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SubjectRE: [PATCH V2] Consider inflight IO in io accounting for high latency devices
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Hi Bart,

Thanks for your review. Can you please see my inline comments?

Regards,
Gulam Mohamed.

-----Original Message-----
From: Bart Van Assche <bvanassche@acm.org>
Sent: Saturday, October 14, 2023 2:43 AM
To: Gulam Mohamed <gulam.mohamed@oracle.com>; axboe@kernel.dk; linux-block@vger.kernel.org; linux-kernel@vger.kernel.org
Subject: Re: [PATCH V2] Consider inflight IO in io accounting for high latency devices

On 10/13/23 12:55, Gulam Mohamed wrote:
> @@ -1015,7 +1018,13 @@ static inline void blk_account_io_start(struct request *req)
> req->part = req->q->disk->part0;
>
> part_stat_lock();
> - update_io_ticks(req->part, jiffies, false);
> +
> + if (req->q->nr_hw_queues == 1) {
> + hctx = xa_load(&req->q->hctx_table, 0);
> + inflight = blk_mq_hctx_has_tags(hctx);
> + }
> +
> + update_io_ticks(req->part, jiffies, inflight);
> part_stat_unlock();
> }
> }

blk_account_io_start() is called by blk_mq_bio_to_request(). So if I/O statistics are enabled and if there is only a single hardware queue,
blk_mq_hctx_has_tags() will be called every time a bio is submitted?
The blk_mq_hctx_has_tags() function iterates over all tags. I would be surprised if anyone would consider the overhead of this approach acceptable.

[GULAM]: Yes, it will be called for every submitted bio but for the high latency devices it will not have much impact. This is indicated by the latency figures I provided in the review mail, with and without our patch.

Thanks,

Bart.
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