linux-kernel - Re: [PATCH v3] lib/raid6: Reduce high latency by using migrate instead of preempt

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Date:   Fri, 17 Dec 2021 14:42:17 +0100
From:   Sebastian Andrzej Siewior <bigeasy@...utronix.de>
To:     Yajun Deng <yajun.deng@...ux.dev>
Cc:     song@...nel.org, masahiroy@...nel.org, williams@...hat.com,
        pmenzel@...gen.mpg.de, linux-kernel@...r.kernel.org,
        linux-rt-users@...r.kernel.org, linux-raid@...r.kernel.org,
        stable@...r.kernel.org, tglx@...utronix.de
Subject: Re: [PATCH v3] lib/raid6: Reduce high latency by using migrate
 instead of preempt

On 2021-12-17 10:16:10 [+0800], Yajun Deng 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.
> 
> 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 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(), the latency won't greater than 100us.
> 
> This patch beneficial for CONFIG_PREEMPT=y or CONFIG_PREEMPT_RT=y, but no
> effect for CONFIG_PREEMPT_VOLUNTARY=y.

Why does it matter? This is only during boot-up/ module loading or do I
miss something? 
The delay is a jiffy so it depends on CONFIG_HZ. You do benchmark for
the best algorithm and if you get preempted during that period then your
results may be wrong and you make a bad selection.

You can either enable one algorithm and or disable
CONFIG_RAID6_PQ_BENCHMARK. I don't see the need for this patch not to
mention the stable tree.

Sebastian