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

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Message-ID: <20131219181948.GD32508@gmail.com>
Date:	Thu, 19 Dec 2013 19:19:48 +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 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...

For the minimal fix I don't think we should change it - but for v3.14 
it looks like a speedup for the from-idle codepath, which is 
performance sensitive.

( It would also be nice to know whether MONITOR loads that cacheline 
  into the CPUs cache, and in what state it loads it. )

Thanks,

	Ingo
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