blk-mq: balance mapping between present CPUs and queues

Spread queues among present CPUs first, then building 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 unbalanced mapping
between present CPUs and queues.

The similar policy has been applied on Managed IRQ affinity.

Cc: Yi Zhang <yi.zhang@redhat.com>
Reported-by: Yi Zhang <yi.zhang@redhat.com>
Reviewed-by: Bob Liu <bob.liu@oracle.com>
Signed-off-by: Ming Lei <ming.lei@redhat.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>

diff --git a/block/blk-mq-cpumap.c b/block/blk-mq-cpumap.c
index f945621a0e8f2864a0829a84f1e8530236854bca..0157f2b3485a1d14da1aaa0239f0cc020dbf20be 100644 (file)
--- a/block/blk-mq-cpumap.c
+++ b/block/blk-mq-cpumap.c
@@ -15,10 +15,10 @@
 #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,
+                      unsigned int nr_queues, const int q)
 {
-       return qmap->queue_offset + (cpu % nr_queues);
+       return qmap->queue_offset + (q % nr_queues);
 }
 
 static int get_first_sibling(unsigned int cpu)
@@ -36,21 +36,36 @@ 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, nr_queues, 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);
+               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] = cpu_to_queue_index(qmap, nr_queues, cpu);
+                               map[cpu] = queue_index(qmap, nr_queues, q++);
                        else
                                map[cpu] = map[first_sibling];
                }