Re:[PATCH] blk-mq: balance mapping between CPUs and queues

[Bob Liu <bob.liu@oracle.com>]

On 7/25/19 4:26 PM, Ming Lei wrote:
> Spread queues among present CPUs first, then building the mapping
> on other non-present CPUs.
> 
> So we can minimize count of dead queues which are mapped by un-present
> CPUs only. Then bad IO performance can be avoided by this unbalanced
> mapping between CPUs and queues.
> 
> The similar policy has been applied on Managed IRQ affinity.
> 
> Reported-by: Yi Zhang <yi.zhang@redhat.com>
> Cc: Yi Zhang <yi.zhang@redhat.com>
> Signed-off-by: Ming Lei <ming.lei@redhat.com>
> ---
>  block/blk-mq-cpumap.c | 34 +++++++++++++++++++++++-----------
>  1 file changed, 23 insertions(+), 11 deletions(-)
> 
> diff --git a/block/blk-mq-cpumap.c b/block/blk-mq-cpumap.c
> index f945621a0e8f..e217f3404dc7 100644
> --- a/block/blk-mq-cpumap.c
> +++ b/block/blk-mq-cpumap.c
> @@ -15,10 +15,9 @@
>  #include "blk.h"
>  #include "blk-mq.h"
>  
> -static int cpu_to_queue_index(struct blk_mq_queue_map *qmap,
> -			      unsigned int nr_queues, const int cpu)
> +static int queue_index(struct blk_mq_queue_map *qmap, const int q)
>  {
> -	return qmap->queue_offset + (cpu % nr_queues);
> +	return qmap->queue_offset + q;
>  }
>  
>  static int get_first_sibling(unsigned int cpu)
> @@ -36,23 +35,36 @@ int blk_mq_map_queues(struct blk_mq_queue_map *qmap)
>  {
>  	unsigned int *map = qmap->mq_map;
>  	unsigned int nr_queues = qmap->nr_queues;
> -	unsigned int cpu, first_sibling;
> +	unsigned int cpu, first_sibling, q = 0;
> +
> +	for_each_possible_cpu(cpu)
> +		map[cpu] = -1;
> +
> +	/*
> +	 * Spread queues among present CPUs first for minimizing
> +	 * count of dead queues which are mapped by all un-present CPUs
> +	 */
> +	for_each_present_cpu(cpu) {
> +		if (q >= nr_queues)
> +			break;
> +		map[cpu] = queue_index(qmap, q++);
> +	}
>  
>  	for_each_possible_cpu(cpu) {
> +		if (map[cpu] != -1)
> +			continue;
>  		/*
>  		 * First do sequential mapping between CPUs and queues.
>  		 * In case we still have CPUs to map, and we have some number of
>  		 * threads per cores then map sibling threads to the same queue
>  		 * for performance optimizations.
>  		 */
> -		if (cpu < nr_queues) {
> -			map[cpu] = cpu_to_queue_index(qmap, nr_queues, cpu);

Why not keep this similarly? 

> +		first_sibling = get_first_sibling(cpu);
> +		if (first_sibling == cpu) {
> +			map[cpu] = queue_index(qmap, q);
> +			q = (q + 1) % nr_queues;
>  		} else {
> -			first_sibling = get_first_sibling(cpu);
> -			if (first_sibling == cpu)
> -				map[cpu] = cpu_to_queue_index(qmap, nr_queues, cpu);
> -			else
> -				map[cpu] = map[first_sibling];
> +			map[cpu] = map[first_sibling];

Then no need to share queue if nr_queues is enough for all possible cpu.

Regards, -Bob