Re: [PATCH] block: fix iolat timestamp and restore accounting semantics

[Josef Bacik <josef@toxicpanda.com>]

On Mon, Dec 10, 2018 at 11:35:10AM -0500, Dennis Zhou wrote:
> The blk-iolatency controller measures the time from rq_qos_throttle() to
> rq_qos_done_bio() and attributes this time to the first bio that needs
> to create the request. This means if a bio is plug-mergeable or
> bio-mergeable, it gets to bypass the blk-iolatency controller.
> 
> The recent series, to tag all bios w/ blkgs in [1] changed the timing
> incorrectly as well. First, the iolatency controller was tagging bios
> and using that information if it should process it in rq_qos_done_bio().
> However, now that all bios are tagged, this caused the atomic_t for the
> struct rq_wait inflight count to underflow resulting in a stall. Second,
> now the timing was using the duration a bio from generic_make_request()
> rather than the timing mentioned above.
> 
> This patch fixes the errors by accounting time separately in a bio
> adding the field bi_start. If this field is set, the bio should be
> processed by blk-iolatency in rq_qos_done_bio().
> 
> [1] https://lore.kernel.org/lkml/20181205171039.73066-1-dennis@kernel.org/
> 
> Signed-off-by: Dennis Zhou <dennis@kernel.org>
> Cc: Josef Bacik <josef@toxicpanda.com>
> ---
>  block/blk-iolatency.c     | 17 ++++++-----------
>  include/linux/blk_types.h | 12 ++++++++++++
>  2 files changed, 18 insertions(+), 11 deletions(-)
> 
> diff --git a/block/blk-iolatency.c b/block/blk-iolatency.c
> index bee092727cad..52d5d7cc387c 100644
> --- a/block/blk-iolatency.c
> +++ b/block/blk-iolatency.c
> @@ -463,6 +463,8 @@ static void blkcg_iolatency_throttle(struct rq_qos *rqos, struct bio *bio)
>  	if (!blk_iolatency_enabled(blkiolat))
>  		return;
>  
> +	bio->bi_start = ktime_get_ns();
> +
>  	while (blkg && blkg->parent) {
>  		struct iolatency_grp *iolat = blkg_to_lat(blkg);
>  		if (!iolat) {
> @@ -480,18 +482,12 @@ static void blkcg_iolatency_throttle(struct rq_qos *rqos, struct bio *bio)
>  }
>  
>  static void iolatency_record_time(struct iolatency_grp *iolat,
> -				  struct bio_issue *issue, u64 now,
> +				  struct bio *bio, u64 now,
>  				  bool issue_as_root)
>  {
> -	u64 start = bio_issue_time(issue);
> +	u64 start = bio->bi_start;
>  	u64 req_time;
>  
> -	/*
> -	 * Have to do this so we are truncated to the correct time that our
> -	 * issue is truncated to.
> -	 */
> -	now = __bio_issue_time(now);
> -
>  	if (now <= start)
>  		return;
>  
> @@ -593,7 +589,7 @@ static void blkcg_iolatency_done_bio(struct rq_qos *rqos, struct bio *bio)
>  	bool enabled = false;
>  
>  	blkg = bio->bi_blkg;
> -	if (!blkg)
> +	if (!blkg || !bio->bi_start)
>  		return;
>  
>  	iolat = blkg_to_lat(bio->bi_blkg);
> @@ -612,8 +608,7 @@ static void blkcg_iolatency_done_bio(struct rq_qos *rqos, struct bio *bio)
>  		atomic_dec(&rqw->inflight);
>  		if (!enabled || iolat->min_lat_nsec == 0)
>  			goto next;
> -		iolatency_record_time(iolat, &bio->bi_issue, now,
> -				      issue_as_root);
> +		iolatency_record_time(iolat, bio, now, issue_as_root);
>  		window_start = atomic64_read(&iolat->window_start);
>  		if (now > window_start &&
>  		    (now - window_start) >= iolat->cur_win_nsec) {
> diff --git a/include/linux/blk_types.h b/include/linux/blk_types.h
> index 46c005d601ac..c2c02ec08d7c 100644
> --- a/include/linux/blk_types.h
> +++ b/include/linux/blk_types.h
> @@ -181,6 +181,18 @@ struct bio {
>  	 */
>  	struct blkcg_gq		*bi_blkg;
>  	struct bio_issue	bi_issue;
> +#ifdef CONFIG_BLK_CGROUP_IOLATENCY
> +	/*
> +	 * blk-iolatency measure the time a bio takes between rq_qos_throttle()
> +	 * and rq_qos_done_bio().  It attributes the time to the bio that gets
> +	 * the request allowing any bios that can tag along via plug merging or
> +	 * bio merging to be free (from blk-iolatency's perspective). This is
> +	 * different from the time a bio takes from generic_make_request() to
> +	 * the end of its life.  So, this also serves as a marker for which bios
> +	 * should be processed by blk-iolatency.
> +	 */
> +	u64			bi_start;
> +#endif /* CONFIG_BLK_CGROUP_IOLATENCY */

So now we have bi_issue and bi_start, both count basically the same thing.  Does
using bi_issue actually matter?  I assume that it's going to be basically the
same as bi_start for the most part, you are just getting us to only care about
the bio's that we care about.

What if we just add a bio flag to indicate that we've gone through io-latency?
Once that's in place do these problems go away?  Or is the extra time counted
from make_request_time to rq_qos_throttle() actually matter?  I feel like it
shouldn't since it's mostly just checks, but I could be mistaken.  Thanks,

Josef