netdev - Re: [RFC PATCH net-next 5/6] net: process RPS/RFS work in kthread context

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Date:   Mon, 21 Sep 2020 10:11:09 +0200
From:   Eric Dumazet <edumazet@...gle.com>
To:     Wei Wang <weiwan@...gle.com>
Cc:     "David S . Miller" <davem@...emloft.net>,
        Linux Kernel Network Developers <netdev@...r.kernel.org>,
        Jakub Kicinski <kuba@...nel.org>,
        Paolo Abeni <pabeni@...hat.com>,
        Hannes Frederic Sowa <hannes@...essinduktion.org>,
        Felix Fietkau <nbd@....name>
Subject: Re: [RFC PATCH net-next 5/6] net: process RPS/RFS work in kthread context

On Sat, Sep 19, 2020 at 12:45 AM Wei Wang <weiwan@...gle.com> wrote:
>
> On Mon, Sep 14, 2020 at 10:26 AM Wei Wang <weiwan@...gle.com> wrote:
> >
> > From: Paolo Abeni <pabeni@...hat.com>
> >
> > This patch adds the missing part to handle RFS/RPS in the napi thread
> > handler and makes sure RPS/RFS works properly when using kthread to do
> > napi poll.
> >
> > Signed-off-by: Paolo Abeni <pabeni@...hat.com>
> > Signed-off-by: Wei Wang <weiwan@...gle.com>
> > ---
>
> With some more thoughts, I think this patch is not needed. RPS/RFS
> uses its own napi (sd->backlog) which currently does not have
> NAPI_STATE_THREADED set. So it is still being handled in softirq
> context by net_rx_action().
> I will remove this patch in the next version if no one objects.

The purpose of the patch was to make sure to kick the IPI

I think we need it, otherwise RPS/RFS might add a lot of jitter.


>
>
> >  net/core/dev.c | 7 +++++++
> >  1 file changed, 7 insertions(+)
> >
> > diff --git a/net/core/dev.c b/net/core/dev.c
> > index be676c21bdc4..ab8af727058b 100644
> > --- a/net/core/dev.c
> > +++ b/net/core/dev.c
> > @@ -6820,6 +6820,7 @@ static int napi_thread_wait(struct napi_struct *napi)
> >  static int napi_threaded_poll(void *data)
> >  {
> >         struct napi_struct *napi = data;
> > +       struct softnet_data *sd;
> >         void *have;
> >
> >         while (!napi_thread_wait(napi)) {
> > @@ -6835,6 +6836,12 @@ static int napi_threaded_poll(void *data)
> >                         __kfree_skb_flush();
> >                         local_bh_enable();
> >
> > +                       sd = this_cpu_ptr(&softnet_data);
> > +                       if (sd_has_rps_ipi_waiting(sd)) {
> > +                               local_irq_disable();
> > +                               net_rps_action_and_irq_enable(sd);
> > +                       }
> > +
> >                         if (!repoll)
> >                                 break;
> >
> > --
> > 2.28.0.618.gf4bc123cb7-goog
> >