Subject: Re: [PATCH RFT 3/3] blk-mq: Optimise blk_mq_queue_tag_busy_iter() for
 shared tags
To:     John Garry <john.garry@huawei.com>, axboe@kernel.dk
Cc:     linux-block@vger.kernel.org, linux-kernel@vger.kernel.org,
        ming.lei@redhat.com, kashyap.desai@broadcom.com
References: <1635852455-39935-1-git-send-email-john.garry@huawei.com>
 <1635852455-39935-4-git-send-email-john.garry@huawei.com>
From:   Hannes Reinecke <hare@suse.de>
Message-ID: <95685711-ee3c-f1ec-4329-47e8e789a08a@suse.de>
Date:   Sun, 28 Nov 2021 11:43:30 +0100
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On 11/2/21 12:27 PM, John Garry wrote:
> Kashyap reports high CPU usage in blk_mq_queue_tag_busy_iter() and callees
> using megaraid SAS RAID card since moving to shared tags [0].
> 
> Previously, when shared tags was shared sbitmap, this function was less
> than optimum since we would iter through all tags for all hctx's,
> yet only ever match upto tagset depth number of rqs.
> 
> Since the change to shared tags, things are even less efficient if we have
> parallel callers of blk_mq_queue_tag_busy_iter(). This is because in
> bt_iter() -> blk_mq_find_and_get_req() there would be more contention on
> accessing each request ref and tags->lock since they are now shared among
> all HW queues.
> 
> Optimise by having separate calls to bt_for_each() for when we're using
> shared tags. In this case no longer pass a hctx, as it is no longer
> relevant, and teach bt_iter() about this.
> 
> Ming suggested something along the lines of this change, apart from a
> different implementation.
> 
> [0] https://lore.kernel.org/linux-block/e4e92abbe9d52bcba6b8cc6c91c442cc@mail.gmail.com/
> 
> Signed-off-by: John Garry <john.garry@huawei.com>
> ---
>   block/blk-mq-tag.c | 52 +++++++++++++++++++++++++++++++---------------
>   1 file changed, 35 insertions(+), 17 deletions(-)
> 
Reviewed-by: Hannes Reinecke <hare@suse.de>

Cheers,

Hannes
-- 
Dr. Hannes Reinecke                Kernel Storage Architect
hare@suse.de                              +49 911 74053 688
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