linux-kernel - Re: [PATCH] x86/topology: Fallback to SMT level only once

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Date:   Mon, 29 Aug 2016 14:20:15 +0200
From:   Jirka Hladky <jhladky@...hat.com>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Jiri Olsa <jolsa@...nel.org>, Jean-Pierre Lozi <jplozi@...ce.fr>,
        Petr Surý <psury@...hat.com>,
        lkml <linux-kernel@...r.kernel.org>,
        Ingo Molnar <mingo@...nel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        "H. Peter Anvin" <hpa@...or.com>
Subject: Re: [PATCH] x86/topology: Fallback to SMT level only once

Hi Peter,

yes, initially I have reported the issue to occur on Intel E5v3 CPU
(that CPU does not have CoD) but it has turned to be a fluctuation of
results. After repeating the test 10 times it has turned out that
Intel E5v3 CPU is not affected. I'm sorry for that.

I have then rerun the test on Opteron 6272 (same CPU as used by
authors of the paper) and there was performance degradation by factor
9x. Jirka has then provided the patch.

Thanks a lot
Jirka

On Mon, Aug 29, 2016 at 12:40 PM, Peter Zijlstra <peterz@...radead.org> wrote:
> On Sun, Aug 28, 2016 at 08:19:46PM +0200, Jiri Olsa wrote:
>> Jirka, Peter and Jean-Pierre reported performance drop on
>> some cpus after making cpu offline and online again.
>>
>> The reason is the kernel logic that falls back to SMT
>> level topology if more than one node is detected within
>> CPU package. During the system boot this logic cuts out
>> the DIE topology level and numa code adds NUMA level
>> on top of this.
>
> Its not SMT topology, and back when I asked if he had CoD enabled or
> such he said not.
>
>
> See also:
>
> http://lkml.kernel.org/r/1471559812-19967-3-git-send-email-srinivas.pandruvada@linux.intel.com
>
> Arguably, that should have been split in two patches, but alas..