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From:   Jason Xing <kerneljasonxing@gmail.com>
Date:   Sun, 26 Mar 2023 18:10:02 +0800
Message-ID: <CAL+tcoCeyqMif1SDUq4MwfV0bBasgQ4LeYuQjPJYDKYLyof=Rw@mail.gmail.com>
Subject: Re: [PATCH net] net: fix raising a softirq on the current cpu with
 rps enabled
To:     Eric Dumazet <edumazet@google.com>
Cc:     davem@davemloft.net, kuba@kernel.org, pabeni@redhat.com,
        netdev@vger.kernel.org, linux-kernel@vger.kernel.org,
        Jason Xing <kernelxing@tencent.com>
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On Sun, Mar 26, 2023 at 12:04=E2=80=AFPM Jason Xing <kerneljasonxing@gmail.=
com> wrote:
>
> On Sat, Mar 25, 2023 at 11:57=E2=80=AFPM Eric Dumazet <edumazet@google.co=
m> wrote:
> >
> > On Sat, Mar 25, 2023 at 8:26=E2=80=AFAM Jason Xing <kerneljasonxing@gma=
il.com> wrote:
> > >
> > > From: Jason Xing <kernelxing@tencent.com>
> > >
> > > Since we decide to put the skb into a backlog queue of another
> > > cpu, we should not raise the softirq for the current cpu. When
> > > to raise a softirq is based on whether we have more data left to
> > > process later. As to the current cpu, there is no indication of
> > > more data enqueued, so we do not need this action. After enqueuing
> > > to another cpu, net_rx_action() function will call ipi and then
> > > another cpu will raise the softirq as expected.
> > >
> > > Also, raising more softirqs which set the corresponding bit field
> > > can make the IRQ mechanism think we probably need to start ksoftirqd
> > > on the current cpu. Actually it shouldn't happen.
> > >
> > > Fixes: 0a9627f2649a ("rps: Receive Packet Steering")
> > > Signed-off-by: Jason Xing <kernelxing@tencent.com>
> > > ---
> > >  net/core/dev.c | 2 --
> > >  1 file changed, 2 deletions(-)
> > >
> > > diff --git a/net/core/dev.c b/net/core/dev.c
> > > index 1518a366783b..bfaaa652f50c 100644
> > > --- a/net/core/dev.c
> > > +++ b/net/core/dev.c
> > > @@ -4594,8 +4594,6 @@ static int napi_schedule_rps(struct softnet_dat=
a *sd)
> > >         if (sd !=3D mysd) {
> > >                 sd->rps_ipi_next =3D mysd->rps_ipi_list;
> > >                 mysd->rps_ipi_list =3D sd;
> > > -
> > > -               __raise_softirq_irqoff(NET_RX_SOFTIRQ);
> > >                 return 1;
> > >         }
> > >  #endif /* CONFIG_RPS */
> > > --
> > > 2.37.3
> > >
> >
> > This is not going to work in some cases. Please take a deeper look.
> >
> > I have to run, if you (or others) do not find the reason, I will give
> > more details when I am done traveling.
>
> I'm wondering whether we could use @mysd instead of @sd like this:
>
> if (!__test_and_set_bit(NAPI_STATE_SCHED, &mysd->backlog.state))
>     __raise_softirq_irqoff(NET_RX_SOFTIRQ);

Ah, I have to add more precise code because the above codes may mislead peo=
ple.

diff --git a/net/core/dev.c b/net/core/dev.c
index 1518a366783b..9ac9b32e392f 100644
--- a/net/core/dev.c
+++ b/net/core/dev.c
@@ -4594,8 +4594,9 @@ static int napi_schedule_rps(struct softnet_data *sd)
        if (sd !=3D mysd) {
                sd->rps_ipi_next =3D mysd->rps_ipi_list;
                mysd->rps_ipi_list =3D sd;
+               if (!__test_and_set_bit(NAPI_STATE_SCHED, &mysd->backlog.st=
ate))
+                       __raise_softirq_irqoff(NET_RX_SOFTIRQ);

-               __raise_softirq_irqoff(NET_RX_SOFTIRQ);
                return 1;
        }
 #endif /* CONFIG_RPS */

Eric, I realized that some paths don't call the ipi to notify another
cpu. If someone grabs the NAPI_STATE_SCHED flag, we know that at the
end of net_rx_action() or the beginning of process_backlog(), the
net_rps_action_and_irq_enable() will handle the information delivery.
However, if no one grabs the flag, in some paths we could not have a
chance immediately to tell another cpu to raise the softirq and then
process those pending data. Thus, I have to make sure if someone owns
the napi poll as shown above.

If I get this wrong, please correct me if you're available. Thanks in advan=
ce.

>
> I traced back to some historical changes and saw some relations with
> this commit ("net: solve a NAPI race"):
> https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit=
/?id=3D39e6c8208d7b6fb9d2047850fb3327db567b564b
>
> Thanks,
> Jason