Re: [PATCH] rcu/nocb: Check rdp_gp->nocb_timer in __call_rcu_nocb_wake()

[Paul E. McKenney]

On Thu, Jan 18, 2024 at 06:51:57AM -0800, Paul E. McKenney wrote:
> On Wed, Jan 17, 2024 at 01:07:25PM +0100, Frederic Weisbecker wrote:
> > Le Wed, Jan 17, 2024 at 06:26:16PM +0800, Zqiang a écrit :
> > > Currently, only rdp_gp->nocb_timer is used, for nocb_timer of
> > > no-rdp_gp structure, the timer_pending() is always return false,
> > > this commit therefore need to check rdp_gp->nocb_timer in
> > > __call_rcu_nocb_wake().
> > > 
> > > Signed-off-by: Zqiang <qiang.zhang1211@xxxxxxxxx>
> > > ---
> > >  kernel/rcu/tree_nocb.h | 3 ++-
> > >  1 file changed, 2 insertions(+), 1 deletion(-)
> > > 
> > > diff --git a/kernel/rcu/tree_nocb.h b/kernel/rcu/tree_nocb.h
> > > index 54971afc3a9b..3f85577bddd4 100644
> > > --- a/kernel/rcu/tree_nocb.h
> > > +++ b/kernel/rcu/tree_nocb.h
> > > @@ -564,6 +564,7 @@ static void __call_rcu_nocb_wake(struct rcu_data *rdp, bool was_alldone,
> > >  	long lazy_len;
> > >  	long len;
> > >  	struct task_struct *t;
> > > +	struct rcu_data *rdp_gp = rdp->nocb_gp_rdp;
> > >  
> > >  	// If we are being polled or there is no kthread, just leave.
> > >  	t = READ_ONCE(rdp->nocb_gp_kthread);
> > > @@ -608,7 +609,7 @@ static void __call_rcu_nocb_wake(struct rcu_data *rdp, bool was_alldone,
> > >  		smp_mb(); /* Enqueue before timer_pending(). */
> > >  		if ((rdp->nocb_cb_sleep ||
> > >  		     !rcu_segcblist_ready_cbs(&rdp->cblist)) &&
> > > -		    !timer_pending(&rdp->nocb_timer)) {
> > > +		    !timer_pending(&rdp_gp->nocb_timer)) {
> > 
> > Hehe, good eyes ;-)
> > 
> > I had that change in mind but while checking that area further I actually
> > wondered what is the actual purpose of this RCU_NOCB_WAKE_FORCE thing. If
> > we reach that place, it means that the nocb_gp kthread should be awaken
> > already (or the timer pending), so what does a force wake up solve in that
> > case?
> > 
> > Paul, any recollection of that?
> 
> Huh.  We never actually do RCU_NOCB_WAKE_FORCE in v6.7, if I followed
> all the code paths correctly.
> 
> Historically, I have been worried about lost wakeups.  Also, there
> used to be code paths in which a wakeup was not needed, for example,
> because we knew that the ending of the current grace period would take
> care of things.  Unless there was some huge pile of callbacks, in which
> case an immediate wakeup could avoid falling behind a callback flood.
> 
> Given that rcutorture does test callback flooding, we appear to be OK,
> but maybe it is time to crank up the flooding more.
> 
> On the other hand, I have started seeing the (very) occasional OOM
> on TREE03.  (In addition to those that show up from time to time on the
> single-CPU TREE09 scenario.)

Oh, and queued for further review and testing, thank you both!

							Thanx, Paul

> > In the meantime:
> > 
> > Reviewed-by: Frederic Weisbecker <frederic@xxxxxxxxxx>