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

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Date:	Tue, 14 Jan 2014 02:31:18 -0500
From:	Len Brown <lenb@...nel.org>
To:	Mel Gorman <mgorman@...e.de>
Cc:	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)

> 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);

thanks,
Len Brown, Intel Open Source Technology Center
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