Received-SPF: pass (google.com: domain of linux-kernel-owner@vger.kernel.org designates 23.128.96.31 as permitted sender) client-ip=23.128.96.31;
MIME-Version: 1.0
References: <20231201083926.1817394-1-judyhsiao@chromium.org>
 <CANn89iJMbMZdnJRP0CUVfEi20whhShBfO+DAmdaerhiXfiTx5A@mail.gmail.com> <CAD=FV=Vf18TxUWpGTN9b=iECq=5BmEoopQjsMH2U6bDX2=T3cQ@mail.gmail.com>
In-Reply-To: <CAD=FV=Vf18TxUWpGTN9b=iECq=5BmEoopQjsMH2U6bDX2=T3cQ@mail.gmail.com>
From:   Eric Dumazet <edumazet@google.com>
Date:   Fri, 1 Dec 2023 16:58:21 +0100
Message-ID: <CANn89iLzmKOGhMeUUxeM=1b2PP3kieTeYsmpfA0GvJdcQMkgtQ@mail.gmail.com>
Subject: Re: [PATCH v1] neighbour: Don't let neigh_forced_gc() disable
 preemption for long
To:     Doug Anderson <dianders@chromium.org>
Cc:     Judy Hsiao <judyhsiao@chromium.org>,
        David Ahern <dsahern@kernel.org>,
        Simon Horman <horms@kernel.org>,
        Brian Haley <haleyb.dev@gmail.com>,
        "David S. Miller" <davem@davemloft.net>,
        Jakub Kicinski <kuba@kernel.org>,
        Joel Granados <joel.granados@gmail.com>,
        Julian Anastasov <ja@ssi.bg>,
        Leon Romanovsky <leon@kernel.org>,
        Luis Chamberlain <mcgrof@kernel.org>,
        Paolo Abeni <pabeni@redhat.com>, linux-kernel@vger.kernel.org,
        netdev@vger.kernel.org
Content-Type: text/plain; charset="UTF-8"
Content-Transfer-Encoding: quoted-printable
Precedence: bulk

On Fri, Dec 1, 2023 at 4:16=E2=80=AFPM Doug Anderson <dianders@chromium.org=
> wrote:
>
> Hi,
>
> On Fri, Dec 1, 2023 at 1:10=E2=80=AFAM Eric Dumazet <edumazet@google.com>=
 wrote:
> >
> > On Fri, Dec 1, 2023 at 9:39=E2=80=AFAM Judy Hsiao <judyhsiao@chromium.o=
rg> wrote:
> > >
> > > We are seeing cases where neigh_cleanup_and_release() is called by
> > > neigh_forced_gc() many times in a row with preemption turned off.
> > > When running on a low powered CPU at a low CPU frequency, this has
> > > been measured to keep preemption off for ~10 ms. That's not great on =
a
> > > system with HZ=3D1000 which expects tasks to be able to schedule in
> > > with ~1ms latency.
> >
> > This will not work in general, because this code runs with BH blocked.
> >
> > jiffies will stay untouched for many more ms on systems with only one C=
PU.
> >
> > I would rather not rely on jiffies here but ktime_get_ns() [1]
> >
> > Also if we break the loop based on time, we might be unable to purge
> > the last elements in gc_list.
> > We might need to use a second list to make sure to cycle over all
> > elements eventually.
> >
> >
> > [1]
> > diff --git a/net/core/neighbour.c b/net/core/neighbour.c
> > index df81c1f0a57047e176b7c7e4809d2dae59ba6be5..e2340e6b07735db8cf6e75d=
23ef09bb4b0db53b4
> > 100644
> > --- a/net/core/neighbour.c
> > +++ b/net/core/neighbour.c
> > @@ -253,9 +253,11 @@ static int neigh_forced_gc(struct neigh_table *tbl=
)
> >  {
> >         int max_clean =3D atomic_read(&tbl->gc_entries) -
> >                         READ_ONCE(tbl->gc_thresh2);
> > +       u64 tmax =3D ktime_get_ns() + NSEC_PER_MSEC;
>
> It might be nice to make the above timeout based on jiffies. On a
> HZ=3D100 system it's probably OK to keep preemption disabled for 10 ms
> but on a HZ=3D1000 system you'd want 1 ms. ...so maybe you'd want to use
> jiffies_to_nsecs(1)?

I do not think so. 10ms would be awfully long.

We have nsec based time service, why downgrading to jiffies resolution ???

>
> One worry might be that we disabled preemption _right before_ we were
> supposed to be scheduled out. In that case we'll end up blocking some
> other task for another full timeslice, but maybe there's not a lot we
> can do there?

Can you tell us in which scenario this gc_list can be so big, other
than fuzzers ?