[RFC PATCH] blk-mq: prefer local cpu on allocating request

[Hillf Danton <hdanton@xxxxxxxx>]

Use local cpu in order to avoid the risk of overloading the first mapped
one if completing requests has to be on the cpu where requests are
dispatched.

Signed-off-by: Hillf Danton <hdanton@xxxxxxxx>
---

--- a/block/blk-mq.c
+++ b/block/blk-mq.c
@@ -426,7 +426,6 @@ struct request *blk_mq_alloc_request_hct
 {
 	struct blk_mq_alloc_data alloc_data = { .flags = flags, .cmd_flags = op };
 	struct request *rq;
-	unsigned int cpu;
 	int ret;
 
 	/*
@@ -454,8 +453,14 @@ struct request *blk_mq_alloc_request_hct
 		blk_queue_exit(q);
 		return ERR_PTR(-EXDEV);
 	}
-	cpu = cpumask_first_and(alloc_data.hctx->cpumask, cpu_online_mask);
-	alloc_data.ctx = __blk_mq_get_ctx(q, cpu);
+
+	/* prefer local cpu if it's mapped to hw queue */
+	if (!cpumask_test_cpu(raw_smp_processor_id(), alloc_data.hctx->cpumask)) {
+		unsigned int cpu;
+		cpu = cpumask_first_and(alloc_data.hctx->cpumask, cpu_online_mask);
+		if (cpu < nr_cpu_ids)
+			alloc_data.ctx = __blk_mq_get_ctx(q, cpu);
+	}
 
 	rq = blk_mq_get_request(q, NULL, &alloc_data);
 	blk_queue_exit(q);