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

[Dexuan Cui]

> From: Dexuan Cui
> Sent: Thursday, December 9, 2021 7:30 PM
> 
> Hi all,
> I found a random high CPU utilization issue with some database benchmark
> program running on a 192-CPU virtual machine (VM). Originally the issue
> was found with RHEL 8.4 and Ubuntu 20.04, and further tests show that the
> issue also reproduces with the latest upstream stable kernel v5.15.7, but
> *not* with v5.16-rc1. It looks like someone resolved the issue in v5.16-rc1
> recently?

I did git-bisect on the linux-block tree's for-5.16/block branch and this patch
resolves the random high CPU utilization issue (I'm not sure how):
	dc5fc361d891 ("block: attempt direct issue of plug list")
	https://git.kernel.org/pub/scm/linux/kernel/git/axboe/linux-block.git/commit/?h=for-5.16/block&id=dc5fc361d891e089dfd9c0a975dc78041036b906

Do you think if it's easy to backport it to earlier versions like 5.10?
It looks like there are a lot of prerequisite patches.

Thanks,
Dexuan