Hi Josef, On Mon, Dec 10, 2018 at 01:25:08PM -0500, Josef Bacik wrote: > 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-den...@kernel.org/ > > > > Signed-off-by: Dennis Zhou <den...@kernel.org> > > Cc: Josef Bacik <jo...@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,
Yeah after talking with Jens about this, it sounds like a good way forward would be to reuse BIO_QUEUE_ENTERED. My initial concern with the flag only approach was the time of stacked drivers. But as they should not be calling into blk_mq_make_request(), tagging in that function should allow us to properly ignore them. It sounds like in the general case where a bio does make it to blk_mq_make_request(), the overhead of the checks prior should be minimal. I'll have a v2 out soon doing tagging with BIO_QUEUE_ENTERED. Thanks, Dennis
