netdev - Re: [PATCH net-next 3/4] net: add skb_defer

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Date:   Mon, 16 May 2022 13:43:22 -0700
From:   Eric Dumazet <edumazet@...gle.com>
To:     Jakub Kicinski <kuba@...nel.org>
Cc:     Eric Dumazet <eric.dumazet@...il.com>,
        "David S . Miller" <davem@...emloft.net>,
        Paolo Abeni <pabeni@...hat.com>,
        netdev <netdev@...r.kernel.org>
Subject: Re: [PATCH net-next 3/4] net: add skb_defer_max sysctl

On Mon, May 16, 2022 at 1:39 PM Jakub Kicinski <kuba@...nel.org> wrote:
>
> On Sun, 15 May 2022 21:24:55 -0700 Eric Dumazet wrote:
> > @@ -6494,16 +6495,21 @@ void skb_attempt_defer_free(struct sk_buff *skb)
> >       int cpu = skb->alloc_cpu;
> >       struct softnet_data *sd;
> >       unsigned long flags;
> > +     unsigned int defer_max;
> >       bool kick;
> >
> >       if (WARN_ON_ONCE(cpu >= nr_cpu_ids) ||
> >           !cpu_online(cpu) ||
> >           cpu == raw_smp_processor_id()) {
> > -             __kfree_skb(skb);
> > +nodefer:     __kfree_skb(skb);
> >               return;
> >       }
> >
> >       sd = &per_cpu(softnet_data, cpu);
> > +     defer_max = READ_ONCE(sysctl_skb_defer_max);
> > +     if (READ_ONCE(sd->defer_count) >= defer_max)
> > +             goto nodefer;
> > +
> >       /* We do not send an IPI or any signal.
> >        * Remote cpu will eventually call skb_defer_free_flush()
> >        */
> > @@ -6513,11 +6519,8 @@ void skb_attempt_defer_free(struct sk_buff *skb)
> >       WRITE_ONCE(sd->defer_list, skb);
> >       sd->defer_count++;
> >
> > -     /* kick every time queue length reaches 128.
> > -      * This condition should hardly be hit under normal conditions,
> > -      * unless cpu suddenly stopped to receive NIC interrupts.
> > -      */
> > -     kick = sd->defer_count == 128;
> > +     /* Send an IPI every time queue reaches half capacity. */
> > +     kick = sd->defer_count == (defer_max >> 1);
>
> nit: it will behave a little strangely for defer_max == 1
> we'll let one skb get onto the list and free the subsequent
> skbs directly but we'll never kick the IPI

Yes, I was aware of this, but decided it was not a big deal.

Presumably people will be interested to disable the thing completely,
I am not sure about defer_max == 1

>
> Moving the sd->defer_count++; should fix it and have no significant
> side effects. I think.

SGTM, thanks !