linux-kernel - Re: [PATCH] xen/blkfront: restart request queue when there is enough persistent_gnts

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Date:	Fri, 9 Jan 2015 16:51:37 +0100
From:	Roger Pau Monné <roger.pau@...rix.com>
To:	Bob Liu <bob.liu@...cle.com>, <xen-devel@...ts.xen.org>
CC:	<konrad.wilk@...cle.com>, <linux-kernel@...r.kernel.org>,
	<david.vrabel@...rix.com>, <junxiao.bi@...cle.com>
Subject: Re: [PATCH] xen/blkfront: restart request queue when there is enough
 persistent_gnts_c

El 06/01/15 a les 14.19, Bob Liu ha escrit:
> When there is no enough free grants, gnttab_alloc_grant_references()
> will fail and block request queue will stop.
> If the system is always lack of grants, blkif_restart_queue_callback() can't be
> scheduled and block request queue can't be restart(block I/O hang).
> 
> But when there are former requests complete, some grants may free to
> persistent_gnts_c, we can give the request queue another chance to restart and
> avoid block hang.
> 
> Reported-by: Junxiao Bi <junxiao.bi@...cle.com>
> Signed-off-by: Bob Liu <bob.liu@...cle.com>
> ---
>  drivers/block/xen-blkfront.c |   11 +++++++++++
>  1 file changed, 11 insertions(+)
> 
> diff --git a/drivers/block/xen-blkfront.c b/drivers/block/xen-blkfront.c
> index 2236c6f..dd30f99 100644
> --- a/drivers/block/xen-blkfront.c
> +++ b/drivers/block/xen-blkfront.c
> @@ -1125,6 +1125,17 @@ static void blkif_completion(struct blk_shadow *s, struct blkfront_info *info,
>  			}
>  		}
>  	}
> +
> +	/*
> +	 * Request queue would be stopped if failed to alloc enough grants and
> +	 * won't be restarted until gnttab_free_count >= info->callback->count.
> +	 *
> +	 * But there is another case, once we have enough persistent grants we
> +	 * can try to restart the request queue instead of continue to wait for
> +	 * 'gnttab_free_count'.
> +	 */
> +	if (info->persistent_gnts_c >= info->callback.count)
> +		schedule_work(&info->work);

I guess I'm missing something here, but blkif_completion is called by
blkif_interrupt, which in turn calls kick_pending_request_queues when
finished, which IMHO should be enough to restart the processing of requests.

Roger.
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