Re: [PATCH] blk-mq: don't call ktime_get_ns() if we don't need it

[Jens Axboe <axboe@kernel.dk>]

On 11/30/18 10:29 AM, Jens Axboe wrote:
> On 11/30/18 10:27 AM, Christoph Hellwig wrote:
>> On Fri, Nov 30, 2018 at 08:56:25AM -0700, Jens Axboe wrote:
>>> We only need the request fields and the end_io time if we have stats
>>> enabled, or if we have a scheduler attached as those may use it for
>>> completion time stats.
>>>
>>> Signed-off-by: Jens Axboe <axboe@kernel.dk>
>>> ---
>>>  block/blk-mq.c | 13 ++++++++++---
>>>  1 file changed, 10 insertions(+), 3 deletions(-)
>>>
>>> diff --git a/block/blk-mq.c b/block/blk-mq.c
>>> index 7dcef565dc0f..2b458e9da946 100644
>>> --- a/block/blk-mq.c
>>> +++ b/block/blk-mq.c
>>> @@ -316,7 +316,8 @@ static struct request *blk_mq_rq_ctx_init(struct blk_mq_alloc_data *data,
>>>  	RB_CLEAR_NODE(&rq->rb_node);
>>>  	rq->rq_disk = NULL;
>>>  	rq->part = NULL;
>>> -	rq->start_time_ns = ktime_get_ns();
>>> +	if (data->q->elevator)
>>> +		rq->start_time_ns = ktime_get_ns();
>>
>> We also seem to use start_time_ns it in blk_account_io_done.
> 
> But only if RQF_STATS is set, in which case we are passing it in correctly.

Oh, needs a stats check for this one, for some reason I was thinking
of the end_io path. Will update.

-- 
Jens Axboe