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Message-ID: <4fcddc26-4f51-4228-9a35-58dc1b5a16bc@kernel.dk>
Date: Sun, 13 Aug 2023 07:54:23 -0600
From: Jens Axboe <axboe@...nel.dk>
To: chengming.zhou@...ux.dev, hch@....de, chuck.lever@...cle.com
Cc: bvanassche@....org, cel@...nel.org, linux-block@...r.kernel.org,
linux-kernel@...r.kernel.org, zhouchengming@...edance.com
Subject: Re: [PATCH] blk-mq: release scheduler resource when request complete
On 8/13/23 7:36 AM, chengming.zhou@...ux.dev wrote:
> From: Chengming Zhou <zhouchengming@...edance.com>
>
> Chuck reported [1] a IO hang problem on NFS exports that reside on SATA
> devices and bisected to commit 615939a2ae73 ("blk-mq: defer to the normal
> submission path for post-flush requests").
>
> We analysed the IO hang problem, found there are two postflush requests
> are waiting for each other.
>
> The first postflush request completed the REQ_FSEQ_DATA sequence, so go to
> the REQ_FSEQ_POSTFLUSH sequence and added in the flush pending list, but
> failed to blk_kick_flush() because of the second postflush request which
> is inflight waiting in scheduler queue.
>
> The second postflush waiting in scheduler queue can't be dispatched because
> the first postflush hasn't released scheduler resource even though it has
> completed by itself.
>
> Fix it by releasing scheduler resource when the first postflush request
> completed, so the second postflush can be dispatched and completed, then
> make blk_kick_flush() succeed.
Change looks good to me. But since we're in here:
> diff --git a/block/blk-mq.c b/block/blk-mq.c
> index f14b8669ac69..5b14f18f9670 100644
> --- a/block/blk-mq.c
> +++ b/block/blk-mq.c
> @@ -682,6 +682,15 @@ struct request *blk_mq_alloc_request_hctx(struct request_queue *q,
> }
> EXPORT_SYMBOL_GPL(blk_mq_alloc_request_hctx);
>
> +static void blk_mq_finish_request(struct request *rq)
> +{
> + struct request_queue *q = rq->q;
> +
> + if ((rq->rq_flags & RQF_USE_SCHED) &&
> + q->elevator->type->ops.finish_request)
> + q->elevator->type->ops.finish_request(rq);
> +}
Any IO scheduler should set ->finish_request(), so this should just be:
static void blk_mq_finish_request(struct request *rq)
{
struct request_queue *q = rq->q;
if (rq->rq_flags & RQF_USE_SCHED)
q->elevator->type->ops.finish_request(rq);
}
and would probably be a good idea to solidify that with a:
if (WARN_ON_ONCE(!e->ops.finish_request))
return -EINVAL;
at the top of elv_register() like we have for insert/dispatch as well.
All 3 IO schedulers do set ->finish_request().
--
Jens Axboe
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