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

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Date:   Mon, 29 Nov 2021 10:19:24 +0800
From:   Ming Lei <ming.lei@...hat.com>
To:     John Garry <john.garry@...wei.com>
Cc:     axboe@...nel.dk, linux-block@...r.kernel.org,
        linux-kernel@...r.kernel.org, kashyap.desai@...adcom.com,
        hare@...e.de
Subject: Re: [PATCH RFT 3/3] blk-mq: Optimise blk_mq_queue_tag_busy_iter()
 for shared tags

On Tue, Nov 02, 2021 at 07:27:35PM +0800, 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@...wei.com>

Looks fine,

Reviewed-by: Ming Lei <ming.lei@...hat.com>


Thanks,
Ming