blk-mq: reduce unnecessary software queue looping

In flush_busy_ctxs() and blk_mq_hctx_has_pending(), regardless of how many
ctxs assigned to one hctx, they will all loop hctx->ctx_map.map_size
times. Here hctx->ctx_map.map_size is a const ALIGN(nr_cpu_ids, 8) / 8.
Especially, flush_busy_ctxs() is in hot code path. And it's unnecessary.
Change ->map_size to contain the actually mapped software queues, so we
only loop for as many iterations as we have to.

And remove cpumask setting and nr_ctx count in blk_mq_init_cpu_queues()
since they are all re-done in blk_mq_map_swqueue().
blk_mq_map_swqueue().

Signed-off-by: Chong Yuan <chong.yuan@memblaze.com>
Reviewed-by: Wenbo Wang <wenbo.wang@memblaze.com>
Updated by me for formatting and commenting.

Signed-off-by: Jens Axboe <axboe@fb.com>

diff --git a/block/blk-mq.c b/block/blk-mq.c
index 1192f85e5ff3ddfdce6b8c2ed0b293f64cf613db..0b49e42e5310087cfd3dc9b91b6c192707b385ba 100644 (file)
--- a/block/blk-mq.c
+++ b/block/blk-mq.c
@@ -1522,8 +1522,6 @@ static int blk_mq_alloc_bitmap(struct blk_mq_ctxmap *bitmap, int node)
        if (!bitmap->map)
                return -ENOMEM;
 
-       bitmap->map_size = num_maps;
-
        total = nr_cpu_ids;
        for (i = 0; i < num_maps; i++) {
                bitmap->map[i].depth = min(total, bitmap->bits_per_word);
@@ -1764,8 +1762,6 @@ static void blk_mq_init_cpu_queues(struct request_queue *q,
                        continue;
 
                hctx = q->mq_ops->map_queue(q, i);
-               cpumask_set_cpu(i, hctx->cpumask);
-               hctx->nr_ctx++;
 
                /*
                 * Set local node, IFF we have more than one hw queue. If
@@ -1802,6 +1798,8 @@ static void blk_mq_map_swqueue(struct request_queue *q)
        }
 
        queue_for_each_hw_ctx(q, hctx, i) {
+               struct blk_mq_ctxmap *map = &hctx->ctx_map;
+
                /*
                 * If no software queues are mapped to this hardware queue,
                 * disable it and free the request entries.
@@ -1817,6 +1815,13 @@ static void blk_mq_map_swqueue(struct request_queue *q)
                        continue;
                }
 
+               /*
+                * Set the map size to the number of mapped software queues.
+                * This is more accurate and more efficient than looping
+                * over all possibly mapped software queues.
+                */
+               map->map_size = hctx->nr_ctx / map->bits_per_word;
+
                /*
                 * Initialize batch roundrobin counts
                 */