From: Vladimir Oltean <vladimir.oltean@nxp.com>
To: Paolo Abeni <pabeni@redhat.com>
Cc: netdev@vger.kernel.org, "David S. Miller" <davem@davemloft.net>,
Eric Dumazet <edumazet@google.com>,
Jakub Kicinski <kuba@kernel.org>,
Jamal Hadi Salim <jhs@mojatatu.com>,
Cong Wang <xiyou.wangcong@gmail.com>,
Jiri Pirko <jiri@resnulli.us>,
Vinicius Costa Gomes <vinicius.gomes@intel.com>,
linux-kernel@vger.kernel.org, intel-wired-lan@lists.osuosl.org,
Muhammad Husaini Zulkifli <muhammad.husaini.zulkifli@intel.com>,
Peilin Ye <yepeilin.cs@gmail.com>,
Pedro Tammela <pctammela@mojatatu.com>
Subject: Re: [PATCH RESEND net-next 2/5] net/sched: taprio: keep child Qdisc refcount elevated at 2 in offload mode
Date: Tue, 6 Jun 2023 18:56:05 +0300 [thread overview]
Message-ID: <20230606155605.so7xpob6zbuugnwv@skbuf> (raw)
In-Reply-To: <6bce1c55e1cd4295a3f36cb4b37398d951ead07b.camel@redhat.com>
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.
next prev parent reply other threads:[~2023-06-06 15:56 UTC|newest]
Thread overview: 25+ messages / expand[flat|nested] mbox.gz Atom feed top
2023-06-02 10:37 [PATCH RESEND net-next 0/5] Improve the taprio qdisc's relationship with its children Vladimir Oltean
2023-06-02 10:37 ` [PATCH RESEND net-next 1/5] net/sched: taprio: don't access q->qdiscs[] in unoffloaded mode during attach() Vladimir Oltean
2023-06-02 10:37 ` [PATCH RESEND net-next 2/5] net/sched: taprio: keep child Qdisc refcount elevated at 2 in offload mode Vladimir Oltean
2023-06-06 10:27 ` Paolo Abeni
2023-06-06 15:56 ` Vladimir Oltean [this message]
2023-06-07 10:05 ` Paolo Abeni
2023-06-07 10:16 ` Vladimir Oltean
2023-06-02 10:37 ` [PATCH RESEND net-next 3/5] net/sched: taprio: try again to report q->qdiscs[] to qdisc_leaf() Vladimir Oltean
2023-06-02 10:37 ` [PATCH RESEND net-next 4/5] net/sched: taprio: delete misleading comment about preallocating child qdiscs Vladimir Oltean
2023-06-02 10:37 ` [PATCH RESEND net-next 5/5] net/sched: taprio: dump class stats for the actual q->qdiscs[] Vladimir Oltean
2023-06-08 18:44 ` Jamal Hadi Salim
2023-06-09 12:10 ` Vladimir Oltean
2023-06-09 14:56 ` Victor Nogueira
2023-06-10 17:49 ` Vladimir Oltean
2023-06-09 16:19 ` Jamal Hadi Salim
2023-06-09 17:56 ` Vladimir Oltean
2023-06-05 12:50 ` [PATCH RESEND net-next 0/5] Improve the taprio qdisc's relationship with its children Vladimir Oltean
2023-06-05 13:27 ` Simon Horman
2023-06-05 15:44 ` Jamal Hadi Salim
2023-06-05 16:53 ` Vladimir Oltean
2023-06-06 15:39 ` Jamal Hadi Salim
2023-06-06 16:31 ` Vladimir Oltean
2023-06-06 17:42 ` Jamal Hadi Salim
2023-06-07 10:09 ` Vladimir Oltean
2023-06-07 10:30 ` Vladimir Oltean
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