Hi Dexuan,

On Sat, Dec 11, 2021 at 03:10:43AM +0000, Dexuan Cui wrote:

quoted

From: Jens Axboe <axboe@kernel.dk>
Sent: Friday, December 10, 2021 6:05 PM
...
It's more likely the real fix is avoiding the repeated plug list scan,
which I guess makes sense. That is this commit:

commit d38a9c04c0d5637a828269dccb9703d42d40d42b
Author: Jens Axboe [off-list ref]
Date:   Thu Oct 14 07:24:07 2021 -0600

    block: only check previous entry for plug merge attempt

If that's the case, try 5.15.x again and do:

echo 2 > /sys/block/<dev>/queue/nomerges

for each drive you are using in the IO test, and see if that gets
rid of the excess CPU usage.

--
Jens Axboe

Thanks for the reply! Unluckily this does not work.

I tried the below command:

for i in `ls /sys/block/*/queue/nomerges`; do echo 2 > $i; done

and verified that the "nomerges" are changed to "2", but the
excess CPU usage can still reproduce easily.

Can you provide the following blk-mq debugfs log?

(cd /sys/kernel/debug/block/dm-N && find . -type f -exec grep -aH . {} \;)

(cd /sys/kernel/debug/block/sdN && find . -type f -exec grep -aH . {} \;)

And it is enough to just collect log from one dm-mpath & one underlying iscsi disk,
so we can understand basic blk-mq setting, such as nr_hw_queues, queue depths, ...



Thanks,
Ming