From mboxrd@z Thu Jan  1 00:00:00 1970
Return-Path: <linux-kernel-owner@vger.kernel.org>
Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand
	id S1756756AbbCEPUj (ORCPT <rfc822;w@1wt.eu>);
	Thu, 5 Mar 2015 10:20:39 -0500
Received: from mail-we0-f171.google.com ([74.125.82.171]:39565 "EHLO
	mail-we0-f171.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org
	with ESMTP id S1751824AbbCEPUh (ORCPT
	<rfc822;linux-kernel@vger.kernel.org>);
	Thu, 5 Mar 2015 10:20:37 -0500
Date: Thu, 5 Mar 2015 16:20:34 +0100
From: Frederic Weisbecker <fweisbec@gmail.com>
To: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Jason Low <jason.low2@hp.com>, Oleg Nesterov <oleg@redhat.com>,
        Peter Zijlstra <peterz@infradead.org>, Ingo Molnar <mingo@kernel.org>,
        "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        Mike Galbraith <umgwanakikbuti@gmail.com>,
        Rik van Riel <riel@redhat.com>, Steven Rostedt <rostedt@goodmis.org>,
        Scott Norton <scott.norton@hp.com>,
        Aswin Chandramouleeswaran <aswin@hp.com>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>
Subject: Re: [PATCH v2] sched, timer: Use atomics for thread_group_cputimer
 to improve scalability
Message-ID: <20150305152032.GC5074@lerouge>
References: <1425321731.5304.14.camel@j-VirtualBox>
 <20150302194033.GA27914@redhat.com>
 <20150302194356.GB27914@redhat.com>
 <1425330975.5304.49.camel@j-VirtualBox>
 <CA+55aFy3Kvyhtr92FCLTcBGLmibP+BgM57zJkPMgL-JfLrZn2A@mail.gmail.com>
MIME-Version: 1.0
Content-Type: text/plain; charset=us-ascii
Content-Disposition: inline
In-Reply-To: <CA+55aFy3Kvyhtr92FCLTcBGLmibP+BgM57zJkPMgL-JfLrZn2A@mail.gmail.com>
User-Agent: Mutt/1.5.23 (2014-03-12)
Sender: linux-kernel-owner@vger.kernel.org
List-ID: <linux-kernel.vger.kernel.org>
X-Mailing-List: linux-kernel@vger.kernel.org

On Mon, Mar 02, 2015 at 01:44:04PM -0800, Linus Torvalds wrote:
> On Mon, Mar 2, 2015 at 1:16 PM, Jason Low <jason.low2@hp.com> wrote:
> >
> > In original code, we set cputimer->running first so it is running while
> > we call update_gt_cputime(). Now in this patch, we swapped the 2 calls
> > such that we set running after calling update_gt_cputime(), so that
> > wouldn't be an issue anymore.
> 
> Hmm. If you actually care about ordering, and 'running' should be
> written to after the other things, then it might be best if you use
> 
>    smp_store_release(&cputimer->running, 1);
> 
> which makes it clear that the store happens *after* what went before it.
> 
> Or at least have a "smp_wmb()" between the atomic64 updates and the
> "WRITE_ONCE()".

FWIW, perhaps it can be reduced with an smp_mb__before_atomic() on the
account_group_*_time() side, paired with smp_wmb() from the thread_group_cputimer()
side. Arming cputime->running shouldn't be too frequent while update cputime
happens at least every tick...

Assuming smp_mb__before_atomic() is more lightweight than smp_load_acquire()
of course. 

> 
> I guess that since you use cmpxchg in update_gt_cputime, the accesses
> end up being ordered anyway, but it might be better to make that thing
> very explicit.
> 
>                    Linus