linux-kernel - Re: [PATCH 2/2] null_blk: restart request processing on completion handler

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Date:	Wed, 3 Jun 2015 00:53:45 +0900
From:	Akinobu Mita <akinobu.mita@...il.com>
To:	LKML <linux-kernel@...r.kernel.org>
Cc:	Akinobu Mita <akinobu.mita@...il.com>, Jens Axboe <axboe@...com>
Subject: Re: [PATCH 2/2] null_blk: restart request processing on completion handler

2015-06-02 8:35 GMT+09:00 Akinobu Mita <akinobu.mita@...il.com>:
> When irqmode=2 (IRQ completion handler is timer) and queue_mode=1
> (Block interface to use is rq), the completion handler should restart
> request handling for any pending requests on a queue because request
> processing stops when the number of commands are queued more than
> hw_queue_depth (null_rq_prep_fn returns BLKPREP_DEFER).
>
> Without this change, the following command cannot finish.
>
>         # modprobe null_blk irqmode=2 queue_mode=1 hw_queue_depth=1
>         # fio --name=t --rw=read --size=1g --direct=1 \
>           --ioengine=libaio --iodepth=64 --filename=/dev/nullb0
>
> Signed-off-by: Akinobu Mita <akinobu.mita@...il.com>
> Cc: Jens Axboe <axboe@...com>
> ---
>  drivers/block/null_blk.c | 12 ++++++++++++
>  1 file changed, 12 insertions(+)
>
> diff --git a/drivers/block/null_blk.c b/drivers/block/null_blk.c
> index 6f0a58e..6f9b753 100644
> --- a/drivers/block/null_blk.c
> +++ b/drivers/block/null_blk.c
> @@ -243,6 +243,17 @@ static enum hrtimer_restart null_cmd_timer_expired(struct hrtimer *timer)
>                         cmd = container_of(entry, struct nullb_cmd, ll_list);
>                         entry = entry->next;
>                         end_cmd(cmd);
> +
> +                       if (cmd->rq) {
> +                               struct request_queue *q = cmd->rq->q;
> +
> +                               if (!q->mq_ops && blk_queue_stopped(q)) {
> +                                       spin_lock(q->queue_lock);
> +                                       if (blk_queue_stopped(q))
> +                                               blk_start_queue(q);

Should 'blk_start_queue(q)' be:

                queue_flag_clear(QUEUE_FLAG_STOPPED, q);
                blk_run_queue_async(q);
?

Otherwise, all pending requests on a queue are completed by this timer
handler execution without needing next timer expiry.

> +                                       spin_unlock(q->queue_lock);
> +                               }
> +                       }
>                 } while (entry);
>         }
>
> @@ -334,6 +345,7 @@ static int null_rq_prep_fn(struct request_queue *q, struct request *req)
>                 req->special = cmd;
>                 return BLKPREP_OK;
>         }
> +       blk_stop_queue(q);
>
>         return BLKPREP_DEFER;
>  }
> --
> 1.9.1
>
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