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

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Message-ID: <BYAPR21MB1270DCE17A0FE017AF3272F1BF729@BYAPR21MB1270.namprd21.prod.outlook.com>
Date:   Sat, 11 Dec 2021 01:29:02 +0000
From:   Dexuan Cui <decui@...rosoft.com>
To:     Jens Axboe <axboe@...nel.dk>,
        "'ming.lei@...hat.com'" <ming.lei@...hat.com>,
        'Christoph Hellwig' <hch@....de>,
        "'linux-block@...r.kernel.org'" <linux-block@...r.kernel.org>
CC:     Long Li <longli@...rosoft.com>,
        "Michael Kelley (LINUX)" <mikelley@...rosoft.com>,
        "'linux-kernel@...r.kernel.org'" <linux-kernel@...r.kernel.org>
Subject: RE: Random high CPU utilization in blk-mq with the none scheduler

> 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