RE: Random high CPU utilization in blk-mq with the none scheduler

Dexuan Cui
2021-12-11T03:44:47Z

> From: Jens Axboe <axboe@kernel.dk>
>
> Just out of curiosity, can you do:
>
> # perf record -a -g -- sleep 3
>
> when you see the excessive CPU usage, then attach the output of
>
> # perf report -g
>
> to a reply?

I ran the commands against the 5.15.7 kernel and generated a
771-MB file. After I ran 'bzip2 -9', the file size is still 22 MB, which
I guess is too big to be shared via the list. I'll try to put it somewhere
and send you a link.

> How confident are you in your bisect result?
>
> --
> Jens Axboe

I'm pretty confident:
1) I can't reproduce the issue with v5.16-rc4 even if I run the test
10 times. Typically the issue repros every time.

2) If I revert the commit

dc5fc361d891 ("block: attempt direct issue of plug list")

and the related patches

ff1552232b36 ("blk-mq: don't issue request directly in case that current is to be blocked")
b22809092c70 ("block: replace always false argument with 'false'")

from v5.16-rc4, I'm able to repro the issue immediately.

Thanks,
Dexuan