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

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Date:	Thu, 19 Dec 2013 18:25:35 +0100
From:	Peter Zijlstra <peterz@...radead.org>
To:	Ingo Molnar <mingo@...nel.org>
Cc:	"H. Peter Anvin" <hpa@...or.com>, 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

On Thu, Dec 19, 2013 at 06:07:41PM +0100, Ingo Molnar wrote:
> 
> * H. Peter Anvin <hpa@...or.com> wrote:
> 
> > On 12/19/2013 08:21 AM, Peter Zijlstra wrote:
> > > 
> > > What's that mb for?
> > > 
> > 
> > It already exists in mwait_idle_with_hints(); I just moved it into 
> > this common function.  It is a bit odd, I have to admit; it seems 
> > like it should be *before* the monitor (and possibly we should have 
> > one after the CLFLUSH as well?)
> 
> Yes, I think we need a barrier before the CLFLUSH, because according 
> to my reading of the Intel documentation CLFLUSH has no implicit 
> ordering so it might get reordered with the store to ->flags in 
> current_set_polling_and_test(), which might result in spurious wakeup 
> problems again.

No it cannot; since current_set_polling_and_test() already has a barrier
to prevent that.

Also, the location patched by hpa doesn't actually call that at all.

That said, I would find it very strange indeed if a CLFLUSH doesn't also
flush the store buffer.

> (And CLFLUSH is a store in a sense, so special in that the regular 
> ordering for stores does not apply.)
> 
> 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.]

I again have to disagree, one would expect monitor to flush all that is
required to start the monitor -- and it actually does so. As is
testified by this extra CLFLUSH being called a bug workaround.
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