netdev - Re: [PATCH net-next 4/4] net: optimize ____napi_schedule() to avoid extra NET_RX

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Date:   Thu, 30 Mar 2023 13:44:16 +0200
From:   Eric Dumazet <edumazet@...gle.com>
To:     Paolo Abeni <pabeni@...hat.com>
Cc:     Jason Xing <kerneljasonxing@...il.com>,
        "David S . Miller" <davem@...emloft.net>,
        Jakub Kicinski <kuba@...nel.org>,
        Jason Xing <kernelxing@...cent.com>, netdev@...r.kernel.org,
        eric.dumazet@...il.com
Subject: Re: [PATCH net-next 4/4] net: optimize ____napi_schedule() to avoid
 extra NET_RX_SOFTIRQ

On Thu, Mar 30, 2023 at 1:39 PM Paolo Abeni <pabeni@...hat.com> wrote:
>
> On Thu, 2023-03-30 at 17:50 +0800, Jason Xing wrote:
> > On Wed, Mar 29, 2023 at 7:53 AM Eric Dumazet <edumazet@...gle.com> wrote:
> > >
> > > ____napi_schedule() adds a napi into current cpu softnet_data poll_list,
> > > then raises NET_RX_SOFTIRQ to make sure net_rx_action() will process it.
> > >
> > > Idea of this patch is to not raise NET_RX_SOFTIRQ when being called indirectly
> > > from net_rx_action(), because we can process poll_list from this point,
> > > without going to full softirq loop.
> > >
> > > This needs a change in net_rx_action() to make sure we restart
> > > its main loop if sd->poll_list was updated without NET_RX_SOFTIRQ
> > > being raised.
> > >
> > > Signed-off-by: Eric Dumazet <edumazet@...gle.com>
> > > Cc: Jason Xing <kernelxing@...cent.com>
> > > ---
> > >  net/core/dev.c | 22 ++++++++++++++++++----
> > >  1 file changed, 18 insertions(+), 4 deletions(-)
> > >
> > > diff --git a/net/core/dev.c b/net/core/dev.c
> > > index f34ce93f2f02e7ec71f5e84d449fa99b7a882f0c..0c4b21291348d4558f036fb05842dab023f65dc3 100644
> > > --- a/net/core/dev.c
> > > +++ b/net/core/dev.c
> > > @@ -4360,7 +4360,11 @@ static inline void ____napi_schedule(struct softnet_data *sd,
> > >         }
> > >
> > >         list_add_tail(&napi->poll_list, &sd->poll_list);
> > > -       __raise_softirq_irqoff(NET_RX_SOFTIRQ);
> > > +       /* If not called from net_rx_action()
> > > +        * we have to raise NET_RX_SOFTIRQ.
> > > +        */
> > > +       if (!sd->in_net_rx_action)
> > > +               __raise_softirq_irqoff(NET_RX_SOFTIRQ);
> > >  }
> > >
> > >  #ifdef CONFIG_RPS
> > > @@ -6648,6 +6652,7 @@ static __latent_entropy void net_rx_action(struct softirq_action *h)
> > >         LIST_HEAD(list);
> > >         LIST_HEAD(repoll);
> > >
> > > +start:
> > >         sd->in_net_rx_action = true;
> > >         local_irq_disable();
> > >         list_splice_init(&sd->poll_list, &list);
> > > @@ -6659,9 +6664,18 @@ static __latent_entropy void net_rx_action(struct softirq_action *h)
> > >                 skb_defer_free_flush(sd);
> > >
> > >                 if (list_empty(&list)) {
> > > -                       sd->in_net_rx_action = false;
> > > -                       if (!sd_has_rps_ipi_waiting(sd) && list_empty(&repoll))
> > > -                               goto end;
> > > +                       if (list_empty(&repoll)) {
> > > +                               sd->in_net_rx_action = false;
> > > +                               barrier();
> > > +                               /* We need to check if ____napi_schedule()
> > > +                                * had refilled poll_list while
> > > +                                * sd->in_net_rx_action was true.
> > > +                                */
> > > +                               if (!list_empty(&sd->poll_list))
> > > +                                       goto start;
> >
> > I noticed that since we decide to go back and restart this loop, it
> > would be better to check the time_limit. More than that,
> > skb_defer_free_flush() can consume some time which is supposed to take
> > into account.
>
> Note that we can have a __napi_schedule() invocation with sd-
> >in_net_rx_action only after executing the napi_poll() call below and
> thus after the related time check (that is - after performing at least
> one full iteration of the main for(;;) loop).
>
> I don't think another check right here is needed.

I was about to say the same thing.

Back to READ_ONCE() and WRITE_ONCE(), I originally had them in my tree,
then simply realized they were not needed and confusing as a matter of fact.

barrier() is the right thing here, and only one is needed, because
others are implicit,
because before hitting reads, we must call out of line functions.