Re: [PATCH 3/8] blk-mq: avoid to write intermediate result to hctx->next_cpu

[Sagi Grimberg]

Looks good,

Reviewed-by: Sagi Grimberg 

Re: [PATCH 3/8] blk-mq: avoid to write intermediate result to hctx->next_cpu

[Christoph Hellwig]

On Sun, Apr 08, 2018 at 05:48:09PM +0800, Ming Lei wrote:
> 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.

Looks good,

Reviewed-by: Christoph Hellwig 

[PATCH 3/8] blk-mq: avoid to write intermediate result to hctx->next_cpu

[Ming Lei]

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 
Cc: Christoph Hellwig 
Cc: Stefan Haberland 
Signed-off-by: Ming Lei 
---
 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