Re: [PATCH] sched/fair: Restore RCU read lock in set_cpu_sd_state_{busy,idle}()

[Peter Zijlstra]

On Thu, May 21, 2026 at 10:51:15PM +0200, Andrea Righi wrote:
> Commit c9d93a73ce87 ("sched/fair: Drop redundant RCU read lock in NOHZ
> kick path") removed the rcu_read_lock()/unlock() pair from
> set_cpu_sd_state_busy() and set_cpu_sd_state_idle() on the assumption
> that all callers run in a safe context for rcu_dereference_all(): IRQs
> disabled or cpus_write_lock() held.
> 
> That assumption is wrong for the CPU hotplug teardown path. When CPUs
> are taken offline, set_cpu_sd_state_busy() is invoked via:
> 
>  cpuhp/N kthread
>    cpuhp_thread_fun()
>      cpuhp_invoke_callback()
>        sched_cpu_deactivate()
>          nohz_balance_exit_idle()
>            set_cpu_sd_state_busy()
>              rcu_dereference_all(per_cpu(sd_llc, cpu))

> 
> Restore the rcu_read_lock()/unlock() pair in both helpers;
> nohz_balancer_kick() is left as is, since its IRQ-disabled context is
> genuinely sufficient.
> 
> Fixes: c9d93a73ce87 ("sched/fair: Drop redundant RCU read lock in NOHZ kick path")
> Reported-by: Marek Szyprowski <m.szyprowski@xxxxxxxxxxx>
> Closes: https://lore.kernel.org/all/38fe0a1d-1a48-435a-910a-c278024d9ac9@xxxxxxxxxxx/
> Signed-off-by: Andrea Righi <arighi@xxxxxxxxxx>

So the obvious alternative is to disable RCU in the one caller that
doesn't play ball.

Was that considered?


--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -8699,7 +8699,8 @@ int sched_cpu_deactivate(unsigned int cp
 	 * Remove CPU from nohz.idle_cpus_mask to prevent participating in
 	 * load balancing when not active
 	 */
-	nohz_balance_exit_idle(rq);
+	scoped_guard (rcu)
+		nohz_balance_exit_idle(rq);
 
 	set_cpu_active(cpu, false);