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 11:28:20 +0800
From:   Chengming Zhou <chengming.zhou@...ux.dev>
To:     Ming Lei <ming.lei@...hat.com>
Cc:     axboe@...nel.dk, tj@...nel.org, hch@....de,
        linux-block@...r.kernel.org, linux-kernel@...r.kernel.org,
        zhouchengming@...edance.com
Subject: Re: [PATCH 1/4] blk-mq: use percpu csd to remote complete instead of
 per-rq csd

On 2023/6/28 10:20, Ming Lei wrote:
> 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));
> 

Good, I will change to use this.

>>  
>>  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.
> 

If this llist_add() see the llist is empty, the consumer function in the softirq
on the remote CPU must have consumed the llist, so smp_call_function_single_async()
won't return -EBUSY ?

Thanks.