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 13:39:27 +0200
From:   Paolo Abeni <pabeni@...hat.com>
To:     Jason Xing <kerneljasonxing@...il.com>,
        Eric Dumazet <edumazet@...gle.com>
Cc:     "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, 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.

Thanks,

Paolo