Re: [PATCH 2/2] blk-mq: simplify queue mapping & schedule with each possisble CPU

["jianchao.wang" <jianchao.w.wang@oracle.com>]

Hi Ming

On 01/12/2018 10:53 AM, Ming Lei wrote:
> From: Christoph Hellwig <hch@lst.de>
> 
> The previous patch assigns interrupt vectors to all possible CPUs, so
> now hctx can be mapped to possible CPUs, this patch applies this fact
> to simplify queue mapping & schedule so that we don't need to handle
> CPU hotplug for dealing with physical CPU plug & unplug. With this
> simplication, we can work well on physical CPU plug & unplug, which
> is a normal use case for VM at least.
> 
> Make sure we allocate blk_mq_ctx structures for all possible CPUs, and
> set hctx->numa_node for possible CPUs which are mapped to this hctx. And
> only choose the online CPUs for schedule.
> 
> Reported-by: Christian Borntraeger <borntraeger@de.ibm.com>
> Tested-by: Christian Borntraeger <borntraeger@de.ibm.com>
> Tested-by: Stefan Haberland <sth@linux.vnet.ibm.com>
> Cc: Thomas Gleixner <tglx@linutronix.de>
> Signed-off-by: Christoph Hellwig <hch@lst.de>
> (merged the three into one because any single one may not work, and fix
> selecting online CPUs for scheduler)
> Signed-off-by: Ming Lei <ming.lei@redhat.com>
> ---
>  block/blk-mq.c | 19 ++++++++-----------
>  1 file changed, 8 insertions(+), 11 deletions(-)
> 
> diff --git a/block/blk-mq.c b/block/blk-mq.c
> index 8000ba6db07d..ef9beca2d117 100644
> --- a/block/blk-mq.c
> +++ b/block/blk-mq.c
> @@ -440,7 +440,7 @@ struct request *blk_mq_alloc_request_hctx(struct request_queue *q,
>  		blk_queue_exit(q);
>  		return ERR_PTR(-EXDEV);
>  	}
> -	cpu = cpumask_first(alloc_data.hctx->cpumask);
> +	cpu = cpumask_first_and(alloc_data.hctx->cpumask, cpu_online_mask);
>  	alloc_data.ctx = __blk_mq_get_ctx(q, cpu);
>  
>  	rq = blk_mq_get_request(q, NULL, op, &alloc_data);
> @@ -1323,9 +1323,10 @@ static int blk_mq_hctx_next_cpu(struct blk_mq_hw_ctx *hctx)
>  	if (--hctx->next_cpu_batch <= 0) {
>  		int next_cpu;
>  
> -		next_cpu = cpumask_next(hctx->next_cpu, hctx->cpumask);
> +		next_cpu = cpumask_next_and(hctx->next_cpu, hctx->cpumask,
> +				cpu_online_mask);
>  		if (next_cpu >= nr_cpu_ids)
> -			next_cpu = cpumask_first(hctx->cpumask);
> +			next_cpu = cpumask_first_and(hctx->cpumask,cpu_online_mask);

the next_cpu here could be >= nr_cpu_ids when the none of on hctx->cpumask is online.
This could be reproduced on NVMe with a patch that could hold some rqs on ctx->rq_list,
meanwhile a script online and offline the cpus. Then a panic occurred in __queue_work().

maybe cpu_possible_mask here, the workers in the pool of the offlined cpu has been unbound.
It should be ok to queue on them.

Thanks
Jianchao 


>  		hctx->next_cpu = next_cpu;
>  		hctx->next_cpu_batch = BLK_MQ_CPU_WORK_BATCH;
> @@ -2219,16 +2220,11 @@ static void blk_mq_init_cpu_queues(struct request_queue *q,
>  		INIT_LIST_HEAD(&__ctx->rq_list);
>  		__ctx->queue = q;
>  
> -		/* If the cpu isn't present, the cpu is mapped to first hctx */
> -		if (!cpu_present(i))
> -			continue;
> -
> -		hctx = blk_mq_map_queue(q, i);
> -
>  		/*
>  		 * Set local node, IFF we have more than one hw queue. If
>  		 * not, we remain on the home node of the device
>  		 */
> +		hctx = blk_mq_map_queue(q, i);
>  		if (nr_hw_queues > 1 && hctx->numa_node == NUMA_NO_NODE)
>  			hctx->numa_node = local_memory_node(cpu_to_node(i));
>  	}
> @@ -2285,7 +2281,7 @@ static void blk_mq_map_swqueue(struct request_queue *q)
>  	 *
>  	 * If the cpu isn't present, the cpu is mapped to first hctx.
>  	 */
> -	for_each_present_cpu(i) {
> +	for_each_possible_cpu(i) {
>  		hctx_idx = q->mq_map[i];
>  		/* unmapped hw queue can be remapped after CPU topo changed */
>  		if (!set->tags[hctx_idx] &&
> @@ -2339,7 +2335,8 @@ static void blk_mq_map_swqueue(struct request_queue *q)
>  		/*
>  		 * Initialize batch roundrobin counts
>  		 */
> -		hctx->next_cpu = cpumask_first(hctx->cpumask);
> +		hctx->next_cpu = cpumask_first_and(hctx->cpumask,
> +				cpu_online_mask);
>  		hctx->next_cpu_batch = BLK_MQ_CPU_WORK_BATCH;
>  	}
>  }
>