Re: [PATCH rcu 03/12] rcu: Add rnp->cbovldmask check in rcutree_migrate_callbacks()

Paul E. McKenney
2022-06-21T22:22:39Z

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@intel.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@intel.com>
> > Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
> > ---
>
> Reviewed-by: Neeraj Upadhyay <quic_neeraju@quicinc.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);