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: Christian Borntraeger <borntrae...@de.ibm.com>
Cc: Christoph Hellwig <h...@lst.de>
Cc: Stefan Haberland <s...@linux.vnet.ibm.com>
Signed-off-by: Ming Lei <ming....@redhat.com>
---
 block/blk-mq.c | 17 +++++++++--------
 1 file changed, 9 insertions(+), 8 deletions(-)

diff --git a/block/blk-mq.c b/block/blk-mq.c
index 9b220dc415ac..a16efa6f2e7f 100644
--- a/block/blk-mq.c
+++ b/block/blk-mq.c
@@ -1345,26 +1345,24 @@ 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;
        }
 
@@ -1372,7 +1370,7 @@ static int blk_mq_hctx_next_cpu(struct blk_mq_hw_ctx 
*hctx)
         * 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;
@@ -1382,10 +1380,13 @@ static int blk_mq_hctx_next_cpu(struct blk_mq_hw_ctx 
*hctx)
                 * 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,
-- 
2.9.5

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