blk-mq: avoid to write intermediate result to hctx->next_cpu

This patch figures out the final selected CPU, then writes
it to hctx->next_cpu once, then we can avoid to intermediate
next cpu observed from other dispatch paths.

Cc: Stefan Haberland <sth@linux.vnet.ibm.com>
Tested-by: Christian Borntraeger <borntraeger@de.ibm.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Sagi Grimberg <sagi@grimberg.me>
Signed-off-by: Ming Lei <ming.lei@redhat.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>

diff --git a/block/blk-mq.c b/block/blk-mq.c
index f489ec9208071e2859777f64d8ebda1308a5d084..db178c577068c9014c8120ed504d26a554454af3 100644 (file)
--- a/block/blk-mq.c
+++ b/block/blk-mq.c
@@ -1344,26 +1344,24 @@ static void __blk_mq_run_hw_queue(struct blk_mq_hw_ctx *hctx)
 static int blk_mq_hctx_next_cpu(struct blk_mq_hw_ctx *hctx)
 {
        bool tried = false;
+       int next_cpu = hctx->next_cpu;
 
        if (hctx->queue->nr_hw_queues == 1)
                return WORK_CPU_UNBOUND;
 
        if (--hctx->next_cpu_batch <= 0) {
-               int next_cpu;
 select_cpu:
-               next_cpu = cpumask_next_and(hctx->next_cpu, hctx->cpumask,
+               next_cpu = cpumask_next_and(next_cpu, hctx->cpumask,
                                cpu_online_mask);
                if (next_cpu >= nr_cpu_ids)
-                       next_cpu = cpumask_first_and(hctx->cpumask,cpu_online_mask);
+                       next_cpu = cpumask_first_and(hctx->cpumask, cpu_online_mask);
 
                /*
                 * No online CPU is found, so have to make sure hctx->next_cpu
                 * is set correctly for not breaking workqueue.
                 */
                if (next_cpu >= nr_cpu_ids)
-                       hctx->next_cpu = cpumask_first(hctx->cpumask);
-               else
-                       hctx->next_cpu = next_cpu;
+                       next_cpu = cpumask_first(hctx->cpumask);
                hctx->next_cpu_batch = BLK_MQ_CPU_WORK_BATCH;
        }
 
@@ -1371,7 +1369,7 @@ select_cpu:
         * Do unbound schedule if we can't find a online CPU for this hctx,
         * and it should only happen in the path of handling CPU DEAD.
         */
-       if (!cpu_online(hctx->next_cpu)) {
+       if (!cpu_online(next_cpu)) {
                if (!tried) {
                        tried = true;
                        goto select_cpu;
@@ -1381,10 +1379,13 @@ select_cpu:
                 * Make sure to re-select CPU next time once after CPUs
                 * in hctx->cpumask become online again.
                 */
+               hctx->next_cpu = next_cpu;
                hctx->next_cpu_batch = 1;
                return WORK_CPU_UNBOUND;
        }
-       return hctx->next_cpu;
+
+       hctx->next_cpu = next_cpu;
+       return next_cpu;
 }
 
 static void __blk_mq_delay_run_hw_queue(struct blk_mq_hw_ctx *hctx, bool async,