Re: [PATCH V2 4/5] blk-mq: re-submit IO in case that hctx is dead

[Hannes Reinecke <hare@suse.de>]

On 8/12/19 3:43 PM, Ming Lei wrote:
> When all CPUs in one hctx are offline, we shouldn't run this hw queue
> for completing request any more.
> 
> So steal bios from the request, and resubmit them, and finally free
> the request in blk_mq_hctx_notify_dead().
> 
> Cc: Bart Van Assche <bvanassche@acm.org>
> Cc: Hannes Reinecke <hare@suse.com>
> Cc: Christoph Hellwig <hch@lst.de>
> Cc: Thomas Gleixner <tglx@linutronix.de>
> Cc: Keith Busch <keith.busch@intel.com>
> Signed-off-by: Ming Lei <ming.lei@redhat.com>
> ---
>  block/blk-mq.c | 48 +++++++++++++++++++++++++++++++++++++++++-------
>  1 file changed, 41 insertions(+), 7 deletions(-)
> 
> diff --git a/block/blk-mq.c b/block/blk-mq.c
> index 6931b2ba2776..ed334fd867c4 100644
> --- a/block/blk-mq.c
> +++ b/block/blk-mq.c
> @@ -2261,10 +2261,30 @@ static int blk_mq_hctx_notify_online(unsigned int cpu, struct hlist_node *node)
>  	return 0;
>  }
>  
> +static void blk_mq_resubmit_io(struct request *rq)
> +{
> +	struct bio_list list;
> +	struct bio *bio;
> +
> +	bio_list_init(&list);
> +	blk_steal_bios(&list, rq);
> +
> +	while (true) {
> +		bio = bio_list_pop(&list);
> +		if (!bio)
> +			break;
> +
> +		generic_make_request(bio);
> +	}
> +
> +	blk_mq_cleanup_rq(rq);
> +	blk_mq_end_request(rq, 0);
> +}
> +
>  /*
> - * 'cpu' is going away. splice any existing rq_list entries from this
> - * software queue to the hw queue dispatch list, and ensure that it
> - * gets run.
> + * 'cpu' has gone away. If this hctx is dead, we can't dispatch request
> + * to the hctx any more, so steal bios from requests of this hctx, and
> + * re-submit them to the request queue, and free these requests finally.
>   */
>  static int blk_mq_hctx_notify_dead(unsigned int cpu, struct hlist_node *node)
>  {
> @@ -2272,6 +2292,8 @@ static int blk_mq_hctx_notify_dead(unsigned int cpu, struct hlist_node *node)
>  	struct blk_mq_ctx *ctx;
>  	LIST_HEAD(tmp);
>  	enum hctx_type type;
> +	bool hctx_dead;
> +	struct request *rq;
>  
>  	hctx = hlist_entry_safe(node, struct blk_mq_hw_ctx, cpuhp_dead);
>  	ctx = __blk_mq_get_ctx(hctx->queue, cpu);
> @@ -2279,6 +2301,9 @@ static int blk_mq_hctx_notify_dead(unsigned int cpu, struct hlist_node *node)
>  
>  	clear_bit(BLK_MQ_S_INTERNAL_STOPPED, &hctx->state);
>  
> +	hctx_dead = cpumask_first_and(hctx->cpumask, cpu_online_mask) >=
> +		nr_cpu_ids;
> +
>  	spin_lock(&ctx->lock);
>  	if (!list_empty(&ctx->rq_lists[type])) {
>  		list_splice_init(&ctx->rq_lists[type], &tmp);
> @@ -2289,11 +2314,20 @@ static int blk_mq_hctx_notify_dead(unsigned int cpu, struct hlist_node *node)
>  	if (list_empty(&tmp))
>  		return 0;
>  
> -	spin_lock(&hctx->lock);
> -	list_splice_tail_init(&tmp, &hctx->dispatch);
> -	spin_unlock(&hctx->lock);
> +	if (!hctx_dead) {
> +		spin_lock(&hctx->lock);
> +		list_splice_tail_init(&tmp, &hctx->dispatch);
> +		spin_unlock(&hctx->lock);
> +		blk_mq_run_hw_queue(hctx, true);
> +		return 0;
> +	}
> +
> +	while (!list_empty(&tmp)) {
> +		rq = list_entry(tmp.next, struct request, queuelist);
> +		list_del_init(&rq->queuelist);
> +		blk_mq_resubmit_io(rq);
> +	}
>  
> -	blk_mq_run_hw_queue(hctx, true);
>  	return 0;
>  }
>  
> 
So what happens when all CPUs assigned to a hardware queue go offline?
Wouldn't blk_steal_bios() etc resend the I/O to the same hw queue,
causing an infinite loop?

Don't we have to rearrange the hardware queues here?

Cheers,

Hannes
-- 
Dr. Hannes Reinecke		   Teamlead Storage & Networking
hare@suse.de			               +49 911 74053 688
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