On Fri, Dec 17, 2021 at 5:42 AM Sebastian Andrzej Siewior bigeasy@linutronix.de wrote:
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?
Yes this only happens on boot-up and module loading.I don't know RT well enough to tell whether latency during module loading is an issue.
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.
With current code, the delay _should be_ 16 jiffies. However, the experiment hits way longer latencies. I agree this may cause inaccurate benchmark results and thus suboptimal RAID algorithm.
I guess the key question is whether long latency at module loading time matters. If that doesn't matter, we should just drop this.
Thanks, Song