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Date:   Wed, 15 Sep 2021 12:36:42 -0700
From:   Jakub Kicinski <kuba@...nel.org>
To:     Cong Wang <xiyou.wangcong@...il.com>
Cc:     David Miller <davem@...emloft.net>,
        Linux Kernel Network Developers <netdev@...r.kernel.org>,
        Jamal Hadi Salim <jhs@...atatu.com>,
        Jiri Pirko <jiri@...nulli.us>,
        Eric Dumazet <edumazet@...gle.com>,
        Matthew Massey <matthewmassey@...com>,
        Dave Taht <dave.taht@...il.com>
Subject: Re: [PATCH net-next 1/3] net: sched: update default qdisc
 visibility after Tx queue cnt changes

On Wed, 15 Sep 2021 09:31:08 -0700 Cong Wang wrote:
> On Mon, Sep 13, 2021 at 3:53 PM Jakub Kicinski <kuba@...nel.org> wrote:
> > diff --git a/net/core/dev.c b/net/core/dev.c
> > index 74fd402d26dd..f930329f0dc2 100644
> > --- a/net/core/dev.c
> > +++ b/net/core/dev.c
> > @@ -2921,6 +2921,8 @@ int netif_set_real_num_tx_queues(struct net_device *dev, unsigned int txq)
> >                 if (dev->num_tc)
> >                         netif_setup_tc(dev, txq);
> >
> > +               dev_qdisc_change_real_num_tx(dev, txq);
> > +  
> 
> Don't we need to flip the device with dev_deactivate()+dev_activate()?
> It looks like the only thing this function resets is qdisc itself, and only
> partially.

We're only making the qdiscs visible, there should be 
no datapath-visible change.

> >                 dev->real_num_tx_queues = txq;
> >
> >                 if (disabling) {

> > diff --git a/net/sched/sch_mq.c b/net/sched/sch_mq.c
> > index e79f1afe0cfd..db18d8a860f9 100644
> > --- a/net/sched/sch_mq.c
> > +++ b/net/sched/sch_mq.c
> > @@ -125,6 +125,29 @@ static void mq_attach(struct Qdisc *sch)
> >         priv->qdiscs = NULL;
> >  }
> >
> > +static void mq_change_real_num_tx(struct Qdisc *sch, unsigned int new_real_tx)  
> 
> This is nearly identical to mqprio_change_real_num_tx(), can we reuse
> it?

Indeed, I was a little unsure where best to place the helper.
Since mq is always built if mqprio is my instinct would be to
export mq_change_real_num_tx and use it in mqprio. But I didn't 
see any existing exports (mq_attach(), mq_queue_get() are also
identical and are not shared) so I just copy&pasted the logic.

LMK if (a) that's fine; (b) I should share the new code; 
(c) I should post a patch to share all the code that's identical;...

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