On 2023/6/28 10:20, Ming Lei wrote:
> On Tue, Jun 27, 2023 at 08:08:51PM +0800, chengming.zhou@xxxxxxxxx wrote:
>> From: Chengming Zhou <zhouchengming@xxxxxxxxxxxxx>
>>
>> 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@xxxxxxxxxxxxx>
>> ---
>> 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.
