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From:   Daniel Vacek <neelx.g@gmail.com>
Date:   Fri, 17 Dec 2021 22:55:40 +0100
Message-ID: <CAA7rmPFnEkKxH2i4yfEGiZ_ngVme0SfrFhok49_AgqLEcdHicQ@mail.gmail.com>
Subject: Re: [PATCH v2] lib/raid6: Reduce high latency by using migrate
 instead of preempt
To:     Song Liu <song@kernel.org>
Cc:     Yajun Deng <yajun.deng@linux.dev>,
        Paul Menzel <pmenzel@molgen.mpg.de>, williams@redhat.com,
        masahiroy@kernel.org, open list <linux-kernel@vger.kernel.org>,
        linux-rt-users <linux-rt-users@vger.kernel.org>,
        linux-raid <linux-raid@vger.kernel.org>
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Precedence: bulk

NACK. This is a BUG.

--nX

On Fri, Dec 17, 2021 at 1:02 AM Song Liu <song@kernel.org> wrote:
>
> On Thu, Dec 16, 2021 at 1:27 AM Yajun Deng <yajun.deng@linux.dev> wrote:
> >
> > We found an abnormally high latency when executing modprobe raid6_pq, the
> > latency is greater than 1.2s when CONFIG_PREEMPT_VOLUNTARY=y, greater than
> > 67ms when CONFIG_PREEMPT=y, and greater than 16ms when CONFIG_PREEMPT_RT=y.
> >
> > This is caused by ksoftirqd fail to scheduled due to disable preemption,
> > this time is too long and unreasonable.
> >
> > Reduce high latency by using migrate_disabl()/emigrate_enable() instead of
> > preempt_disable()/preempt_enable().
> >
> > How to reproduce:
> >  - Install cyclictest
> >      sudo apt install rt-tests
> >  - Run cyclictest example in one terminal
> >      sudo cyclictest -S -p 95 -d 0 -i 1000 -D 24h -m
> >  - Modprobe raid6_pq in another terminal
> >       sudo modprobe raid6_pq
> >
> > This patch beneficial for CONFIG_PREEMPT=y and CONFIG_PREEMPT_RT=y, but
> > no effect for CONFIG_PREEMPT_VOLUNTARY=y.
> >
> > Fixes: fe5cbc6e06c7 ("md/raid6 algorithms: delta syndrome functions")
> > Fixes: cc4589ebfae6 ("Rename raid6 files now they're in a 'raid6' directory.")
> > Link: https://lore.kernel.org/linux-raid/b06c5e3ef3413f12a2c2b2a241005af9@linux.dev/T/#t
> > Signed-off-by: Yajun Deng <yajun.deng@linux.dev>
>
> Updated the patch and CC'ed stable. It should make to LTS kernels.
>
> Thanks,
> Song