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
