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Message-ID: <CAD=FV=Vf18TxUWpGTN9b=iECq=5BmEoopQjsMH2U6bDX2=T3cQ@mail.gmail.com> Date: Fri, 1 Dec 2023 07:16:30 -0800 From: Doug Anderson <dianders@...omium.org> To: Eric Dumazet <edumazet@...gle.com> 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 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; It might be nice to make the above timeout based on jiffies. On a HZ=100 system it's probably OK to keep preemption disabled for 10 ms but on a HZ=1000 system you'd want 1 ms. ...so maybe you'd want to use jiffies_to_nsecs(1)? 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? -Doug
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