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

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Date:   Wed, 28 Jun 2023 10:20:00 +0800
From:   Ming Lei <ming.lei@...hat.com>
To:     chengming.zhou@...ux.dev
Cc:     axboe@...nel.dk, tj@...nel.org, hch@....de,
        linux-block@...r.kernel.org, linux-kernel@...r.kernel.org,
        zhouchengming@...edance.com, ming.lei@...hat.com
Subject: Re: [PATCH 1/4] blk-mq: use percpu csd to remote complete instead of
 per-rq csd

On Tue, Jun 27, 2023 at 08:08:51PM +0800, chengming.zhou@...ux.dev wrote:
> 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.
> 
> Signed-off-by: Chengming Zhou <zhouchengming@...edance.com>
> ---
>  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..a36822479b94 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(struct __call_single_data, blk_cpu_csd);

It might be better to use call_single_data, given:

/* Use __aligned() to avoid to use 2 cache lines for 1 csd */
  typedef struct __call_single_data call_single_data_t
  	__aligned(sizeof(struct __call_single_data));

>  
>  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;
> +	struct __call_single_data *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);
>  }

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.

thanks
Ming