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

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Date:	Thu, 12 Dec 2013 16:51:53 +0100
From:	Peter Zijlstra <peterz@...radead.org>
To:	"H. Peter Anvin" <hpa@...or.com>
Cc:	Ingo Molnar <mingo@...nel.org>, Borislav Petkov <bp@...en8.de>,
	Mike Galbraith <bitbucket@...ine.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

On Thu, Dec 12, 2013 at 07:06:57AM -0800, H. Peter Anvin wrote:
> On 12/12/2013 05:28 AM, Ingo Molnar wrote:
> >>
> >> The reason we use thread_info::flags is because we need to write
> >> TIF_NEED_RESCHED into it to wake up anyhow.
> >>
> >> Using another cacheline would mean the wakeup path would need to write a
> >> second cross cpu cacheline -- that is badness too.
> >>
> >> So no, I don't think we want to listen to another line.
> > 
> 
> Right, okay, so that's the implicit wakeup.  However, I would think the
> CLFLUSH would hurt a lot more.

Maybe, but still, who cares? Its only a few broken cpus that actually
need the clflush, normal cpus do not. We should not optimize for the
broken case.


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