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Message-ID: <CANn89iLuxfSZs+HV6-3=2FJL_KHg3=7WLZ109VXqsXO2-k+KvQ@mail.gmail.com>
Date: Tue, 5 Dec 2023 09:00:33 +0100
From: Eric Dumazet <edumazet@...gle.com>
To: Doug Anderson <dianders@...omium.org>
Cc: Judy Hsiao <judyhsiao@...omium.org>, David Ahern <dsahern@...nel.org>,
Simon Horman <horms@...nel.org>, Brian Haley <haleyb.dev@...il.com>,
"David S. Miller" <davem@...emloft.net>, Jakub Kicinski <kuba@...nel.org>,
Joel Granados <joel.granados@...il.com>, Julian Anastasov <ja@....bg>, Leon Romanovsky <leon@...nel.org>,
Luis Chamberlain <mcgrof@...nel.org>, Paolo Abeni <pabeni@...hat.com>, linux-kernel@...r.kernel.org,
netdev@...r.kernel.org
Subject: Re: [PATCH v1] neighbour: Don't let neigh_forced_gc() disable
preemption for long
On Tue, Dec 5, 2023 at 12:40 AM Doug Anderson <dianders@...omium.org> wrote:
>
> Hi,
>
> On Fri, Dec 1, 2023 at 1:10 AM Eric Dumazet <edumazet@...gle.com> wrote:
> >
> > On Fri, Dec 1, 2023 at 9:39 AM Judy Hsiao <judyhsiao@...omium.org> 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=1000 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 CPU.
> >
> > 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..e2340e6b07735db8cf6e75d23ef09bb4b0db53b4
> > 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 = atomic_read(&tbl->gc_entries) -
> > READ_ONCE(tbl->gc_thresh2);
> > + u64 tmax = ktime_get_ns() + NSEC_PER_MSEC;
> > unsigned long tref = jiffies - 5 * HZ;
> > struct neighbour *n, *tmp;
> > int shrunk = 0;
> > + int loop = 0;
> >
> > NEIGH_CACHE_STAT_INC(tbl, forced_gc_runs);
> >
> > @@ -279,10 +281,16 @@ static int neigh_forced_gc(struct neigh_table *tbl)
> > if (shrunk >= max_clean)
> > break;
> > }
> > + if (++loop == 16) {
> > + if (ktime_get_ns() > tmax)
> > + goto unlock;
> > + loop = 0;
> > + }
> > }
> >
> > WRITE_ONCE(tbl->last_flush, jiffies);
> >
> > +unlock:
> > write_unlock_bh(&tbl->lock);
>
> I'm curious what the plan here is. Your patch looks OK to me and I
> could give it a weak Reviewed-by, but I don't know the code well
> enough to know if we also need to address your second comment that we
> need to "use a second list to make sure to cycle over all elements
> eventually". Is that something you'd expect to get resolved before
> landing?
Please Judy send a V2 of the patch.
I gave feedback, my intention was not to author the patch.
This is standard procedure, I now realize this was Judy first
contribution, sorry for not making this clear.
I will simply add a "Reviewed-by: ..." tag when I agree with the result.
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