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

[Neeraj Upadhyay]

On 6/21/2022 3:50 AM, Paul E. McKenney wrote:
> From: Zqiang <qiang1.zhang@xxxxxxxxx>
>
> 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/

> appropriately.
>
> Signed-off-by: Zqiang <qiang1.zhang@xxxxxxxxx>
> Signed-off-by: Paul E. McKenney <paulmck@xxxxxxxxxx>
> ---

Reviewed-by: Neeraj Upadhyay <quic_neeraju@xxxxxxxxxxx>


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);