From 0c3240a1ef2e840aaa17f593326e3642bc857aa7 Mon Sep 17 00:00:00 2001 From: Peter Zijlstra Date: Thu, 11 Aug 2016 18:54:13 +0200 Subject: RFC: FROMLIST: cgroup: avoid synchronize_sched() in __cgroup_procs_write() The current percpu-rwsem read side is entirely free of serializing insns at the cost of having a synchronize_sched() in the write path. The latency of the synchronize_sched() is too high for cgroups. The commit 1ed1328792ff talks about the write path being a fairly cold path but this is not the case for Android which moves task to the foreground cgroup and back around binder IPC calls from foreground processes to background processes, so it is significantly hotter than human initiated operations. Switch cgroup_threadgroup_rwsem into the slow mode for now to avoid the problem, hopefully it should not be that slow after another commit 80127a39681b ("locking/percpu-rwsem: Optimize readers and reduce global impact"). We could just add rcu_sync_enter() into cgroup_init() but we do not want another synchronize_sched() at boot time, so this patch adds the new helper which doesn't block but currently can only be called before the first use. Cc: Tejun Heo Cc: Paul McKenney Reported-by: John Stultz Reported-by: Dmitry Shmidt Signed-off-by: Peter Zijlstra (Intel) Signed-off-by: Oleg Nesterov [jstultz: backported to 4.4] Change-Id: I34aa9c394d3052779b56976693e96d861bd255f2 Mailing-list-URL: https://lkml.org/lkml/2016/8/11/557 Signed-off-by: John Stultz --- kernel/cgroup.c | 6 ++++++ 1 file changed, 6 insertions(+) (limited to 'kernel/cgroup.c') diff --git a/kernel/cgroup.c b/kernel/cgroup.c index a57b7c86871c..bd541053efc2 100644 --- a/kernel/cgroup.c +++ b/kernel/cgroup.c @@ -5346,6 +5346,12 @@ int __init cgroup_init(void) BUG_ON(cgroup_init_cftypes(NULL, cgroup_dfl_base_files)); BUG_ON(cgroup_init_cftypes(NULL, cgroup_legacy_base_files)); + /* + * The latency of the synchronize_sched() is too high for cgroups, + * avoid it at the cost of forcing all readers into the slow path. + */ + rcu_sync_enter_start(&cgroup_threadgroup_rwsem.rss); + mutex_lock(&cgroup_mutex); /* Add init_css_set to the hash table */ -- cgit v1.2.3