linux-kernel - Re: [PATCH] mq-deadline: the dd->dispatch queue follows a FIFO policy

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Message-ID: <CAAWJmAb0+Xhm7Wy2kqH7E4VGLUqZa_2ohBYP4ttw6zzjD109JA@mail.gmail.com>
Date: Tue, 9 Dec 2025 10:06:47 +0800
From: Tao pilgrim <pilgrimtao@...il.com>
To: Bart Van Assche <bvanassche@....org>
Cc: axboe@...nel.dk, linux-block@...r.kernel.org, linux-kernel@...r.kernel.org, 
	Chengkaitao <chengkaitao@...inos.cn>
Subject: Re: [PATCH] mq-deadline: the dd->dispatch queue follows a FIFO policy

On Tue, Dec 9, 2025 at 12:24 AM Bart Van Assche <bvanassche@....org> wrote:
>
> On 12/8/25 4:02 AM, chengkaitao wrote:
> > From: Chengkaitao <chengkaitao@...inos.cn>
> >
> > In the initial implementation, the 'list_add(&rq->queuelist, ...' statement
> > added to the dd_insert_request function was designed to differentiate
> > priorities among various IO-requests within the same linked list. For
> > example, 'Commit 945ffb60c11d ("mq-deadline: add blk-mq adaptation of the
> > deadline IO scheduler")', introduced this 'list_add' operation to ensure
> > that requests with the at_head flag would always be dispatched before
> > requests without the REQ_TYPE_FS flag.
> >
> > Since 'Commit 7687b38ae470 ("bfq/mq-deadline: remove redundant check for
> > passthrough request")', removed blk_rq_is_passthrough, the dd->dispatch
> > list now contains only requests with the at_head flag. In this context,
> > all at_head requests should be treated as having equal priority, and a
> > first-in-first-out (FIFO) policy better aligns with the current situation.
> > Therefore, replacing list_add with list_add_tail is more appropriate.
> >
> > Signed-off-by: Chengkaitao <chengkaitao@...inos.cn>
> > ---
> >   block/mq-deadline.c | 2 +-
> >   1 file changed, 1 insertion(+), 1 deletion(-)
> >
> > diff --git a/block/mq-deadline.c b/block/mq-deadline.c
> > index 3e3719093aec..dcd7f4f1ecd2 100644
> > --- a/block/mq-deadline.c
> > +++ b/block/mq-deadline.c
> > @@ -661,7 +661,7 @@ static void dd_insert_request(struct blk_mq_hw_ctx *hctx, struct request *rq,
> >       trace_block_rq_insert(rq);
> >
> >       if (flags & BLK_MQ_INSERT_AT_HEAD) {
> > -             list_add(&rq->queuelist, &dd->dispatch);
> > +             list_add_tail(&rq->queuelist, &dd->dispatch);
> >               rq->fifo_time = jiffies;
> >       } else {
> >               deadline_add_rq_rb(per_prio, rq);
>
> I think the current behavior (LIFO) is on purpose and also that it only
> should be changed if there is a strong reason to change it. I don't see
> a strong reason being mentioned in the patch description.
>
Previously, the dd->dispatch queue contained both requests with the
BLK_MQ_INSERT_AT_HEAD flag and those without it. The implementation
placed requests with the BLK_MQ_INSERT_AT_HEAD flag at the head of
the dd->dispatch queue, while requests without this flag were placed
at the tail. However, now all requests in the dd->dispatch list carry
the BLK_MQ_INSERT_AT_HEAD flag, and I can't find any justification for
continuing to use a LIFO (last-in-first-out) policy.

Additionally, the dispatch queue has recently been moved from struct
dd_per_prio to struct deadline_data, switching back to a single dispatch
list. This means more requests will queue in the dispatch list, and
continuing with the LIFO policy makes earlier-arriving requests more
susceptible to starvation. Could anyone explain the rationale behind
maintaining this approach?

-- 
Yours,
Kaitao Cheng