On Mon, Dec 17, 2018 at 8:04 AM Dennis Zhou <dennis@xxxxxxxxxx> 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. > Hi, I have a question about merging in plug list, since plug merges are done before rq_qos_throttle(), why would plug-mergeable bios bypass the controller? thanks, liubo > The recent series [1], to tag all bios w/ blkgs undermined how iolatency > was determining which bios it was charging and should process in > rq_qos_done_bio(). Because all bios are being tagged, this caused the > atomic_t for the struct rq_wait inflight count to underflow and result > in a stall. > > This patch adds a new flag BIO_TRACKED to let controllers know that a > bio is going through the rq_qos path. blk-iolatency now checks if this > flag is set to see if it should process the bio in rq_qos_done_bio(). > > Overloading BLK_QUEUE_ENTERED works, but makes the flag rules confusing. > BIO_THROTTLED was another candidate, but the flag is set for all bios > that have gone through blk-throttle code. Overloading a flag comes with > the burden of making sure that when either implementation changes, a > change in setting rules for one doesn't cause a bug in the other. So > here, we unfortunately opt for adding a new flag. > > [1] https://lore.kernel.org/lkml/20181205171039.73066-1-dennis@xxxxxxxxxx/ > > Fixes: 5cdf2e3fea5e ("blkcg: associate blkg when associating a device") > Signed-off-by: Dennis Zhou <dennis@xxxxxxxxxx> > Cc: Josef Bacik <josef@xxxxxxxxxxxxxx> > --- > block/blk-iolatency.c | 2 +- > block/blk-rq-qos.h | 5 +++++ > include/linux/blk_types.h | 1 + > 3 files changed, 7 insertions(+), 1 deletion(-) > > diff --git a/block/blk-iolatency.c b/block/blk-iolatency.c > index bee092727cad..fc714ef402a6 100644 > --- a/block/blk-iolatency.c > +++ b/block/blk-iolatency.c > @@ -593,7 +593,7 @@ 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_flagged(bio, BIO_TRACKED)) > return; > > iolat = blkg_to_lat(bio->bi_blkg); > diff --git a/block/blk-rq-qos.h b/block/blk-rq-qos.h > index fd8a0c5debd3..58f62483b537 100644 > --- a/block/blk-rq-qos.h > +++ b/block/blk-rq-qos.h > @@ -170,6 +170,11 @@ static inline void rq_qos_done_bio(struct request_queue *q, struct bio *bio) > > static inline void rq_qos_throttle(struct request_queue *q, struct bio *bio) > { > + /* > + * BIO_TRACKED lets controllers know that a bio went through the > + * normal rq_qos path. > + */ > + bio_set_flag(bio, BIO_TRACKED); > if (q->rq_qos) > __rq_qos_throttle(q->rq_qos, bio); > } > diff --git a/include/linux/blk_types.h b/include/linux/blk_types.h > index 46c005d601ac..fc99474ac968 100644 > --- a/include/linux/blk_types.h > +++ b/include/linux/blk_types.h > @@ -228,6 +228,7 @@ struct bio { > #define BIO_TRACE_COMPLETION 10 /* bio_endio() should trace the final completion > * of this bio. */ > #define BIO_QUEUE_ENTERED 11 /* can use blk_queue_enter_live() */ > +#define BIO_TRACKED 12 /* set if bio goes through the rq_qos path */ > > /* See BVEC_POOL_OFFSET below before adding new flags */ > > -- > 2.17.1 >
