cgroup: Make !percpu threadgroup_rwsem operations optional

3942a9bd7b ("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>

init/Kconfig

@@ -936,6 +936,16 @@ if CGROUPS
 config PAGE_COUNTER
 	bool
 
+config CGROUP_FAVOR_DYNMODS
+        bool "Favor dynamic modification latency reduction by default"
+        help
+          This option enables the "favordynmods" mount option by default
+          which reduces the latencies of dynamic cgroup modifications such
+          as task migrations and controller on/offs at the cost of making
+          hot path operations such as forks and exits more expensive.
+
+          Say N if unsure.
+
 config MEMCG
 	bool "Memory controller"
 	select PAGE_COUNTER