The ->lazy_len is only checked locklessly. Recheck again under the
->nocb_lock to avoid spending more time on flushing/waking if not
necessary. The ->lazy_len can still increment concurrently (from 1 to
infinity) but under the ->nocb_lock we at least know for sure if there
are lazy callbacks at all (->lazy_len > 0).
Signed-off-by: Frederic Weisbecker <[email protected]>
---
kernel/rcu/tree_nocb.h | 16 ++++++++++++----
1 file changed, 12 insertions(+), 4 deletions(-)
diff --git a/kernel/rcu/tree_nocb.h b/kernel/rcu/tree_nocb.h
index c321fce2af8e..dfa9c10d6727 100644
--- a/kernel/rcu/tree_nocb.h
+++ b/kernel/rcu/tree_nocb.h
@@ -1358,12 +1358,20 @@ lazy_rcu_shrink_scan(struct shrinker *shrink, struct shrink_control *sc)
if (!rcu_rdp_is_offloaded(rdp))
continue;
+ if (!READ_ONCE(rdp->lazy_len))
+ continue;
+
+ rcu_nocb_lock_irqsave(rdp, flags);
+ /*
+ * Recheck under the nocb lock. Since we are not holding the bypass
+ * lock we may still race with increments from the enqueuer but still
+ * we know for sure if there is at least one lazy callback.
+ */
_count = READ_ONCE(rdp->lazy_len);
-
- if (_count == 0)
+ if (!_count) {
+ rcu_nocb_unlock_irqrestore(rdp, flags);
continue;
-
- rcu_nocb_lock_irqsave(rdp, flags);
+ }
WARN_ON_ONCE(!rcu_nocb_flush_bypass(rdp, NULL, jiffies, false));
rcu_nocb_unlock_irqrestore(rdp, flags);
wake_nocb_gp(rdp, false);
--
2.34.1
On Wed, Mar 29, 2023 at 06:02:02PM +0200, Frederic Weisbecker wrote:
> The ->lazy_len is only checked locklessly. Recheck again under the
> ->nocb_lock to avoid spending more time on flushing/waking if not
> necessary. The ->lazy_len can still increment concurrently (from 1 to
> infinity) but under the ->nocb_lock we at least know for sure if there
> are lazy callbacks at all (->lazy_len > 0).
>
> Signed-off-by: Frederic Weisbecker <[email protected]>
> ---
> kernel/rcu/tree_nocb.h | 16 ++++++++++++----
> 1 file changed, 12 insertions(+), 4 deletions(-)
>
> diff --git a/kernel/rcu/tree_nocb.h b/kernel/rcu/tree_nocb.h
> index c321fce2af8e..dfa9c10d6727 100644
> --- a/kernel/rcu/tree_nocb.h
> +++ b/kernel/rcu/tree_nocb.h
> @@ -1358,12 +1358,20 @@ lazy_rcu_shrink_scan(struct shrinker *shrink, struct shrink_control *sc)
> if (!rcu_rdp_is_offloaded(rdp))
> continue;
>
> + if (!READ_ONCE(rdp->lazy_len))
> + continue;
Do you depend on the ordering of the above read of ->lazy_len against
anything in the following, aside from the re-read of ->lazy_len? (Same
variable, both READ_ONCE() or stronger, so you do get that ordering.)
If you do need that ordering, the above READ_ONCE() needs to instead
be smp_load_acquire() or similar. If you don't need that ordering,
what you have is good.
> + rcu_nocb_lock_irqsave(rdp, flags);
> + /*
> + * Recheck under the nocb lock. Since we are not holding the bypass
> + * lock we may still race with increments from the enqueuer but still
> + * we know for sure if there is at least one lazy callback.
> + */
> _count = READ_ONCE(rdp->lazy_len);
> -
> - if (_count == 0)
> + if (!_count) {
> + rcu_nocb_unlock_irqrestore(rdp, flags);
> continue;
> -
> - rcu_nocb_lock_irqsave(rdp, flags);
> + }
> WARN_ON_ONCE(!rcu_nocb_flush_bypass(rdp, NULL, jiffies, false));
> rcu_nocb_unlock_irqrestore(rdp, flags);
> wake_nocb_gp(rdp, false);
> --
> 2.34.1
>
On Wed, Mar 29, 2023 at 01:54:20PM -0700, Paul E. McKenney wrote:
> On Wed, Mar 29, 2023 at 06:02:02PM +0200, Frederic Weisbecker wrote:
> > The ->lazy_len is only checked locklessly. Recheck again under the
> > ->nocb_lock to avoid spending more time on flushing/waking if not
> > necessary. The ->lazy_len can still increment concurrently (from 1 to
> > infinity) but under the ->nocb_lock we at least know for sure if there
> > are lazy callbacks at all (->lazy_len > 0).
> >
> > Signed-off-by: Frederic Weisbecker <[email protected]>
> > ---
> > kernel/rcu/tree_nocb.h | 16 ++++++++++++----
> > 1 file changed, 12 insertions(+), 4 deletions(-)
> >
> > diff --git a/kernel/rcu/tree_nocb.h b/kernel/rcu/tree_nocb.h
> > index c321fce2af8e..dfa9c10d6727 100644
> > --- a/kernel/rcu/tree_nocb.h
> > +++ b/kernel/rcu/tree_nocb.h
> > @@ -1358,12 +1358,20 @@ lazy_rcu_shrink_scan(struct shrinker *shrink, struct shrink_control *sc)
> > if (!rcu_rdp_is_offloaded(rdp))
> > continue;
> >
> > + if (!READ_ONCE(rdp->lazy_len))
> > + continue;
>
> Do you depend on the ordering of the above read of ->lazy_len against
> anything in the following, aside from the re-read of ->lazy_len? (Same
> variable, both READ_ONCE() or stronger, so you do get that ordering.)
>
> If you do need that ordering, the above READ_ONCE() needs to instead
> be smp_load_acquire() or similar. If you don't need that ordering,
> what you have is good.
No ordering dependency intended here. The early ->lazy_len read is really just
an optimization here to avoid locking if it *seems* there is nothing to do with
this rdp. But what follows doesn't depend on that read.
Thanks.
On Wed, Mar 29, 2023 at 11:22:45PM +0200, Frederic Weisbecker wrote:
> On Wed, Mar 29, 2023 at 01:54:20PM -0700, Paul E. McKenney wrote:
> > On Wed, Mar 29, 2023 at 06:02:02PM +0200, Frederic Weisbecker wrote:
> > > The ->lazy_len is only checked locklessly. Recheck again under the
> > > ->nocb_lock to avoid spending more time on flushing/waking if not
> > > necessary. The ->lazy_len can still increment concurrently (from 1 to
> > > infinity) but under the ->nocb_lock we at least know for sure if there
> > > are lazy callbacks at all (->lazy_len > 0).
> > >
> > > Signed-off-by: Frederic Weisbecker <[email protected]>
> > > ---
> > > kernel/rcu/tree_nocb.h | 16 ++++++++++++----
> > > 1 file changed, 12 insertions(+), 4 deletions(-)
> > >
> > > diff --git a/kernel/rcu/tree_nocb.h b/kernel/rcu/tree_nocb.h
> > > index c321fce2af8e..dfa9c10d6727 100644
> > > --- a/kernel/rcu/tree_nocb.h
> > > +++ b/kernel/rcu/tree_nocb.h
> > > @@ -1358,12 +1358,20 @@ lazy_rcu_shrink_scan(struct shrinker *shrink, struct shrink_control *sc)
> > > if (!rcu_rdp_is_offloaded(rdp))
> > > continue;
> > >
> > > + if (!READ_ONCE(rdp->lazy_len))
> > > + continue;
> >
> > Do you depend on the ordering of the above read of ->lazy_len against
> > anything in the following, aside from the re-read of ->lazy_len? (Same
> > variable, both READ_ONCE() or stronger, so you do get that ordering.)
> >
> > If you do need that ordering, the above READ_ONCE() needs to instead
> > be smp_load_acquire() or similar. If you don't need that ordering,
> > what you have is good.
>
> No ordering dependency intended here. The early ->lazy_len read is really just
> an optimization here to avoid locking if it *seems* there is nothing to do with
> this rdp. But what follows doesn't depend on that read.
Full steam ahead with READ_ONCE(), then! ;-)
Thanx, Paul