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Date: Thu, 14 Jul 2016 16:58:44 +0200
From: Oleg Nesterov <oleg@redhat.com>
To: Peter Zijlstra <peterz@infradead.org>
Cc: Tejun Heo <tj@kernel.org>, John Stultz <john.stultz@linaro.org>,
        Ingo Molnar <mingo@redhat.com>, lkml <linux-kernel@vger.kernel.org>,
        Dmitry Shmidt <dimitrysh@google.com>,
        Rom Lemarchand <romlem@google.com>, Colin Cross <ccross@google.com>,
        Todd Kjos <tkjos@google.com>,
        "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>
Subject: Re: Severe performance regression w/ 4.4+ on Android due to cgroup
	locking changes
Message-ID: <20160714145844.GA7760@redhat.com>
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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
part?

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?

Oleg.

--- 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);
+
 	get_user_ns(init_cgroup_ns.user_ns);
 
 	mutex_lock(&cgroup_mutex);