netdev - Re: [Patch net-next] sch_htb: redefine htb qdisc overlimits

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Date:   Sat, 4 May 2019 10:49:05 -0700
From:   Cong Wang <xiyou.wangcong@...il.com>
To:     Eric Dumazet <eric.dumazet@...il.com>
Cc:     Linux Kernel Network Developers <netdev@...r.kernel.org>,
        Eric Dumazet <edumazet@...gle.com>
Subject: Re: [Patch net-next] sch_htb: redefine htb qdisc overlimits

On Sat, May 4, 2019 at 8:41 AM Eric Dumazet <eric.dumazet@...il.com> wrote:
>
>
>
> On 5/2/19 2:06 PM, Cong Wang wrote:
> > In commit 3c75f6ee139d ("net_sched: sch_htb: add per class overlimits counter")
> > we added an overlimits counter for each HTB class which could
> > properly reflect how many times we use up all the bandwidth
> > on each class. However, the overlimits counter in HTB qdisc
> > does not, it is way bigger than the sum of each HTB class.
> > In fact, this qdisc overlimits counter increases when we have
> > no skb to dequeue, which happens more often than we run out of
> > bandwidth.
> >
> > It makes more sense to make this qdisc overlimits counter just
> > be a sum of each HTB class, in case people still get confused.
> >
> > I have verified this patch with one single HTB class, where HTB
> > qdisc counters now always match HTB class counters as expected.
> >
> > Cc: Eric Dumazet <edumazet@...gle.com>
> > Signed-off-by: Cong Wang <xiyou.wangcong@...il.com>
> > ---
> >  net/sched/sch_htb.c | 7 +++++--
> >  1 file changed, 5 insertions(+), 2 deletions(-)
> >
> > diff --git a/net/sched/sch_htb.c b/net/sched/sch_htb.c
> > index d27d9bc9d010..cece0d455985 100644
> > --- a/net/sched/sch_htb.c
> > +++ b/net/sched/sch_htb.c
> > @@ -177,6 +177,7 @@ struct htb_sched {
> >       int                     row_mask[TC_HTB_MAXDEPTH];
> >
> >       struct htb_level        hlevel[TC_HTB_MAXDEPTH];
> > +     u32                     overlimits;
> >  };
>
>
> Hi Cong, your patch makes sense, but I am not sure about the location of this field,
> as this might need another cache line miss.
>
> Maybe instead reduce 'long            direct_pkts;'  to 'u32 direct_pkts;' since we only export
> direct_pkts as 32bit quantity in the struct tc_htb_glob.
>
> Then move the 'u32 overlimits;' right after direct_pkts to fill the new hole.

Sure, v2 is coming. :)

Thanks for catching it.