4),
+	CGRP_ROOT_CPUSET_V2_MODE = (1 << 16),
 
 	/*
 	 * Enable legacy local memory.events.
 	 */
-	CGRP_ROOT_MEMORY_LOCAL_EVENTS = (1 << 5),
+	CGRP_ROOT_MEMORY_LOCAL_EVENTS = (1 << 17),
 
 	/*
 	 * Enable recursive subtree protection
 	 */
-	CGRP_ROOT_MEMORY_RECURSIVE_PROT = (1 << 6),
+	CGRP_ROOT_MEMORY_RECURSIVE_PROT = (1 << 18),
 };
 
 /* cftype->flags */