linux-kernel - Re: [PATCH RFT 0/3] blk-mq: Optimise blk_mq_queue_tag_busy

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Date:   Mon, 15 Nov 2021 15:56:16 +0000
From:   John Garry <john.garry@...wei.com>
To:     <axboe@...nel.dk>, <kashyap.desai@...adcom.com>
CC:     <linux-block@...r.kernel.org>, <linux-kernel@...r.kernel.org>,
        <ming.lei@...hat.com>, <hare@...e.de>
Subject: Re: [PATCH RFT 0/3] blk-mq: Optimise blk_mq_queue_tag_busy_iter() for
 shared tags

On 02/11/2021 11:27, John Garry wrote:

Hi Kashyap,

Any chance you can try this series or give an update on the issue 
reported earlier?

thanks @ Ming for the reviews.

Cheers,
John

> In [0], Kashyap reports high CPU usage for blk_mq_queue_tag_busy_iter()
> and callees for shared tags.
> 
> Indeed blk_mq_queue_tag_busy_iter() would be less optimum for moving to
> shared tags, but it was not optimum previously.
> 
> So I would like this series tested, and also to know what is triggering
> blk_mq_queue_tag_busy_iter() from userspace to cause such high CPU
> loading.
> 
> As suggested by Ming, reading /proc/diskstats in a while true loop
> can trigger blk_mq_queue_tag_busy_iter(); I do so in a test with 2x
> separate consoles, and here are the results:
> 
> v5.15
> blk_mq_queue_tag_busy_iter() 6.2%
> part_stat_read_all() 6.7
> 
> pre-v5.16 (Linus' master branch @ commit bfc484fe6abb)
> blk_mq_queue_tag_busy_iter() 4.5%
> part_stat_read_all() 6.2
> 
> pre-v5.16+this series
> blk_mq_queue_tag_busy_iter() not shown in top users
> part_stat_read_all() 7.5%
> 
> These results are from perf top, on a system with 7x
> disks, with hisi_sas which has 16x HW queues.
> 
> [0] https://lore.kernel.org/linux-block/e4e92abbe9d52bcba6b8cc6c91c442cc@mail.gmail.com/
> 
> John Garry (3):
>    blk-mq: Drop busy_iter_fn blk_mq_hw_ctx argument
>    blk-mq: Delete busy_iter_fn
>    blk-mq: Optimise blk_mq_queue_tag_busy_iter() for shared tags
> 
>   block/blk-mq-tag.c     | 58 +++++++++++++++++++++++++++---------------
>   block/blk-mq-tag.h     |  2 +-
>   block/blk-mq.c         | 17 ++++++-------
>   include/linux/blk-mq.h |  2 --
>   4 files changed, 47 insertions(+), 32 deletions(-)
>