linux-kernel - Re: 50 Watt idle power regression bisected to Linux-3.10

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Message-ID: <20131211174710.GA12431@gmail.com>
Date:	Wed, 11 Dec 2013 18:47:10 +0100
From:	Ingo Molnar <mingo@...nel.org>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	Mike Galbraith <bitbucket@...ine.de>,
	Borislav Petkov <bp@...en8.de>,
	Thomas Gleixner <tglx@...utronix.de>,
	Len Brown <lenb@...nel.org>,
	Linux PM list <linux-pm@...r.kernel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Jeremy Eder <jeder@...hat.com>, x86@...nel.org
Subject: Re: 50 Watt idle power regression bisected to Linux-3.10


* Peter Zijlstra <peterz@...radead.org> wrote:

> On Wed, Dec 11, 2013 at 03:42:38PM +0100, Ingo Molnar wrote:
> > Another thing that is required I think is to issue a write barrier 
> > before CLFLUSH instruction. By my (possibly incorrect ...) reading of 
> > the documentation CLFLUSH does not appear to be ordered (at all), so 
> > it might execute before the modification to the affected memory?
> > 
> > 
> > So something like:
> > 
> > 		if (this_cpu_has(X86_FEATURE_CLFLUSH_MONITOR)) {
> > 			smp_wmb(); /* order CLFLUSH */
> > 			clflush(&current_thread_info()->flags);
> > 		}
> 
> smp_wmb() is a NO-OP on x86 remember :-)

Well, it's a compiler barrier but yes - I suspect a smp_mb() might be 
needed - at least according to the CLFLUSH documentation it has no 
implicit guaranteed ordering wrt. preceding writes.

Thanks,

	Ingo
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