linux-kernel - Re: [PATCH] x86 idle: repair large-server 50-watt idle-power regression

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Message-ID: <20131219182314.GE32508@gmail.com>
Date:	Thu, 19 Dec 2013 19:23:14 +0100
From:	Ingo Molnar <mingo@...nel.org>
To:	"H. Peter Anvin" <hpa@...or.com>
Cc:	Peter Zijlstra <peterz@...radead.org>, Len Brown <lenb@...nel.org>,
	x86@...nel.org, linux-pm@...r.kernel.org,
	linux-kernel@...r.kernel.org, Len Brown <len.brown@...el.com>,
	stable@...r.kernel.org,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Mike Galbraith <efault@....de>, Borislav Petkov <bp@...en8.de>
Subject: Re: [PATCH] x86 idle: repair large-server 50-watt idle-power
 regression


* H. Peter Anvin <hpa@...or.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
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