Re: [Patch net-next v2 2/4] net_sched: update hierarchical backlog too

[Eric Dumazet <eric.dumazet@gmail.com>]

On Mon, 2015-11-02 at 13:09 -0800, Cong Wang wrote:
> (Sorry for the delay)
> 
> On Fri, Oct 30, 2015 at 12:30 PM, Eric Dumazet <eric.dumazet@gmail.com> wrote:
> > On Fri, 2015-10-30 at 11:22 -0700, Cong Wang wrote:
> >> diff --git a/net/sched/sch_sfq.c b/net/sched/sch_sfq.c
> >> index 3abab53..498f0a2 100644
> >> --- a/net/sched/sch_sfq.c
> >> +++ b/net/sched/sch_sfq.c
> >> @@ -346,7 +346,7 @@ static int
> >>  sfq_enqueue(struct sk_buff *skb, struct Qdisc *sch)
> >>  {
> >>       struct sfq_sched_data *q = qdisc_priv(sch);
> >> -     unsigned int hash;
> >> +     unsigned int hash, dropped;
> >>       sfq_index x, qlen;
> >>       struct sfq_slot *slot;
> >>       int uninitialized_var(ret);
> >> @@ -461,7 +461,7 @@ sfq_enqueue(struct sk_buff *skb, struct Qdisc *sch)
> >>               return NET_XMIT_SUCCESS;
> >>
> >>       qlen = slot->qlen;
> >> -     sfq_drop(sch);
> >> +     dropped = sfq_drop(sch);
> >>       /* Return Congestion Notification only if we dropped a packet
> >>        * from this flow.
> >>        */
> >> @@ -469,7 +469,7 @@ sfq_enqueue(struct sk_buff *skb, struct Qdisc *sch)
> >>               return NET_XMIT_CN;
> >
> >
> > I believe you missed the NET_XMIT_CN cases.
> >
> > SFQ can drop a prior packet, and queue current packet.
> >
> > qdisc_tree_reduce_backlog() wont be called to update parents.
> >
> > Not sure about other qdisc(s)
> 
> Are you saying some qdisc_tree_reduce_backlog() is missing? It could be,
> since I don't audit that, if so, I'd do that in a separated patch, because this
> patch is supposed to fix existing qdisc_tree_reduce_backlog().
> 
> Does this make sense for you?

Well, before your changes, the qdisc_tree_decrease_qlen(sch, 0) would
have been useless, since qdisc_tree_decrease_qlen() does nothing in this
case [1]

But after your changes, we need to change the backlog, even if the qlen
does not change.

Maybe I missed something, but your patch is not really obvious ;)

[1]
void qdisc_tree_decrease_qlen(struct Qdisc *sch, unsigned int n)
{
        ...
        if (n == 0)
                return;