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Message-ID: <35e3e173-8018-42d8-a6e8-7ba994ff6b17@acm.org>
Date: Fri, 13 Oct 2023 14:13:08 -0700
From: Bart Van Assche <bvanassche@....org>
To: Gulam Mohamed <gulam.mohamed@...cle.com>, axboe@...nel.dk,
linux-block@...r.kernel.org, linux-kernel@...r.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.
Thanks,
Bart.
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