linux-kernel - [PATCH v2 1/4] blk-mq: use percpu csd to remote complete instead of per-rq csd

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Date:   Thu, 29 Jun 2023 19:03:56 +0800
From:   chengming.zhou@...ux.dev
To:     axboe@...nel.dk, ming.lei@...hat.com, hch@....de, tj@...nel.org
Cc:     linux-block@...r.kernel.org, linux-kernel@...r.kernel.org,
        Chengming Zhou <zhouchengming@...edance.com>
Subject: [PATCH v2 1/4] blk-mq: use percpu csd to remote complete instead of per-rq csd

From: Chengming Zhou <zhouchengming@...edance.com>

If request need to be completed remotely, we insert it into percpu llist,
and smp_call_function_single_async() if llist is empty previously.

We don't need to use per-rq csd, percpu csd is enough. And the size of
struct request is decreased by 24 bytes.

This way is cleaner, and looks correct, given block softirq is guaranteed to be
scheduled to consume the list if one new request is added to this percpu list,
either smp_call_function_single_async() returns -EBUSY or 0.

Signed-off-by: Chengming Zhou <zhouchengming@...edance.com>
---
v2:
 - Change to use call_single_data_t, which avoid to use 2 cache lines
   for 1 csd, as suggested by Ming Lei.
 - Improve the commit log, the explanation is copied from Ming Lei.
---
 block/blk-mq.c         | 12 ++++++++----
 include/linux/blk-mq.h |  5 +----
 2 files changed, 9 insertions(+), 8 deletions(-)

diff --git a/block/blk-mq.c b/block/blk-mq.c
index decb6ab2d508..e52200edd2b1 100644
--- a/block/blk-mq.c
+++ b/block/blk-mq.c
@@ -43,6 +43,7 @@
 #include "blk-ioprio.h"
 
 static DEFINE_PER_CPU(struct llist_head, blk_cpu_done);
+static DEFINE_PER_CPU(call_single_data_t, blk_cpu_csd);
 
 static void blk_mq_insert_request(struct request *rq, blk_insert_t flags);
 static void blk_mq_request_bypass_insert(struct request *rq,
@@ -1156,13 +1157,13 @@ static void blk_mq_complete_send_ipi(struct request *rq)
 {
 	struct llist_head *list;
 	unsigned int cpu;
+	call_single_data_t *csd;
 
 	cpu = rq->mq_ctx->cpu;
 	list = &per_cpu(blk_cpu_done, cpu);
-	if (llist_add(&rq->ipi_list, list)) {
-		INIT_CSD(&rq->csd, __blk_mq_complete_request_remote, rq);
-		smp_call_function_single_async(cpu, &rq->csd);
-	}
+	csd = &per_cpu(blk_cpu_csd, cpu);
+	if (llist_add(&rq->ipi_list, list))
+		smp_call_function_single_async(cpu, csd);
 }
 
 static void blk_mq_raise_softirq(struct request *rq)
@@ -4796,6 +4797,9 @@ static int __init blk_mq_init(void)
 
 	for_each_possible_cpu(i)
 		init_llist_head(&per_cpu(blk_cpu_done, i));
+	for_each_possible_cpu(i)
+		INIT_CSD(&per_cpu(blk_cpu_csd, i),
+			 __blk_mq_complete_request_remote, NULL);
 	open_softirq(BLOCK_SOFTIRQ, blk_done_softirq);
 
 	cpuhp_setup_state_nocalls(CPUHP_BLOCK_SOFTIRQ_DEAD,
diff --git a/include/linux/blk-mq.h b/include/linux/blk-mq.h
index f401067ac03a..070551197c0e 100644
--- a/include/linux/blk-mq.h
+++ b/include/linux/blk-mq.h
@@ -182,10 +182,7 @@ struct request {
 		rq_end_io_fn		*saved_end_io;
 	} flush;
 
-	union {
-		struct __call_single_data csd;
-		u64 fifo_time;
-	};
+	u64 fifo_time;
 
 	/*
 	 * completion callback.
-- 
2.39.2