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From: John Stultz <john.stultz@linaro.org>
Date: Thu, 14 Jul 2016 09:43:40 -0700
Message-ID: <CALAqxLWdS4jv8a=bcF433DivjzH-sWsM6BCvW3Apy+JDdN3nMw@mail.gmail.com>
Subject: Re: Severe performance regression w/ 4.4+ on Android due to cgroup
 locking changes
To: Peter Zijlstra <peterz@infradead.org>
Cc: Tejun Heo <tj@kernel.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>, Oleg Nesterov <oleg@redhat.com>,
        "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>
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On Thu, Jul 14, 2016 at 6:18 AM, Peter Zijlstra <peterz@infradead.org> wrote:
> On Wed, Jul 13, 2016 at 10:51:02PM +0200, Peter Zijlstra wrote:
>> So, IIRC, the trade-off is a full memory barrier in read_lock and
>> read_unlock() vs sync_sched() in write.
>>
>> Full memory barriers are expensive and while the combined cost might
>> well exceed the cost of the sync_sched() it doesn't suffer the latency
>> issues.
>>
>> Not sure if we can frob the two in a single codebase, but I can have a
>> poke if Oleg or Paul doesn't beat me to it.
>
> 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.

Unfortunately it didn't apply cleanly to the 4.4 based tree I'm
working with, so I had to manually apply the entirety of the
percpu-rwsem.c changes myself. Hopefully I didn't screw it up.

So running with this, I'm still seeing some pretty large delays. 80ms
peak, with lots of >20ms values as well.
So it doesn't seem to have the positive effect that Paul's change provided.

thanks
-john