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 S1756117Ab3LSSXo (ORCPT <rfc822;w@1wt.eu>);
	Thu, 19 Dec 2013 13:23:44 -0500
Received: from mail-we0-f178.google.com ([74.125.82.178]:35872 "EHLO
	mail-we0-f178.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org
	with ESMTP id S1755821Ab3LSSXT (ORCPT
	<rfc822;linux-kernel@vger.kernel.org>);
	Thu, 19 Dec 2013 13:23:19 -0500
Date: Thu, 19 Dec 2013 19:23:14 +0100
From: Ingo Molnar <mingo@kernel.org>
To: "H. Peter Anvin" <hpa@zytor.com>
Cc: Peter Zijlstra <peterz@infradead.org>, Len Brown <lenb@kernel.org>,
        x86@kernel.org, linux-pm@vger.kernel.org, linux-kernel@vger.kernel.org,
        Len Brown <len.brown@intel.com>, stable@vger.kernel.org,
        Linus Torvalds <torvalds@linux-foundation.org>,
        Thomas Gleixner <tglx@linutronix.de>, Mike Galbraith <efault@gmx.de>,
        Borislav Petkov <bp@alien8.de>
Subject: Re: [PATCH] x86 idle: repair large-server 50-watt idle-power
 regression
Message-ID: <20131219182314.GE32508@gmail.com>
References: <baff264285f6e585df757d58b17788feabc68918.1387403066.git.len.brown@intel.com>
 <20131219122257.GC11279@gmail.com>
 <52B316FF.50906@zytor.com>
 <20131219160210.GA28426@gmail.com>
 <52B31B21.6010901@zytor.com>
 <20131219162136.GM16438@laptop.programming.kicks-ass.net>
 <52B323BE.7090108@zytor.com>
 <20131219170741.GB30382@gmail.com>
 <52B33640.3020204@zytor.com>
 <52B338AA.1020307@zytor.com>
MIME-Version: 1.0
Content-Type: text/plain; charset=us-ascii
Content-Disposition: inline
In-Reply-To: <52B338AA.1020307@zytor.com>
User-Agent: Mutt/1.5.21 (2010-09-15)
Sender: linux-kernel-owner@vger.kernel.org
List-ID: <linux-kernel.vger.kernel.org>
X-Mailing-List: linux-kernel@vger.kernel.org


* H. Peter Anvin <hpa@zytor.com> wrote:

> On 12/19/2013 10:09 AM, H. Peter Anvin wrote:
> > On 12/19/2013 09:07 AM, Ingo Molnar wrote:
> >>
> >> Likewise, having a barrier before the MONITOR looks sensible as well. 
> >> Having it _after_ monitor looks weird and is probably wrong. [It might 
> >> have been the effects of someone seeing the spurious wakeup problems 
> >> with realizing the true source, or so.]
> >>
> > 
> > Does anyone know the history of this barrier after the monitor?  I know
> > Len is looking for a minimal patchset that can go into -stable, and it
> > seems prudent to not preturb the code more than necessary, but going
> > forward it would be nice to know...
> > 
> 
> Hmm... it *looks* like it is intended to be part of the construct:
> 
> 	smp_mb();
> 	if (!need_resched())
> 		...
> 
> I found a note in the HLT variant of the function saying:
> 
> /*
>  * TS_POLLING-cleared state must be visible before we
>  * test NEED_RESCHED:
>  */

Yes, that makes sense: the need_resched test is a load, and MONITOR is 
a load as well. Can the two ever cross, or does the CPU guarantee that 
because it's the same address, the loads don't cross?

Thanks,

	Ingo