linux-kernel - Re: [PATCH v2] lib/raid6: Reduce high latency by using migrate instead of preempt

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Date:   Thu, 16 Dec 2021 11:23:12 -0800
From:   Song Liu <song@...nel.org>
To:     Yajun Deng <yajun.deng@...ux.dev>
Cc:     Paul Menzel <pmenzel@...gen.mpg.de>, williams@...hat.com,
        masahiroy@...nel.org, open list <linux-kernel@...r.kernel.org>,
        linux-rt-users@...r.kernel.org,
        linux-raid <linux-raid@...r.kernel.org>
Subject: Re: [PATCH v2] lib/raid6: Reduce high latency by using migrate
 instead of preempt

On Thu, Dec 16, 2021 at 1:27 AM Yajun Deng <yajun.deng@...ux.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@...ux.dev>

Updated the patch and CC'ed stable. It should make to LTS kernels.

Thanks,
Song