Re: [PATCH v1] neighbour: Don't let neigh_forced_gc() disable preemption for long

[Eric Dumazet]

On Tue, Dec 5, 2023 at 12:40 AM Doug Anderson <dianders@xxxxxxxxxxxx> wrote:
>
> Hi,
>
> On Fri, Dec 1, 2023 at 1:10 AM Eric Dumazet <edumazet@xxxxxxxxxx> wrote:
> >
> > On Fri, Dec 1, 2023 at 9:39 AM Judy Hsiao <judyhsiao@xxxxxxxxxxxx> 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.