[RESEND,net-next,2/5] net/sched: taprio: keep child Qdisc refcount elevated at 2 in offload mode

  The taprio qdisc currently lives in the following equilibrium.

In the software scheduling case, taprio attaches itself to all TXQs,
thus having a refcount of 1 + the number of TX queues. In this mode,
q->qdiscs[] is not visible directly to the Qdisc API. The lifetime of
the Qdiscs from this private array lasts until qdisc_destroy() ->
taprio_destroy().

In the fully offloaded case, the root taprio has a refcount of 1, and
all child q->qdiscs[] also have a refcount of 1. The child q->qdiscs[]
are visible to the Qdisc API (they are attached to the netdev TXQs
directly), however taprio loses a reference to them very early - during
qdisc_graft(parent==NULL) -> taprio_attach(). At that time, taprio frees
the q->qdiscs[] array to not leak memory, but interestingly, it does not
release a reference on these qdiscs because it doesn't effectively own
them - they are created by taprio but owned by the Qdisc core, and will
be freed by qdisc_graft(parent==NULL, new==NULL) -> qdisc_put(old) when
the Qdisc is deleted or when the child Qdisc is replaced with something
else.

My interest is to change this equilibrium such that taprio also owns a
reference on the q->qdiscs[] child Qdiscs for the lifetime of the root
Qdisc, including in full offload mode. I want this because I would like
taprio_leaf(), taprio_dump_class(), taprio_dump_class_stats() to have
insight into q->qdiscs[] for the software scheduling mode - currently
they look at dev_queue->qdisc_sleeping, which is, as mentioned, the same
as the root taprio.

The following set of changes is necessary:
- don't free q->qdiscs[] early in taprio_attach(), free it late in
  taprio_destroy() for consistency with software mode. But:
- currently that's not possible, because taprio doesn't own a reference
  on q->qdiscs[]. So hold that reference - once during the initial
  attach() and once during subsequent graft() calls when the child is
  changed.
- always keep track of the current child in q->qdiscs[], even for full
  offload mode, so that we free in taprio_destroy() what we should, and
  not something stale.

Signed-off-by: Vladimir Oltean <vladimir.oltean@nxp.com>
---
 net/sched/sch_taprio.c | 28 +++++++++++++++++-----------
 1 file changed, 17 insertions(+), 11 deletions(-)
  

On Fri, 2023-06-02 at 13:37 +0300, Vladimir Oltean wrote:
> The taprio qdisc currently lives in the following equilibrium.
> 
> In the software scheduling case, taprio attaches itself to all TXQs,
> thus having a refcount of 1 + the number of TX queues. In this mode,
> q->qdiscs[] is not visible directly to the Qdisc API. The lifetime of
> the Qdiscs from this private array lasts until qdisc_destroy() ->
> taprio_destroy().
> 
> In the fully offloaded case, the root taprio has a refcount of 1, and
> all child q->qdiscs[] also have a refcount of 1. The child q->qdiscs[]
> are visible to the Qdisc API (they are attached to the netdev TXQs
> directly), however taprio loses a reference to them very early - during
> qdisc_graft(parent==NULL) -> taprio_attach(). At that time, taprio frees
> the q->qdiscs[] array to not leak memory, but interestingly, it does not
> release a reference on these qdiscs because it doesn't effectively own
> them - they are created by taprio but owned by the Qdisc core, and will
> be freed by qdisc_graft(parent==NULL, new==NULL) -> qdisc_put(old) when
> the Qdisc is deleted or when the child Qdisc is replaced with something
> else.
> 
> My interest is to change this equilibrium such that taprio also owns a
> reference on the q->qdiscs[] child Qdiscs for the lifetime of the root
> Qdisc, including in full offload mode. I want this because I would like
> taprio_leaf(), taprio_dump_class(), taprio_dump_class_stats() to have
> insight into q->qdiscs[] for the software scheduling mode - currently
> they look at dev_queue->qdisc_sleeping, which is, as mentioned, the same
> as the root taprio.
> 
> The following set of changes is necessary:
> - don't free q->qdiscs[] early in taprio_attach(), free it late in
>   taprio_destroy() for consistency with software mode. But:
> - currently that's not possible, because taprio doesn't own a reference
>   on q->qdiscs[]. So hold that reference - once during the initial
>   attach() and once during subsequent graft() calls when the child is
>   changed.
> - always keep track of the current child in q->qdiscs[], even for full
>   offload mode, so that we free in taprio_destroy() what we should, and
>   not something stale.
> 
> Signed-off-by: Vladimir Oltean <vladimir.oltean@nxp.com>
> ---
>  net/sched/sch_taprio.c | 28 +++++++++++++++++-----------
>  1 file changed, 17 insertions(+), 11 deletions(-)
> 
> diff --git a/net/sched/sch_taprio.c b/net/sched/sch_taprio.c
> index b1c611c72aa4..8807fc915b79 100644
> --- a/net/sched/sch_taprio.c
> +++ b/net/sched/sch_taprio.c
> @@ -2138,23 +2138,20 @@ static void taprio_attach(struct Qdisc *sch)
>  
>  			qdisc->flags |= TCQ_F_ONETXQUEUE | TCQ_F_NOPARENT;
>  			old = dev_graft_qdisc(dev_queue, qdisc);
> +			/* Keep refcount of q->qdiscs[ntx] at 2 */
> +			qdisc_refcount_inc(qdisc);
>  		} else {
>  			/* In software mode, attach the root taprio qdisc
>  			 * to all netdev TX queues, so that dev_qdisc_enqueue()
>  			 * goes through taprio_enqueue().
>  			 */
>  			old = dev_graft_qdisc(dev_queue, sch);
> +			/* Keep root refcount at 1 + num_tx_queues */
>  			qdisc_refcount_inc(sch);
>  		}
>  		if (old)
>  			qdisc_put(old);
>  	}
> -
> -	/* access to the child qdiscs is not needed in offload mode */
> -	if (FULL_OFFLOAD_IS_ENABLED(q->flags)) {
> -		kfree(q->qdiscs);
> -		q->qdiscs = NULL;
> -	}
>  }
>  
>  static struct netdev_queue *taprio_queue_get(struct Qdisc *sch,
> @@ -2183,15 +2180,24 @@ static int taprio_graft(struct Qdisc *sch, unsigned long cl,
>  	if (dev->flags & IFF_UP)
>  		dev_deactivate(dev);
>  
> -	if (FULL_OFFLOAD_IS_ENABLED(q->flags)) {
> +	/* In software mode, the root taprio qdisc is still the one attached to
> +	 * all netdev TX queues, and hence responsible for taprio_enqueue() to
> +	 * forward the skbs to the child qdiscs from the private q->qdiscs[]
> +	 * array. So only attach the new qdisc to the netdev queue in offload
> +	 * mode, where the enqueue must bypass taprio. However, save the
> +	 * reference to the new qdisc in the private array in both cases, to
> +	 * have an up-to-date reference to our children.
> +	 */
> +	if (FULL_OFFLOAD_IS_ENABLED(q->flags))
>  		*old = dev_graft_qdisc(dev_queue, new);
> -	} else {
> +	else
>  		*old = q->qdiscs[cl - 1];
> -		q->qdiscs[cl - 1] = new;
> -	}
>  
> -	if (new)
> +	q->qdiscs[cl - 1] = new;
> +	if (new) {
> +		qdisc_refcount_inc(new);
>  		new->flags |= TCQ_F_ONETXQUEUE | TCQ_F_NOPARENT;
> +	}
>  
Isn't the above leaking a reference to old with something alike:

tc qdisc replace dev eth0 handle 8001: parent root stab overhead 24 taprio flags 0x2 #...
	# each q in q->qdiscs has refcnt == 2
tc qdisc replace dev eth0 parent 8001:8 cbs #...
	# -> taprio_graft(..., cbs, ...)
	# cbs refcnt is 2
	# 'old' refcnt decreases by 1, refcount will not reach 0?!?

kmemleak should be able to see that.

BTW, what about including your tests from the cover letter somewhere under tc-testing?

Thanks!

Paolo
  

On Tue, Jun 06, 2023 at 12:27:09PM +0200, Paolo Abeni wrote:
> On Fri, 2023-06-02 at 13:37 +0300, Vladimir Oltean wrote:
> > The taprio qdisc currently lives in the following equilibrium.
> > 
> > In the software scheduling case, taprio attaches itself to all TXQs,
> > thus having a refcount of 1 + the number of TX queues. In this mode,
> > q->qdiscs[] is not visible directly to the Qdisc API. The lifetime of
> > the Qdiscs from this private array lasts until qdisc_destroy() ->
> > taprio_destroy().
> > 
> > In the fully offloaded case, the root taprio has a refcount of 1, and
> > all child q->qdiscs[] also have a refcount of 1. The child q->qdiscs[]
> > are visible to the Qdisc API (they are attached to the netdev TXQs
> > directly), however taprio loses a reference to them very early - during
> > qdisc_graft(parent==NULL) -> taprio_attach(). At that time, taprio frees
> > the q->qdiscs[] array to not leak memory, but interestingly, it does not
> > release a reference on these qdiscs because it doesn't effectively own
> > them - they are created by taprio but owned by the Qdisc core, and will
> > be freed by qdisc_graft(parent==NULL, new==NULL) -> qdisc_put(old) when
> > the Qdisc is deleted or when the child Qdisc is replaced with something
> > else.
> > 
> > My interest is to change this equilibrium such that taprio also owns a
> > reference on the q->qdiscs[] child Qdiscs for the lifetime of the root
> > Qdisc, including in full offload mode. I want this because I would like
> > taprio_leaf(), taprio_dump_class(), taprio_dump_class_stats() to have
> > insight into q->qdiscs[] for the software scheduling mode - currently
> > they look at dev_queue->qdisc_sleeping, which is, as mentioned, the same
> > as the root taprio.
> > 
> > The following set of changes is necessary:
> > - don't free q->qdiscs[] early in taprio_attach(), free it late in
> >   taprio_destroy() for consistency with software mode. But:
> > - currently that's not possible, because taprio doesn't own a reference
> >   on q->qdiscs[]. So hold that reference - once during the initial
> >   attach() and once during subsequent graft() calls when the child is
> >   changed.
> > - always keep track of the current child in q->qdiscs[], even for full
> >   offload mode, so that we free in taprio_destroy() what we should, and
> >   not something stale.
> > 
> > Signed-off-by: Vladimir Oltean <vladimir.oltean@nxp.com>
> > ---
> >  net/sched/sch_taprio.c | 28 +++++++++++++++++-----------
> >  1 file changed, 17 insertions(+), 11 deletions(-)
> > 
> > diff --git a/net/sched/sch_taprio.c b/net/sched/sch_taprio.c
> > index b1c611c72aa4..8807fc915b79 100644
> > --- a/net/sched/sch_taprio.c
> > +++ b/net/sched/sch_taprio.c
> > @@ -2138,23 +2138,20 @@ static void taprio_attach(struct Qdisc *sch)
> >  
> >  			qdisc->flags |= TCQ_F_ONETXQUEUE | TCQ_F_NOPARENT;
> >  			old = dev_graft_qdisc(dev_queue, qdisc);
> > +			/* Keep refcount of q->qdiscs[ntx] at 2 */
> > +			qdisc_refcount_inc(qdisc);
> >  		} else {
> >  			/* In software mode, attach the root taprio qdisc
> >  			 * to all netdev TX queues, so that dev_qdisc_enqueue()
> >  			 * goes through taprio_enqueue().
> >  			 */
> >  			old = dev_graft_qdisc(dev_queue, sch);
> > +			/* Keep root refcount at 1 + num_tx_queues */
> >  			qdisc_refcount_inc(sch);
> >  		}
> >  		if (old)
> >  			qdisc_put(old);
> >  	}
> > -
> > -	/* access to the child qdiscs is not needed in offload mode */
> > -	if (FULL_OFFLOAD_IS_ENABLED(q->flags)) {
> > -		kfree(q->qdiscs);
> > -		q->qdiscs = NULL;
> > -	}
> >  }
> >  
> >  static struct netdev_queue *taprio_queue_get(struct Qdisc *sch,
> > @@ -2183,15 +2180,24 @@ static int taprio_graft(struct Qdisc *sch, unsigned long cl,
> >  	if (dev->flags & IFF_UP)
> >  		dev_deactivate(dev);
> >  
> > -	if (FULL_OFFLOAD_IS_ENABLED(q->flags)) {
> > +	/* In software mode, the root taprio qdisc is still the one attached to
> > +	 * all netdev TX queues, and hence responsible for taprio_enqueue() to
> > +	 * forward the skbs to the child qdiscs from the private q->qdiscs[]
> > +	 * array. So only attach the new qdisc to the netdev queue in offload
> > +	 * mode, where the enqueue must bypass taprio. However, save the
> > +	 * reference to the new qdisc in the private array in both cases, to
> > +	 * have an up-to-date reference to our children.
> > +	 */
> > +	if (FULL_OFFLOAD_IS_ENABLED(q->flags))
> >  		*old = dev_graft_qdisc(dev_queue, new);
> > -	} else {
> > +	else
> >  		*old = q->qdiscs[cl - 1];
> > -		q->qdiscs[cl - 1] = new;
> > -	}
> >  
> > -	if (new)
> > +	q->qdiscs[cl - 1] = new;
> > +	if (new) {
> > +		qdisc_refcount_inc(new);
> >  		new->flags |= TCQ_F_ONETXQUEUE | TCQ_F_NOPARENT;
> > +	}
> >  
> Isn't the above leaking a reference to old with something alike:
> 
> tc qdisc replace dev eth0 handle 8001: parent root stab overhead 24 taprio flags 0x2 #...
> 	# each q in q->qdiscs has refcnt == 2
> tc qdisc replace dev eth0 parent 8001:8 cbs #...
> 	# -> taprio_graft(..., cbs, ...)
> 	# cbs refcnt is 2
> 	# 'old' refcnt decreases by 1, refcount will not reach 0?!?
> 
> kmemleak should be able to see that.

You're right - in full offload mode, the refcount of "old" (pfifo, parent 8001:8)
does not drop to 0 after grafting something else to 8001:8.

I believe this other implementation should work in all cases.

static int taprio_graft(struct Qdisc *sch, unsigned long cl,
			struct Qdisc *new, struct Qdisc **old,
			struct netlink_ext_ack *extack)
{
	struct taprio_sched *q = qdisc_priv(sch);
	struct net_device *dev = qdisc_dev(sch);
	struct netdev_queue *dev_queue = taprio_queue_get(sch, cl);

	if (!dev_queue)
		return -EINVAL;

	if (dev->flags & IFF_UP)
		dev_deactivate(dev);

	/* In offload mode, the child Qdisc is directly attached to the netdev
	 * TX queue, and thus, we need to keep its refcount elevated in order
	 * to counteract qdisc_graft()'s call to qdisc_put() once per TX queue.
	 * However, save the reference to the new qdisc in the private array in
	 * both software and offload cases, to have an up-to-date reference to
	 * our children.
	 */
	if (FULL_OFFLOAD_IS_ENABLED(q->flags)) {
		*old = dev_graft_qdisc(dev_queue, new);
		if (new)
			qdisc_refcount_inc(new);
		if (*old)
			qdisc_put(*old);
	} else {
		*old = q->qdiscs[cl - 1];
	}

	q->qdiscs[cl - 1] = new;
	if (new)
		new->flags |= TCQ_F_ONETXQUEUE | TCQ_F_NOPARENT;

	if (dev->flags & IFF_UP)
		dev_activate(dev);

	return 0;
}

> BTW, what about including your tests from the cover letter somewhere under tc-testing?

I don't know about that. Does it involve adding taprio hw offload to netdevsim,
so that both code paths are covered?

To be honest with you, I never ran tdc, and I worry about what I may find there,
unrelated to what I'm being asked to add (at a cursory glance I see overlapping
TXQs in taprio.json), and which has the possibility of turning this into a huge
time pit.
  

On Tue, 2023-06-06 at 18:56 +0300, Vladimir Oltean wrote:
> On Tue, Jun 06, 2023 at 12:27:09PM +0200, Paolo Abeni wrote:
> > On Fri, 2023-06-02 at 13:37 +0300, Vladimir Oltean wrote:
> > > The taprio qdisc currently lives in the following equilibrium.
> > > 
> > > In the software scheduling case, taprio attaches itself to all TXQs,
> > > thus having a refcount of 1 + the number of TX queues. In this mode,
> > > q->qdiscs[] is not visible directly to the Qdisc API. The lifetime of
> > > the Qdiscs from this private array lasts until qdisc_destroy() ->
> > > taprio_destroy().
> > > 
> > > In the fully offloaded case, the root taprio has a refcount of 1, and
> > > all child q->qdiscs[] also have a refcount of 1. The child q->qdiscs[]
> > > are visible to the Qdisc API (they are attached to the netdev TXQs
> > > directly), however taprio loses a reference to them very early - during
> > > qdisc_graft(parent==NULL) -> taprio_attach(). At that time, taprio frees
> > > the q->qdiscs[] array to not leak memory, but interestingly, it does not
> > > release a reference on these qdiscs because it doesn't effectively own
> > > them - they are created by taprio but owned by the Qdisc core, and will
> > > be freed by qdisc_graft(parent==NULL, new==NULL) -> qdisc_put(old) when
> > > the Qdisc is deleted or when the child Qdisc is replaced with something
> > > else.
> > > 
> > > My interest is to change this equilibrium such that taprio also owns a
> > > reference on the q->qdiscs[] child Qdiscs for the lifetime of the root
> > > Qdisc, including in full offload mode. I want this because I would like
> > > taprio_leaf(), taprio_dump_class(), taprio_dump_class_stats() to have
> > > insight into q->qdiscs[] for the software scheduling mode - currently
> > > they look at dev_queue->qdisc_sleeping, which is, as mentioned, the same
> > > as the root taprio.
> > > 
> > > The following set of changes is necessary:
> > > - don't free q->qdiscs[] early in taprio_attach(), free it late in
> > >   taprio_destroy() for consistency with software mode. But:
> > > - currently that's not possible, because taprio doesn't own a reference
> > >   on q->qdiscs[]. So hold that reference - once during the initial
> > >   attach() and once during subsequent graft() calls when the child is
> > >   changed.
> > > - always keep track of the current child in q->qdiscs[], even for full
> > >   offload mode, so that we free in taprio_destroy() what we should, and
> > >   not something stale.
> > > 
> > > Signed-off-by: Vladimir Oltean <vladimir.oltean@nxp.com>
> > > ---
> > >  net/sched/sch_taprio.c | 28 +++++++++++++++++-----------
> > >  1 file changed, 17 insertions(+), 11 deletions(-)
> > > 
> > > diff --git a/net/sched/sch_taprio.c b/net/sched/sch_taprio.c
> > > index b1c611c72aa4..8807fc915b79 100644
> > > --- a/net/sched/sch_taprio.c
> > > +++ b/net/sched/sch_taprio.c
> > > @@ -2138,23 +2138,20 @@ static void taprio_attach(struct Qdisc *sch)
> > >  
> > >  			qdisc->flags |= TCQ_F_ONETXQUEUE | TCQ_F_NOPARENT;
> > >  			old = dev_graft_qdisc(dev_queue, qdisc);
> > > +			/* Keep refcount of q->qdiscs[ntx] at 2 */
> > > +			qdisc_refcount_inc(qdisc);
> > >  		} else {
> > >  			/* In software mode, attach the root taprio qdisc
> > >  			 * to all netdev TX queues, so that dev_qdisc_enqueue()
> > >  			 * goes through taprio_enqueue().
> > >  			 */
> > >  			old = dev_graft_qdisc(dev_queue, sch);
> > > +			/* Keep root refcount at 1 + num_tx_queues */
> > >  			qdisc_refcount_inc(sch);
> > >  		}
> > >  		if (old)
> > >  			qdisc_put(old);
> > >  	}
> > > -
> > > -	/* access to the child qdiscs is not needed in offload mode */
> > > -	if (FULL_OFFLOAD_IS_ENABLED(q->flags)) {
> > > -		kfree(q->qdiscs);
> > > -		q->qdiscs = NULL;
> > > -	}
> > >  }
> > >  
> > >  static struct netdev_queue *taprio_queue_get(struct Qdisc *sch,
> > > @@ -2183,15 +2180,24 @@ static int taprio_graft(struct Qdisc *sch, unsigned long cl,
> > >  	if (dev->flags & IFF_UP)
> > >  		dev_deactivate(dev);
> > >  
> > > -	if (FULL_OFFLOAD_IS_ENABLED(q->flags)) {
> > > +	/* In software mode, the root taprio qdisc is still the one attached to
> > > +	 * all netdev TX queues, and hence responsible for taprio_enqueue() to
> > > +	 * forward the skbs to the child qdiscs from the private q->qdiscs[]
> > > +	 * array. So only attach the new qdisc to the netdev queue in offload
> > > +	 * mode, where the enqueue must bypass taprio. However, save the
> > > +	 * reference to the new qdisc in the private array in both cases, to
> > > +	 * have an up-to-date reference to our children.
> > > +	 */
> > > +	if (FULL_OFFLOAD_IS_ENABLED(q->flags))
> > >  		*old = dev_graft_qdisc(dev_queue, new);
> > > -	} else {
> > > +	else
> > >  		*old = q->qdiscs[cl - 1];
> > > -		q->qdiscs[cl - 1] = new;
> > > -	}
> > >  
> > > -	if (new)
> > > +	q->qdiscs[cl - 1] = new;
> > > +	if (new) {
> > > +		qdisc_refcount_inc(new);
> > >  		new->flags |= TCQ_F_ONETXQUEUE | TCQ_F_NOPARENT;
> > > +	}
> > >  
> > Isn't the above leaking a reference to old with something alike:
> > 
> > tc qdisc replace dev eth0 handle 8001: parent root stab overhead 24 taprio flags 0x2 #...
> > 	# each q in q->qdiscs has refcnt == 2
> > tc qdisc replace dev eth0 parent 8001:8 cbs #...
> > 	# -> taprio_graft(..., cbs, ...)
> > 	# cbs refcnt is 2
> > 	# 'old' refcnt decreases by 1, refcount will not reach 0?!?
> > 
> > kmemleak should be able to see that.
> 
> You're right - in full offload mode, the refcount of "old" (pfifo, parent 8001:8)
> does not drop to 0 after grafting something else to 8001:8.
> 
> I believe this other implementation should work in all cases.
> 
> static int taprio_graft(struct Qdisc *sch, unsigned long cl,
> 			struct Qdisc *new, struct Qdisc **old,
> 			struct netlink_ext_ack *extack)
> {
> 	struct taprio_sched *q = qdisc_priv(sch);
> 	struct net_device *dev = qdisc_dev(sch);
> 	struct netdev_queue *dev_queue = taprio_queue_get(sch, cl);
> 
> 	if (!dev_queue)
> 		return -EINVAL;
> 
> 	if (dev->flags & IFF_UP)
> 		dev_deactivate(dev);
> 
> 	/* In offload mode, the child Qdisc is directly attached to the netdev
> 	 * TX queue, and thus, we need to keep its refcount elevated in order
> 	 * to counteract qdisc_graft()'s call to qdisc_put() once per TX queue.
> 	 * However, save the reference to the new qdisc in the private array in
> 	 * both software and offload cases, to have an up-to-date reference to
> 	 * our children.
> 	 */
> 	if (FULL_OFFLOAD_IS_ENABLED(q->flags)) {
> 		*old = dev_graft_qdisc(dev_queue, new);
> 		if (new)
> 			qdisc_refcount_inc(new);
> 		if (*old)
> 			qdisc_put(*old);
> 	} else {
> 		*old = q->qdiscs[cl - 1];
> 	}

Perhaps the above chunk could be:

	*old = q->qdiscs[cl - 1];
	if (FULL_OFFLOAD_IS_ENABLED(q->flags)) {
		WARN_ON_ONCE(dev_graft_qdisc(dev_queue, new) != *old);
		if (new)
			qdisc_refcount_inc(new);
		if (*old)
			qdisc_put(*old);
	}

(boldly assuming I'm not completely lost, which looks a wild bet ;)

> 	q->qdiscs[cl - 1] = new;
> 	if (new)
> 		new->flags |= TCQ_F_ONETXQUEUE | TCQ_F_NOPARENT;
> 
> 	if (dev->flags & IFF_UP)
> 		dev_activate(dev);
> 
> 	return 0;
> }
> 
> > BTW, what about including your tests from the cover letter somewhere under tc-testing?
> 
> I don't know about that. Does it involve adding taprio hw offload to netdevsim,
> so that both code paths are covered?

I guess I underlooked the needed effort and we could live without new
tests here.


Cheers,

Paolo
  

On Wed, Jun 07, 2023 at 12:05:19PM +0200, Paolo Abeni wrote:
> Perhaps the above chunk could be:
> 
> 	*old = q->qdiscs[cl - 1];
> 	if (FULL_OFFLOAD_IS_ENABLED(q->flags)) {
> 		WARN_ON_ONCE(dev_graft_qdisc(dev_queue, new) != *old);
> 		if (new)
> 			qdisc_refcount_inc(new);
> 		if (*old)
> 			qdisc_put(*old);
> 	}
> 
> (boldly assuming I'm not completely lost, which looks a wild bet ;)

Yeah, could be like that. In full offload mode, q->qdiscs[cl - 1] is
also what's grafted to the TXQ, so the WARN_ON() would be indicative of
a serious bug if it triggered.

> > > BTW, what about including your tests from the cover letter somewhere under tc-testing?
> > 
> > I don't know about that. Does it involve adding taprio hw offload to netdevsim,
> > so that both code paths are covered?
> 
> I guess I underlooked the needed effort and we could live without new
> tests here.

Let's see how the discussion progresses.