On Wed, Sep 21, 2022 at 11:52:45PM +0000, Joel Fernandes wrote:
[..]
> > > Anyway, for testing this should be good...
> > >
> > > ---8<-----------------------
> > >
> > > diff --git a/kernel/rcu/tree_nocb.h b/kernel/rcu/tree_nocb.h
> > > index bd8f39ee2cd0..e3344c262672 100644
> > > --- a/kernel/rcu/tree_nocb.h
> > > +++ b/kernel/rcu/tree_nocb.h
> > > @@ -382,15 +382,19 @@ static bool rcu_nocb_flush_bypass(struct rcu_data *rdp, struct rcu_head *rhp,
> > > unsigned long j, unsigned long flush_flags)
> > > {
> > > bool ret;
> > > + bool was_alldone;
> > >
> > > if (!rcu_rdp_is_offloaded(rdp))
> > > return true;
> > > rcu_lockdep_assert_cblist_protected(rdp);
> > > rcu_nocb_bypass_lock(rdp);
> > > + if (flush_flags & FLUSH_BP_WAKE)
> > > + was_alldone = !rcu_segcblist_pend_cbs(&rdp->cblist);
> > > +
> >
> > You can check that outside bypass lock (but you still need nocb_lock).
> >
> > > ret = rcu_nocb_do_flush_bypass(rdp, rhp, j, flush_flags);
> > >
> > > - if (flush_flags & FLUSH_BP_WAKE)
> > > - wake_nocb_gp(rdp, true);
> > > + if (flush_flags & FLUSH_BP_WAKE && was_alldone)
> > > + wake_nocb_gp(rdp, false);
> >
> > That doesn't check if the bypass list was empty.
>
> I am ending up with something like the below for v6, after discussing with
> Paul on IRC he pointed out we only need to do the rcu_barrier() related
> wakeup when all the CBs are lazy in the bypass list. Otherwise timer goes
> off. I think Frederic mentioned something similar above in different words.
>
> I prefer to keep this logic in tree_nocb.h since rcu_barrier_entrain()
> shouldn't have to deal with nocb internals (in theory anyway).
>
> Looks Ok?
And I did miss something so here is the updated version, testing it further
now, I think this is the last 'concern' I had before making the other API
changes we discussed so should hopefully be smooth sailing from here. Thank you!
---8<-----------------------
Subject: [PATCH for v6] fixup! rcu: Introduce call_rcu_lazy() API implementation
Signed-off-by: Joel Fernandes (Google) <joel@xxxxxxxxxxxxxxxxx>
---
kernel/rcu/tree_nocb.h | 15 ++++++++++++---
1 file changed, 12 insertions(+), 3 deletions(-)
diff --git a/kernel/rcu/tree_nocb.h b/kernel/rcu/tree_nocb.h
index c197534d0c99..472ceb8af6d3 100644
--- a/kernel/rcu/tree_nocb.h
+++ b/kernel/rcu/tree_nocb.h
@@ -375,18 +375,27 @@ static bool rcu_nocb_flush_bypass(struct rcu_data *rdp, struct rcu_head *rhp,
unsigned long j, unsigned long flush_flags)
{
bool ret;
- bool was_alldone;
+ bool was_alldone = false;
+ bool bypass_all_lazy = false;
+ long bypass_ncbs;
if (!rcu_rdp_is_offloaded(rdp))
return true;
rcu_lockdep_assert_cblist_protected(rdp);
rcu_nocb_bypass_lock(rdp);
- if (flush_flags & FLUSH_BP_WAKE)
+
+ if (flush_flags & FLUSH_BP_WAKE) {
was_alldone = !rcu_segcblist_pend_cbs(&rdp->cblist);
+ bypass_ncbs = rcu_cblist_n_cbs(&rdp->nocb_bypass);
+ bypass_all_lazy = bypass_ncbs && (bypass_ncbs == rdp->lazy_len);
+ }
ret = rcu_nocb_do_flush_bypass(rdp, rhp, j, flush_flags);
- if (flush_flags & FLUSH_BP_WAKE && was_alldone)
+ // Wake up the nocb GP thread if needed. GP thread could be sleeping
+ // while waiting for lazy timer to expire (otherwise rcu_barrier may
+ // end up waiting for the duration of the lazy timer).
+ if (flush_flags & FLUSH_BP_WAKE && was_alldone && bypass_all_lazy)
wake_nocb_gp(rdp, false);
return ret;
--
2.37.3.998.g577e59143f-goog
