linux-kernel - Re: [PATCH v2] blk-mq: release scheduler resource when request complete

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Date:   Thu, 17 Aug 2023 23:29:37 +0800
From:   Chengming Zhou <chengming.zhou@...ux.dev>
To:     Bart Van Assche <bvanassche@....org>, axboe@...nel.dk, hch@....de
Cc:     oe-lkp@...ts.linux.dev, lkp@...el.com,
        Chuck Lever <chuck.lever@...cle.com>,
        linux-block@...r.kernel.org, cel@...nel.org,
        linux-kernel@...r.kernel.org,
        kernel test robot <oliver.sang@...el.com>,
        chengming.zhou@...ux.dev
Subject: Re: [PATCH v2] blk-mq: release scheduler resource when request
 complete

On 2023/8/17 22:50, Bart Van Assche wrote:
> On 8/17/23 07:41, kernel test robot wrote:
>> [  222.622837][ T2216] statistics for priority 1: i 276 m 0 d 276 c 278
>> [ 222.629307][ T2216] WARNING: CPU: 0 PID: 2216 at block/mq-deadline.c:680 dd_exit_sched (block/mq-deadline.c:680 (discriminator 3))
> 
> The above information shows that dd_inserted_request() has been called
> 276 times and also that dd_finish_request() has been called 278 times.

Thanks much for your help.

This patch indeed introduced a regression, postflush requests will be completed
twice, so here dd_finish_request() is more than dd_inserted_request().

diff --git a/block/blk-mq.c b/block/blk-mq.c
index a8c63bef8ff1..7cd47ffc04ce 100644
--- a/block/blk-mq.c
+++ b/block/blk-mq.c
@@ -686,8 +686,10 @@ static void blk_mq_finish_request(struct request *rq)
 {
        struct request_queue *q = rq->q;

-       if (rq->rq_flags & RQF_USE_SCHED)
+       if (rq->rq_flags & RQF_USE_SCHED) {
                q->elevator->type->ops.finish_request(rq);
+               rq->rq_flags &= ~RQF_USE_SCHED;
+       }
 }


Clear RQF_USE_SCHED flag here should fix this problem, which should be ok
since finish_request() is the last callback, this flag isn't needed anymore.

Jens, should I send this diff as another patch or resend updated v3?

Thanks.

> Calling dd_finish_request() more than once per request breaks the code
> for priority handling since that code checks how many requests are
> pending per priority level by subtracting the number of completion calls
> from the number of insertion calls (see also dd_queued()). I think the
> above output indicates that this patch introduced a regression.
> 
> Bart.
>