Re: [PATCH 2/2] block: cache current nsec time in struct blk_plug

[Kanchan Joshi <joshi.k@samsung.com>]

On 1/16/2024 3:23 AM, Jens Axboe wrote:

> diff --git a/block/blk-core.c b/block/blk-core.c
> index 11342af420d0..cc4db4d92c75 100644
> --- a/block/blk-core.c
> +++ b/block/blk-core.c
> @@ -1073,6 +1073,7 @@ void blk_start_plug_nr_ios(struct blk_plug *plug, unsigned short nr_ios)
>   	if (tsk->plug)
>   		return;
>   
> +	plug->cur_ktime = 0;
>   	plug->mq_list = NULL;
>   	plug->cached_rq = NULL;
>   	plug->nr_ios = min_t(unsigned short, nr_ios, BLK_MAX_REQUEST_COUNT);
> diff --git a/include/linux/blkdev.h b/include/linux/blkdev.h
> index 2f9ceea0e23b..23c237b22071 100644
> --- a/include/linux/blkdev.h
> +++ b/include/linux/blkdev.h
> @@ -942,6 +942,7 @@ struct blk_plug {
>   
>   	/* if ios_left is > 1, we can batch tag/rq allocations */
>   	struct request *cached_rq;
> +	u64 cur_ktime;
>   	unsigned short nr_ios;
>   
>   	unsigned short rq_count;
> @@ -977,7 +978,15 @@ long nr_blockdev_pages(void);
>   
>   static inline u64 blk_time_get_ns(void)
>   {
> -	return ktime_get_ns();
> +	struct blk_plug *plug = current->plug;
> +
> +	if (!plug)
> +		return ktime_get_ns();
> +	if (!(plug->cur_ktime & 1ULL)) {
> +		plug->cur_ktime = ktime_get_ns();
> +		plug->cur_ktime |= 1ULL;
> +	}
> +	return plug->cur_ktime;

I did not understand the relevance of 1ULL here.
If ktime_get_ns() returns even value, it will turn that into an odd 
value before caching. And that value will be returned for the subsequent 
calls.
But how is that better compared to just caching whatever ktime_get_ns() 
returned.