linux-kernel - very low IOPS due to "block: reduce kblockd_mod_delayed_work

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Date:   Sat, 18 Dec 2021 13:57:15 -0500
From:   "Alex Xu (Hello71)" <alex_y_xu@...oo.ca>
To:     Jens Axboe <axboe@...nel.dk>, linux-block@...r.kernel.org,
        Dexuan Cui <decui@...rosoft.com>, ming.lei@...hat.com,
        hch@....de, Long Li <longli@...rosoft.com>,
        "Michael Kelley (LINUX)" <mikelley@...rosoft.com>,
        linux-kernel@...r.kernel.org
Subject: very low IOPS due to "block: reduce kblockd_mod_delayed_work_on() CPU
 consumption"

Hi,

I recently noticed that between 6441998e2e and 9eaa88c703, I/O became 
much slower on my machine using ext4 on dm-crypt on NVMe with bfq 
scheduler. Checking iostat during heavy usage (find / -xdev and fstrim 
-v /), maximum IOPS had fallen from ~10000 to ~100. Reverting cb2ac2912a 
("block: reduce kblockd_mod_delayed_work_on() CPU consumption") resolves 
the issue.

Thanks,
Alex.