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

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Message-ID: <20131219175037.GK2480@laptop.programming.kicks-ass.net>
Date:	Thu, 19 Dec 2013 18:50:37 +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 06:25:35PM +0100, Peter Zijlstra wrote:
> On Thu, Dec 19, 2013 at 06:07:41PM +0100, Ingo Molnar wrote:
> > Likewise, having a barrier before the MONITOR looks sensible as well. 
> 
> 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.

SDM states that MONITOR is ordered like a LOAD, and a LOAD cannot pass
a previous STORE to the same address.

That said; there's enough holes in there to swim a titanic through,
seeing how MONITOR stares at an entire cacheline and LOAD/STORE order is
specified on location, whatever that means.


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