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Date: Thu, 14 Jul 2016 18:37:31 +0200
From: Peter Zijlstra <peterz@infradead.org>
To: Oleg Nesterov <oleg@redhat.com>
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
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On Thu, Jul 14, 2016 at 04:58:44PM +0200, Oleg Nesterov wrote:
> 
> But note that we do not need RCU_NONE. All we need is the trivial change
> below.

Hurm, maybe. So having that unbalanced keeps us in GP_PASSED state and
since we'll never drop gp_count back to 0 nothing will ever happen.

Cute, yes.

> 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?

Indeed, I rebased that patch on top of the current tree and had to add
support for down_trylock() but otherwise much the same thing.

I can send it out again.