Re: [PATCH 1/4] block: add scalable completion tracking of requests

[Jens Axboe <axboe@fb.com>]

On 11/03/2016 05:17 AM, Ming Lei wrote:
>> diff --git a/block/blk-core.c b/block/blk-core.c
>> index 0bfaa54d3e9f..ca77c725b4e5 100644
>> --- a/block/blk-core.c
>> +++ b/block/blk-core.c
>> @@ -2462,6 +2462,8 @@ void blk_start_request(struct request *req)
>>  {
>>         blk_dequeue_request(req);
>>
>> +       blk_stat_set_issue_time(&req->issue_stat);
>> +
>>         /*
>>          * We are now handing the request to the hardware, initialize
>>          * resid_len to full count and add the timeout handler.
>> @@ -2529,6 +2531,8 @@ bool blk_update_request(struct request *req, int error, unsigned int nr_bytes)
>>
>>         trace_block_rq_complete(req->q, req, nr_bytes);
>>
>> +       blk_stat_add(&req->q->rq_stats[rq_data_dir(req)], req);
>
> blk_update_request() is often called lockless, so it isn't good to
> do it here.

It's not really a concern, not for the legacy path here nor the mq one
where it is per sw context. The collisions are rare enough that it'll
skew the latencies a bit for that short window, but then go away again.
I'd much rather take that, than adding locking for this part.

-- 
Jens Axboe