From: "Paul E. McKenney" <paulmck@kernel.org>
To: Frederic Weisbecker <frederic@kernel.org>
Cc: LKML <linux-kernel@vger.kernel.org>, rcu <rcu@vger.kernel.org>,
Uladzislau Rezki <urezki@gmail.com>,
Neeraj Upadhyay <quic_neeraju@quicinc.com>,
Boqun Feng <boqun.feng@gmail.com>,
Joel Fernandes <joel@joelfernandes.org>
Subject: Re: [PATCH 1/4] rcu/nocb: Protect lazy shrinker against concurrent (de-)offloading
Date: Sun, 26 Mar 2023 14:45:18 -0700 [thread overview]
Message-ID: <dd853e13-ffcd-4579-adad-80b014e906ef@paulmck-laptop> (raw)
In-Reply-To: <ZCCknqnceazfyzvr@localhost.localdomain>
On Sun, Mar 26, 2023 at 10:01:34PM +0200, Frederic Weisbecker wrote:
> Le Fri, Mar 24, 2023 at 03:51:54PM -0700, Paul E. McKenney a écrit :
> > On Fri, Mar 24, 2023 at 11:09:08PM +0100, Frederic Weisbecker wrote:
> > > Le Wed, Mar 22, 2023 at 04:18:24PM -0700, Paul E. McKenney a écrit :
> > > > > @@ -1336,13 +1336,25 @@ lazy_rcu_shrink_scan(struct shrinker *shrink, struct shrink_control *sc)
> > > > > unsigned long flags;
> > > > > unsigned long count = 0;
> > > > >
> > > > > + /*
> > > > > + * Protect against concurrent (de-)offloading. Otherwise nocb locking
> > > > > + * may be ignored or imbalanced.
> > > > > + */
> > > > > + mutex_lock(&rcu_state.barrier_mutex);
> > > >
> > > > I was worried about this possibly leading to out-of-memory deadlock,
> > > > but if I recall correctly, the (de-)offloading process never allocates
> > > > memory, so this should be OK?
> > >
> > > Good point. It _should_ be fine but like you, Joel and Hillf pointed out
> > > it's asking for trouble.
> > >
> > > We could try Joel's idea to use mutex_trylock() as a best effort, which
> > > should be fine as it's mostly uncontended.
> > >
> > > The alternative is to force nocb locking and check the offloading state
> > > right after. So instead of:
> > >
> > > rcu_nocb_lock_irqsave(rdp, flags);
> > > //flush stuff
> > > rcu_nocb_unlock_irqrestore(rdp, flags);
> > >
> > > Have:
> > >
> > > raw_spin_lock_irqsave(rdp->nocb_lock, flags);
> > > if (!rcu_rdp_is_offloaded(rdp))
> > > raw_spin_unlock_irqrestore(rdp->nocb_lock, flags);
> > > continue;
> > > }
> > > //flush stuff
> > > rcu_nocb_unlock_irqrestore(rdp, flags);
> > >
> > > But it's not pretty and also disqualifies the last two patches as
> > > rcu_nocb_mask can't be iterated safely anymore.
> > >
> > > What do you think?
> >
> > The mutex_trylock() approach does have the advantage of simplicity,
> > and as you say should do well given low contention.
> >
> > Which reminds me, what sort of test strategy did you have in mind?
> > Memory exhaustion can have surprising effects.
>
> The best I can do is to trigger the count and scan callbacks through
> the shrinker debugfs and see if it crashes or not :-)
Sounds like a good start. Maybe also a good finish? ;-)
> > > > > /* Snapshot count of all CPUs */
> > > > > for_each_possible_cpu(cpu) {
> > > > > struct rcu_data *rdp = per_cpu_ptr(&rcu_data, cpu);
> > > > > - int _count = READ_ONCE(rdp->lazy_len);
> > > > > + int _count;
> > > > > +
> > > > > + if (!rcu_rdp_is_offloaded(rdp))
> > > > > + continue;
> > > >
> > > > If the CPU is offloaded, isn't ->lazy_len guaranteed to be zero?
> > > >
> > > > Or can it contain garbage after a de-offloading operation?
> > >
> > > If it's deoffloaded, ->lazy_len is indeed (supposed to be) guaranteed to be zero.
> > > Bypass is flushed and disabled atomically early on de-offloading and the
> > > flush resets ->lazy_len.
> >
> > Whew! At the moment, I don't feel strongly about whether or not
> > the following code should (1) read the value, (2) warn on non-zero,
> > (3) assume zero without reading, or (4) some other option that is not
> > occurring to me. Your choice!
>
> (2) looks like a good idea!
Sounds good to me!
Thanx, Paul
next prev parent reply other threads:[~2023-03-26 21:45 UTC|newest]
Thread overview: 20+ messages / expand[flat|nested] mbox.gz Atom feed top
2023-03-22 19:44 [PATCH 0/4] rcu/nocb: Shrinker related boring fixes Frederic Weisbecker
2023-03-22 19:44 ` [PATCH 1/4] rcu/nocb: Protect lazy shrinker against concurrent (de-)offloading Frederic Weisbecker
2023-03-22 23:18 ` Paul E. McKenney
2023-03-24 0:55 ` Joel Fernandes
2023-03-24 1:06 ` Paul E. McKenney
2023-03-24 22:09 ` Frederic Weisbecker
2023-03-24 22:51 ` Paul E. McKenney
2023-03-26 20:01 ` Frederic Weisbecker
2023-03-26 21:45 ` Paul E. McKenney [this message]
2023-03-29 16:07 ` Frederic Weisbecker
2023-03-29 20:45 ` Paul E. McKenney
2023-03-22 19:44 ` [PATCH 2/4] rcu/nocb: Fix shrinker race against callback enqueuer Frederic Weisbecker
2023-03-22 23:19 ` Paul E. McKenney
2023-03-22 19:44 ` [PATCH 3/4] rcu/nocb: Recheck lazy callbacks under the ->nocb_lock from shrinker Frederic Weisbecker
2023-03-22 23:21 ` Paul E. McKenney
2023-03-22 19:44 ` [PATCH 4/4] rcu/nocb: Make shrinker to iterate only NOCB CPUs Frederic Weisbecker
2023-03-24 0:41 ` Joel Fernandes
2023-03-29 16:01 [PATCH 0/4 v2] rcu/nocb: Shrinker related boring fixes Frederic Weisbecker
2023-03-29 16:02 ` [PATCH 1/4] rcu/nocb: Protect lazy shrinker against concurrent (de-)offloading Frederic Weisbecker
2023-03-29 20:44 ` Paul E. McKenney
2023-03-29 21:18 ` Frederic Weisbecker
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