On Wed, Sep 07, 2022 at 09:44:01AM -0400, Joel Fernandes wrote:
> Hi Frederic,
>
> On 9/7/2022 5:40 AM, Frederic Weisbecker wrote:
> >>
> >> 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).
>
> Right, ok. I can make it outside the bypass lock, and the nocb_lock is implied
> by rcu_lockdep_assert_cblist_protected().
>
> >
> >> 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.
>
> thanks, will add your suggested optimization, however I have a general question:
>
> For the case where the bypass list is empty, where does rcu_barrier() do a wake
> up of the nocb GP thread after entrain()?
>
> I don't see a call to __call_rcu_nocb_wake() like rcutree_migrate_callbacks()
> does. Is the wake up done in some other path?
Actually I can't find it. You're right it may be missing. It's not even just
about the bypass list but also the entrain'ed callback. Would you be willing to
cook a fix?
Thanks!
>
> thanks,
>
> - Joel
