linux-kernel - Re: [LKP] Re: [sched/hotplug] 2558aacff8: will-it-scale.per_thread

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Date:   Tue, 15 Dec 2020 09:37:32 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Xing Zhengjun <zhengjun.xing@...ux.intel.com>
Cc:     kernel test robot <oliver.sang@...el.com>,
        Valentin Schneider <valentin.schneider@....com>,
        Daniel Bristot de Oliveira <bristot@...hat.com>,
        LKML <linux-kernel@...r.kernel.org>, x86@...nel.org,
        lkp@...ts.01.org, lkp@...el.com, zhengjun.xing@...el.com,
        aubrey.li@...ux.intel.com, yu.c.chen@...el.com
Subject: Re: [LKP] Re: [sched/hotplug] 2558aacff8:
 will-it-scale.per_thread_ops -1.6% regression

On Tue, Dec 15, 2020 at 01:35:46PM +0800, Xing Zhengjun wrote:
> On 12/11/2020 12:14 AM, Peter Zijlstra wrote:
> > On Thu, Dec 10, 2020 at 04:18:59PM +0800, kernel test robot wrote:
> > > FYI, we noticed a -1.6% regression of will-it-scale.per_thread_ops due to commit:
> > > commit: 2558aacff8586699bcd248b406febb28b0a25de2 ("sched/hotplug: Ensure only per-cpu kthreads run during hotplug")
> > 
> > Mooo, weird but whatever. Does the below help at all?
> 
> I test the patch

Thanks!

> , the regression reduced to -0.6%.
> 
> =========================================================================================
> tbox_group/testcase/rootfs/kconfig/compiler/nr_task/mode/test/cpufreq_governor/ucode:
> 
> lkp-cpl-4sp1/will-it-scale/debian-10.4-x86_64-20200603.cgz/x86_64-rhel-8.3/gcc-9/100%/thread/sched_yield/performance/0x700001e
> 
> commit:
>   565790d28b1e33ee2f77bad5348b99f6dfc366fd
>   2558aacff8586699bcd248b406febb28b0a25de2
>   4b26139b8db627a55043183614a32b0aba799d27 (this test patch)
> 
> 565790d28b1e33ee 2558aacff8586699bcd248b406f 4b26139b8db627a55043183614a
> ---------------- --------------------------- ---------------------------
>          %stddev     %change         %stddev     %change         %stddev
>              \          |                \          |                \
>  4.011e+08            -1.6%  3.945e+08            -0.6%  3.989e+08 will-it-scale.144.threads
>    2785455            -1.6%    2739520            -0.6%    2769967 will-it-scale.per_thread_ops
>  4.011e+08            -1.6%  3.945e+08            -0.6%  3.989e+08 will-it-scale.workload

Well, that's better. But I'm rather confused now, because with this new
patch, the actual hot paths are identical, so I've no idea what is
actually causing the regression :/

The above numbers don't seem to have variance, how sure are we the
results are stable? The thing is, when I tried reproducing this locally,
I was mostly looking at noise.

> > ---
> >   kernel/sched/core.c  | 40 +++++++++++++++-------------------------
> >   kernel/sched/sched.h | 13 +++++--------
> >   2 files changed, 20 insertions(+), 33 deletions(-)

Anyway, let me queue this in sched/urgent, it's simpler code and has
less regression.