linux-kernel - Re: Idle power fix regresses ebizzy performance (was 3.12-stable backport of NUMA balancing patches)

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Message-ID: <1389686473.5951.11.camel@marge.simpson.net>
Date:	Tue, 14 Jan 2014 09:01:13 +0100
From:	Mike Galbraith <bitbucket@...ine.de>
To:	Len Brown <lenb@...nel.org>
Cc:	Mel Gorman <mgorman@...e.de>, Len Brown <len.brown@...el.com>,
	athorlton@....com, Rik van Riel <riel@...hat.com>,
	chegu_vinod@...com, Greg KH <gregkh@...uxfoundation.org>,
	"H. Peter Anvin" <hpa@...ux.intel.com>,
	LKML <linux-kernel@...r.kernel.org>, stable@...r.kernel.org
Subject: Re: Idle power fix regresses ebizzy performance (was 3.12-stable
 backport of NUMA balancing patches)

On Tue, 2014-01-14 at 02:31 -0500, Len Brown wrote: 
> > This is a false alarm.
> 
> Thanks for the follow-up, Mel.
> 
> Agreed, it makes no sense for ebizzy measure 'throughput', when a
> library debug bottleneck
> prevents it from scaling past 3% CPU utilization.
> 
> Still, the broken configuration did find a difference due to the
> addition of CLFLUSH on this box.
> It makes me wonder if we will find issues on workloads that may depend
> on the latency
> of idle entry/exit, or perhaps sensitivity to the state of the cache
> line containing thread_info->flags.
> 
> If somebody runs into such a workload, please try changing this 1 line
> of intel_idle.c to limit
> the CLFLUSH to C-states deeper than C1E, and let me know what you see.
> 
> - if (this_cpu_has(X86_FEATURE_CLFLUSH_MONITOR))
> + if ((eax > 1) && this_cpu_has(X86_FEATURE_CLFLUSH_MONITOR))
>           clflush((void *)&current_thread_info()->flags);

Hm, seems any high frequency switcher scheduling cross-core (pipe-test,
or maybe a tbench pair) should show the cost to an affected box.

-Mike

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