Re: [PATCH] block: build default queue map via irq_create_affinity_masks

[Ming Lei <ming.lei@redhat.com>]

On Wed, Jun 30, 2021 at 11:51:53AM +0800, Ming Lei wrote:
> The default queue mapping builder of blk_mq_map_queues doesn't take NUMA
> topo into account, so the built mapping is pretty bad, since CPUs
> belonging to different NUMA node are assigned to same queue. It is
> observed that IOPS drops by ~30% when running two jobs on same hctx
> of null_blk from two CPUs belonging to two NUMA nodes compared with
> from same NUMA node.
> 
> Address the issue by reusing irq_create_affinity_masks() for building
> the default queue mapping, so that we can re-use the mapping created
> for managed irq.
> 
> Lots of drivers may benefit from the change, such as nvme pci poll,
> nvme tcp, ...
> 
> Signed-off-by: Ming Lei <ming.lei@redhat.com>
> ---
>  block/blk-mq-cpumap.c | 60 +++++++++----------------------------------
>  1 file changed, 12 insertions(+), 48 deletions(-)
> 
> diff --git a/block/blk-mq-cpumap.c b/block/blk-mq-cpumap.c
> index 3db84d3197f1..946e373296a3 100644
> --- a/block/blk-mq-cpumap.c
> +++ b/block/blk-mq-cpumap.c
> @@ -10,67 +10,31 @@
>  #include <linux/mm.h>
>  #include <linux/smp.h>
>  #include <linux/cpu.h>
> +#include <linux/interrupt.h>
>  
>  #include <linux/blk-mq.h>
>  #include "blk.h"
>  #include "blk-mq.h"
>  
> -static int queue_index(struct blk_mq_queue_map *qmap,
> -		       unsigned int nr_queues, const int q)
> -{
> -	return qmap->queue_offset + (q % nr_queues);
> -}
> -
> -static int get_first_sibling(unsigned int cpu)
> -{
> -	unsigned int ret;
> -
> -	ret = cpumask_first(topology_sibling_cpumask(cpu));
> -	if (ret < nr_cpu_ids)
> -		return ret;
> -
> -	return cpu;
> -}
> -
>  int blk_mq_map_queues(struct blk_mq_queue_map *qmap)
>  {
> +	struct irq_affinity_desc *masks = NULL;
> +	struct irq_affinity affd = {0};
>  	unsigned int *map = qmap->mq_map;
>  	unsigned int nr_queues = qmap->nr_queues;
> -	unsigned int cpu, first_sibling, q = 0;
> +	unsigned int q;
>  
> -	for_each_possible_cpu(cpu)
> -		map[cpu] = -1;
> +	masks = irq_create_affinity_masks(nr_queues, &affd);
> +	if (!masks)
> +		return -ENOMEM;
>  
> -	/*
> -	 * 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, nr_queues, q++);
> -	}
> +	for (q = 0; q < nr_queues; q++) {
> +		unsigned int cpu;
>  
> -	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 (q < nr_queues) {
> -			map[cpu] = queue_index(qmap, nr_queues, q++);
> -		} else {
> -			first_sibling = get_first_sibling(cpu);
> -			if (first_sibling == cpu)
> -				map[cpu] = queue_index(qmap, nr_queues, q++);
> -			else
> -				map[cpu] = map[first_sibling];
> -		}
> +		for_each_cpu(cpu, &masks[q].mask)
> +			map[cpu] = q;

oops, the above line should have been:

			map[cpu] = qmap->queue_offset + q;

Thanks,
Ming