On Mon, Jan 15, 2024 at 02:53:55PM -0700, Jens Axboe wrote:
> If the block plug gets flushed, eg on preempt or schedule out, then
> we invalidate the cached clock.
There must be something implicitly happening that I am missing. Where is
the 'cur_time' cached clock invalidated on a plug flush?
> 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;
> }
> #else /* CONFIG_BLOCK */
> struct blk_plug {
