Re: [PATCH v2] blk-mq: don't call ktime_get_ns() if we don't need it

[Omar Sandoval <osandov@osandov.com>]

On Fri, Nov 30, 2018 at 02:13:54PM -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.

Reviewed-by: Omar Sandoval <osandov@fb.com>

> Signed-off-by: Jens Axboe <axboe@kernel.dk>
> 
> ---
> 
> v2: add helper, use it in both spots. also clear ->start_time_ns
>     so merging doesn't read garbage.
> 
> diff --git a/block/blk-mq.c b/block/blk-mq.c
> index 7dcef565dc0f..e09d7f500077 100644
> --- a/block/blk-mq.c
> +++ b/block/blk-mq.c
> @@ -281,6 +281,15 @@ bool blk_mq_can_queue(struct blk_mq_hw_ctx *hctx)
>  }
>  EXPORT_SYMBOL(blk_mq_can_queue);
>  
> +/*
> + * Only need start/end time stamping if we have stats enabled, or using
> + * an IO scheduler.
> + */
> +static inline bool blk_mq_need_time_stamp(struct request *rq)
> +{
> +	return (rq->rq_flags & RQF_IO_STAT) || rq->q->elevator;
> +}
> +
>  static struct request *blk_mq_rq_ctx_init(struct blk_mq_alloc_data *data,
>  		unsigned int tag, unsigned int op)
>  {
> @@ -316,7 +325,10 @@ 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 (blk_mq_need_time_stamp(rq))
> +		rq->start_time_ns = ktime_get_ns();
> +	else
> +		rq->start_time_ns = 0;
>  	rq->io_start_time_ns = 0;
>  	rq->nr_phys_segments = 0;
>  #if defined(CONFIG_BLK_DEV_INTEGRITY)
> @@ -522,7 +534,10 @@ EXPORT_SYMBOL_GPL(blk_mq_free_request);
>  
>  inline void __blk_mq_end_request(struct request *rq, blk_status_t error)
>  {
> -	u64 now = ktime_get_ns();
> +	u64 now = 0;
> +
> +	if (blk_mq_need_time_stamp(rq))
> +		now = ktime_get_ns();
>  
>  	if (rq->rq_flags & RQF_STATS) {
>  		blk_mq_poll_stats_start(rq->q);
> -- 
> Jens Axboe
>