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Date: Tue, 14 Jan 2014 02:31:18 -0500
Message-ID: <CAJvTdKniGkxxzvPvEDSU9BgtubR-5Fn3YsepjSNAwn5XkL3nVA@mail.gmail.com>
Subject: Re: Idle power fix regresses ebizzy performance (was 3.12-stable
 backport of NUMA balancing patches)
From: Len Brown <lenb@kernel.org>
To: Mel Gorman <mgorman@suse.de>
Cc: Len Brown <len.brown@intel.com>, athorlton@sgi.com,
        Rik van Riel <riel@redhat.com>, chegu_vinod@hp.com,
        Greg KH <gregkh@linuxfoundation.org>,
        "H. Peter Anvin" <hpa@linux.intel.com>,
        LKML <linux-kernel@vger.kernel.org>, stable@vger.kernel.org
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Sender: linux-kernel-owner@vger.kernel.org

> 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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