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