Offlining offloaded CPUs don't migrate their callbacks just like non-offloaded CPUs do. It's up to their CB/GP kthread to handle what remains.
Therefore we can't afford to de-offload an offline CPU that still has pending work to do, or the callbacks would be ignored. NOTE: The long term solution will be to wait for all pending callbacks to be processed before completing a CPU down operation. Suggested-by: Paul E. McKenney <[email protected]> Signed-off-by: Frederic Weisbecker <[email protected]> Cc: Paul E. McKenney <[email protected]> Cc: Josh Triplett <[email protected]> Cc: Steven Rostedt <[email protected]> Cc: Mathieu Desnoyers <[email protected]> Cc: Lai Jiangshan <[email protected]> Cc: Joel Fernandes <[email protected]> Cc: Neeraj Upadhyay <[email protected]> --- kernel/rcu/tree_plugin.h | 8 ++++++++ 1 file changed, 8 insertions(+) diff --git a/kernel/rcu/tree_plugin.h b/kernel/rcu/tree_plugin.h index 09caf319a4a9..33e9d53d2181 100644 --- a/kernel/rcu/tree_plugin.h +++ b/kernel/rcu/tree_plugin.h @@ -2228,6 +2228,14 @@ static int __rcu_nocb_rdp_deoffload(struct rcu_data *rdp) printk("De-offloading %d\n", rdp->cpu); rcu_nocb_lock_irqsave(rdp, flags); + /* + * If there are still pending work offloaded, the offline + * CPU won't help much handling them. + */ + if (cpu_is_offline(rdp->cpu) && !rcu_segcblist_empty(&rdp->cblist)) { + rcu_nocb_unlock_irqrestore(rdp, flags); + return -EBUSY; + } raw_spin_lock_rcu_node(rnp); rcu_segcblist_offload(cblist, false); raw_spin_unlock_rcu_node(rnp); -- 2.25.1
