linux-kernel - Re: [PATCH rcu 03/12] rcu: Add rnp->cbovldmask check in rcutree_migrate

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Date:   Tue, 21 Jun 2022 15:22:30 -0700
From:   "Paul E. McKenney" <paulmck@...nel.org>
To:     Neeraj Upadhyay <quic_neeraju@...cinc.com>
Cc:     rcu@...r.kernel.org, linux-kernel@...r.kernel.org,
        kernel-team@...com, rostedt@...dmis.org,
        Zqiang <qiang1.zhang@...el.com>
Subject: Re: [PATCH rcu 03/12] rcu: Add rnp->cbovldmask check in
 rcutree_migrate_callbacks()

On Tue, Jun 21, 2022 at 11:27:28AM +0530, Neeraj Upadhyay wrote:
> 
> 
> On 6/21/2022 3:50 AM, Paul E. McKenney wrote:
> > From: Zqiang <qiang1.zhang@...el.com>
> > 
> > Currently, the rcu_node structure's ->cbovlmask field is set in call_rcu()
> > when a given CPU is suffering from callback overload.  But if that CPU
> > goes offline, the outgoing CPU's callbacks is migrated to the running
> > CPU, which is likely to overload the running CPU.  However, that CPU's
> > bit in its leaf rcu_node structure's ->cbovlmask field remains zero.
> > 
> > Initially, this is OK because the outgoing CPU's bit remains set.
> > However, that bit will be cleared at the next end of a grace period,
> > at which time it is quite possible that the running CPU will still
> > be overloaded.  If the running CPU invokes call_rcu(), then overload
> > will be checked for and the bit will be set.  Except that there is no
> > guarantee that the running CPU will invoke call_rcu(), in which case the
> > next grace period will fail to take the running CPU's overload condition
> > into account.  Plus, because the bit is not set, the end of the grace
> > period won't check for overload on this CPU.
> > 
> > This commit therefore adds a call to check_cb_ovld_locked() in
> > check_cb_ovld_locked() to set the running CPU's ->cbovlmask bit
> 
> Nit: s/check_cb_ovld_locked/rcutree_migrate_callbacks/

Good catch, fixed!

> > appropriately.
> > 
> > Signed-off-by: Zqiang <qiang1.zhang@...el.com>
> > Signed-off-by: Paul E. McKenney <paulmck@...nel.org>
> > ---
> 
> Reviewed-by: Neeraj Upadhyay <quic_neeraju@...cinc.com>

Thank you, applied.

							Thanx, Paul

> Thanks
> Neeraj
> 
> >   kernel/rcu/tree.c | 1 +
> >   1 file changed, 1 insertion(+)
> > 
> > diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
> > index c19d5926886fb..f4a37f2032664 100644
> > --- a/kernel/rcu/tree.c
> > +++ b/kernel/rcu/tree.c
> > @@ -4491,6 +4491,7 @@ void rcutree_migrate_callbacks(int cpu)
> >   	needwake = needwake || rcu_advance_cbs(my_rnp, my_rdp);
> >   	rcu_segcblist_disable(&rdp->cblist);
> >   	WARN_ON_ONCE(rcu_segcblist_empty(&my_rdp->cblist) != !rcu_segcblist_n_cbs(&my_rdp->cblist));
> > +	check_cb_ovld_locked(my_rdp, my_rnp);
> >   	if (rcu_rdp_is_offloaded(my_rdp)) {
> >   		raw_spin_unlock_rcu_node(my_rnp); /* irqs remain disabled. */
> >   		__call_rcu_nocb_wake(my_rdp, true, flags);