Received-SPF: pass (google.com: domain of linux-kernel-owner@vger.kernel.org designates 2620:137:e000::1:20 as permitted sender) client-ip=2620:137:e000::1:20;
MIME-Version: 1.0
References: <20230325152417.5403-1-kerneljasonxing@gmail.com> <CANn89iJaVrObJNDC9TrnSUC3XQeo-zfmUXLVrNVcsbRDPuSNtA@mail.gmail.com>
In-Reply-To: <CANn89iJaVrObJNDC9TrnSUC3XQeo-zfmUXLVrNVcsbRDPuSNtA@mail.gmail.com>
From:   Jason Xing <kerneljasonxing@gmail.com>
Date:   Sun, 26 Mar 2023 12:04:07 +0800
Message-ID: <CAL+tcoDVCywXXt0Whnx+o0PcULmdms0osJf0qUb0HKvVwuE6oQ@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>
Content-Type: text/plain; charset="UTF-8"
Content-Transfer-Encoding: quoted-printable
Precedence: bulk

On Sat, Mar 25, 2023 at 11:57=E2=80=AFPM Eric Dumazet <edumazet@google.com>=
 wrote:
>
> On Sat, Mar 25, 2023 at 8:26=E2=80=AFAM Jason Xing <kerneljasonxing@gmail=
.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_data =
*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);

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