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

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Date:   Thu, 6 Jul 2023 15:07:35 +0200
From:   Christoph Hellwig <hch@....de>
To:     chengming.zhou@...ux.dev
Cc:     axboe@...nel.dk, ming.lei@...hat.com, hch@....de, tj@...nel.org,
        linux-block@...r.kernel.org, linux-kernel@...r.kernel.org,
        Chengming Zhou <zhouchengming@...edance.com>
Subject: Re: [PATCH v2 1/4] blk-mq: use percpu csd to remote complete
 instead of per-rq csd

On Thu, Jun 29, 2023 at 07:03:56PM +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.
> 
> 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.

Please trim your commit logs to 73 characters per line so that they
are readable in git log output.

>  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);
>  }

No need for the list and csd variables here as they are only used
once.

But I think this code has a rpboem when it is preemptd between
the llist_add and smp_call_function_single_async.  We either need a
get_cpu/put_cpu around them, or instroduce a structure with the list
and csd, and then you can use one pointer from per_cpu and still ensure
the list and csd are for the same CPU.