linux-kernel - Re: [PATCH] block: Consider inflight IO in io accounting for high latency devices

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Date:   Fri, 8 Sep 2023 08:33:47 -0600
From:   Jens Axboe <axboe@...nel.dk>
To:     Gulam Mohamed <gulam.mohamed@...cle.com>,
        linux-block@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] block: Consider inflight IO in io accounting for high
 latency devices

On 9/7/23 3:45 PM, Gulam Mohamed wrote:
> diff --git a/block/blk-mq.c b/block/blk-mq.c
> index ec922c6bccbe..70e5763fb799 100644
> --- a/block/blk-mq.c
> +++ b/block/blk-mq.c
> @@ -1000,6 +1000,8 @@ static inline void blk_account_io_done(struct request *req, u64 now)
>  
>  static inline void blk_account_io_start(struct request *req)
>  {
> +	bool delta = false;
> +

This is an odd name for this variable...

> @@ -1015,7 +1017,10 @@ 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) {
> +			delta = !!part_in_flight(req->part);
> +		}

No parens needed here. But that aside, I think this could be a lot
better. You don't really care about the number of requests inflight,
only if there are some. A better helper than part_in_flight() could do
that ala:

static bool part_any_in_flight(struct block_device *part)
{
	int cpu;

	for_each_possible_cpu(cpu) {
		if (part_stat_local_read_cpu(part, in_flight[0], cpu) ||
		    part_stat_local_read_cpu(part, in_flight[1], cpu))
			return true;
	}

	return false;
}

But I do wonder if it's just missed state checking for the request
itself that's missing this, and this is fixing it entirely the wrong way
around.

-- 
Jens Axboe