On 11/14/2014 09:58 AM, Christoph Hellwig wrote:
> Paul, what's the best way to figure out these CPU stalls?
> 
> The second oops is in blk_mq_map_queue() which is a trivial
> two level cpu lookup.  I wonder if there's something odd about
> cpu numbers on these big old sparc systems?
> 
> Something like the debug patch below might shed some light on where the
> index goes wrong, but it'll be horribly verbose.
> 
> 
> diff --git a/block/blk-mq.c b/block/blk-mq.c
> index b5896d4..ef4b35b 100644
> --- a/block/blk-mq.c
> +++ b/block/blk-mq.c
> @@ -1270,7 +1270,12 @@ run_queue:
>   */
>  struct blk_mq_hw_ctx *blk_mq_map_queue(struct request_queue *q, const int cpu)
>  {
> -	return q->queue_hw_ctx[q->mq_map[cpu]];
> +	int idx;
> +
> +	printk("cpu: %d\n", cpu);
> +	idx = q->mq_map[cpu];
> +	printk("queue: %d\n", idx);
> +	return q->queue_hw_ctx[idx];
>  }
>  EXPORT_SYMBOL(blk_mq_map_queue);

It'd probably be better to shove this debug stuff into the map building
code instead, ala attached.

-- 
Jens Axboe