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Date: Wed, 11 Dec 2013 17:44:03 +0100
From: Peter Zijlstra <peterz@infradead.org>
To: Ingo Molnar <mingo@kernel.org>
Cc: Mike Galbraith <bitbucket@online.de>, Borislav Petkov <bp@alien8.de>,
        Thomas Gleixner <tglx@linutronix.de>, Len Brown <lenb@kernel.org>,
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        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
        Jeremy Eder <jeder@redhat.com>, x86@kernel.org
Subject: Re: 50 Watt idle power regression bisected to Linux-3.10
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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 :-)

Also, a wmb doesn't actually need to flush the store buffers.