diff mbox series

[v3] block: fix blk-iolatency accounting underflow

Message ID 20181217160351.27903-1-dennis@kernel.org (mailing list archive)
State New, archived
Headers show
Series [v3] block: fix blk-iolatency accounting underflow | expand

Commit Message

Dennis Zhou Dec. 17, 2018, 4:03 p.m. UTC 
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 [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@kernel.org/

Fixes: 5cdf2e3fea5e ("blkcg: associate blkg when associating a device")
Signed-off-by: Dennis Zhou <dennis@kernel.org>
Cc: Josef Bacik <josef@toxicpanda.com>
---
 block/blk-iolatency.c     | 2 +-
 block/blk-rq-qos.h        | 5 +++++
 include/linux/blk_types.h | 1 +
 3 files changed, 7 insertions(+), 1 deletion(-)

Comments

Jens Axboe Dec. 17, 2018, 4:09 p.m. UTC | #1 
On 12/17/18 9:03 AM, 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 [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.

I think this is better than (ab)using QUEUE_ENTERED, but this is an area
that needs some love and cleanup in the future.
Liu Bo Dec. 17, 2018, 7:42 p.m. UTC | #2 
On Mon, Dec 17, 2018 at 8:04 AM Dennis Zhou <dennis@kernel.org> 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@kernel.org/
>
> Fixes: 5cdf2e3fea5e ("blkcg: associate blkg when associating a device")
> Signed-off-by: Dennis Zhou <dennis@kernel.org>
> Cc: Josef Bacik <josef@toxicpanda.com>
> ---
>  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
>
Dennis Zhou Dec. 17, 2018, 9:28 p.m. UTC | #3 
On Mon, Dec 17, 2018 at 11:42:28AM -0800, Liu Bo wrote:
> On Mon, Dec 17, 2018 at 8:04 AM Dennis Zhou <dennis@kernel.org> 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
> 

Hi Liubo,

BIO_TRACKED is tagging the bio that is responsible for allocating a new
request, so that rq_qos controllers can decide whether or not they want
to process the bio any part of the way. I should have phrased that a
little better in the commit message. It's not that the bio itself is
bypassing the blk-iolatency controller, but the blk-iolatency controller
deciding to not do anything based on the BIO_TRACKED flag. This doesn't
change any of the function calls made on a bio/request.

Thanks,
Dennis
Liu Bo Dec. 17, 2018, 11:23 p.m. UTC | #4 
On Mon, Dec 17, 2018 at 1:28 PM Dennis Zhou <dennis@kernel.org> wrote:
>
> On Mon, Dec 17, 2018 at 11:42:28AM -0800, Liu Bo wrote:
> > On Mon, Dec 17, 2018 at 8:04 AM Dennis Zhou <dennis@kernel.org> 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
> >
>
> Hi Liubo,
>
> BIO_TRACKED is tagging the bio that is responsible for allocating a new
> request, so that rq_qos controllers can decide whether or not they want
> to process the bio any part of the way. I should have phrased that a
> little better in the commit message. It's not that the bio itself is
> bypassing the blk-iolatency controller, but the blk-iolatency controller
> deciding to not do anything based on the BIO_TRACKED flag. This doesn't
> change any of the function calls made on a bio/request.
>

Thanks for the explanation.
I see it now, so the mentioned series had associated all bios with a
blkg if possible so that done_bio() bumps up inflight counter even in
case a bio has been merged in a previous request.

BIO_TRACKED seems to be too generic to use, but otherwise it makes sense to me,
Reviewed-by: Liu Bo <bo.liu@linux.alibaba.com>

thanks,
liubo

> Thanks,
> Dennis
diff mbox series

Patch

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 */