Re: Severe performance regression w/ 4.4+ on Android due to cgroup locking changes

From: Oleg Nesterov
Date: Thu Jul 14 2016 - 10:58:32 EST

On 07/14, Peter Zijlstra wrote:
> OK, not too horrible if I say so myself :-)
> The below is a compile tested only first draft so far. I'll go give it
> some runtime next.

Yes, thanks.

But note that we do not need RCU_NONE. All we need is the trivial change
below. Damn, I am trying to find my old rcu-sync patches which I didn't
send, but can't... OK, this almost off-topic right now, just this "enter"
is ugly and we can't switch the slow/fast modes dynamically.

The rest of you patch is "optimize the slow path" and we already discussed
it before, I personally like it. Perhaps you can redo it without RCU_NONE

Of course, this leads to another question: do we really need rcu-sync at
all, or should we change percpu-rwsem to always work in the "slow" mode
which is not that slow with your change... I'd like to keep it ;)

What do you think?


--- a/kernel/cgroup.c
+++ b/kernel/cgroup.c
@@ -5605,6 +5605,8 @@ 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));

+ rcu_sync_enter(&cgroup_threadgroup_rwsem.rss);