linux-kernel - Re: [PATCH v2 1/2] workqueue: new struct io

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Message-ID: <ZoLoJ-uBo9qyAlMg@slm.duckdns.org>
Date: Mon, 1 Jul 2024 07:32:23 -1000
From: Tejun Heo <tj@...nel.org>
To: Yi Sun <yi.sun@...soc.com>
Cc: sunyibuaa@...il.com, jiangshanlai@...il.com, jaegeuk@...nel.org,
	chao@...nel.org, ebiggers@...gle.com,
	linux-f2fs-devel@...ts.sourceforge.net,
	linux-kernel@...r.kernel.org, kent.overstreet@...ux.dev,
	niuzhiguo84@...il.com, Hao_hao.Wang@...soc.com,
	yunlongxing23@...il.com
Subject: Re: [PATCH v2 1/2] workqueue: new struct io_work

Hello,

On Mon, Jul 01, 2024 at 03:51:37PM +0800, Yi Sun wrote:
> +/*
> + * If a work may do disk IO, it is recommended to use struct io_work
> + * instead of struct work_struct.
> + */
> +struct io_work {
> +	struct work_struct work;
> +
> +	/* If the work does submit_bio, io priority may be needed. */
> +	unsigned short ioprio;
> +	/* Record kworker's original io priority. */
> +	unsigned short ori_ioprio;
> +	/* Whether the work has set io priority? */
> +	long ioprio_flag;
> +};

There are fundamental limitations to this approach in terms of
prioritization. If you tag each work items with priority and then send them
to the same workqueue, there's nothing preventing a low priority issuer from
flooding the workqueue and causing a priority inversion. ie. To solve this
properly, you need per-issuer-class workqueue so that the concurrency limit
is not shared across different priorities.

Now, this limited implementation, while incomplete and easy to defeat, may
still be useful. After all, ioprio itself, I think, is flawed in the same
way. If f2fs wants to implement this internally, that's okay, I suppose, but
as a generic mechanism, I don't think this makes a lot of sense.

Thanks.

-- 
tejun