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

[Christoph Hellwig <hch@infradead.org>]

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.

Also update it in attempt_merge, which might have some odd results
for completly uninitialized fields.