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 14:43:58 +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 12:50, Ming Lei wrote:
> On Wed, Jun 28, 2023 at 11:28:20AM +0800, Chengming Zhou wrote:
>> 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 ?
> 
> block softirq can be scheduled from other code path, such as blk_mq_raise_softirq()
> for single queue's remote completion, where no percpu csd schedule is needed, so
> two smp_call_function_single_async() could be called, and the 2nd one
> may return -EBUSY.

Thanks for your very clear explanation! I understand what you mean.

Yes, the 2nd smp_call_function_single_async() will return -EBUSY, but it's ok since
the 1st will do the right thing.

> 
> Not mention csd_unlock() could be called after the callback returns, see
> __flush_smp_call_function_queue().

Ok, CSD_TYPE_SYNC will csd_unlock() after csd_do_func() returns, our CSD_TYPE_ASYNC
will csd_unlock() before csd_do_func().

> 
> But that is fine, if there is pending block softirq, the llist is
> guaranteed to be consumed because the csd callback just raises block
> softirq, and request/llist is consumed in softirq handler.
> 

Agree, it's fine even the 2nd return -EBUSY when the 1st function is raising block softirq,
our llist will be consumed in softirq handler.

Thanks!