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

Song Liu
2021-12-16T19:23:28Z

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