Re: [PATCH 1/6] sched/fair: Use guard(rcu) for sched_domain RCU sections

[K Prateek Nayak]

Hello Andrea,

On 4/28/2026 10:46 AM, Andrea Righi wrote:
> Use the scoped guard(rcu)() helper to safely access sched_domain
> pointers.
> 
> No functional change intended, this is preparation for topology work
> where sched_domain lifetimes are easier to reason about with explicit,
> scope-bounded RCU critical sections.
> 
> Suggested-by: K Prateek Nayak <kprateek.nayak@xxxxxxx>
> Signed-off-by: Andrea Righi <arighi@xxxxxxxxxx>
> ---
>  kernel/sched/fair.c | 141 ++++++++++++++++++++++----------------------
>  1 file changed, 71 insertions(+), 70 deletions(-)
> 
> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index 69361c63353ad..fc0828150c780 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -8083,6 +8083,8 @@ static int select_idle_sibling(struct task_struct *p, int prev, int target)
>  	 */
>  	lockdep_assert_irqs_disabled();
>  
> +	guard(rcu)();

Since IRQs are disabled, we don't need an addition RCU read lock here.
See a03fee333a2f ("sched/fair: Remove superfluous rcu_read_lock()")

> +
>  	if (choose_idle_cpu(target, p) &&
>  	    asym_fits_cpu(task_util, util_min, util_max, target))
>  		return target;
> @@ -12701,55 +12703,16 @@ static void kick_ilb(unsigned int flags)
>  }
>  
>  /*
> - * Current decision point for kicking the idle load balancer in the presence
> - * of idle CPUs in the system.
> + * Decide whether the ILB needs a stats and/or balance kick based on
> + * sched_domain state.
>   */
> -static void nohz_balancer_kick(struct rq *rq)
> +static bool nohz_balancer_needs_kick(struct rq *rq)
>  {
> -	unsigned long now = jiffies;
>  	struct sched_domain_shared *sds;
>  	struct sched_domain *sd;
>  	int nr_busy, i, cpu = rq->cpu;
> -	unsigned int flags = 0;
> -
> -	if (unlikely(rq->idle_balance))
> -		return;
> -
> -	/*
> -	 * We may be recently in ticked or tickless idle mode. At the first
> -	 * busy tick after returning from idle, we will update the busy stats.
> -	 */
> -	nohz_balance_exit_idle(rq);
> -
> -	if (READ_ONCE(nohz.has_blocked_load) &&
> -	    time_after(now, READ_ONCE(nohz.next_blocked)))
> -		flags = NOHZ_STATS_KICK;
> -
> -	/*
> -	 * Most of the time system is not 100% busy. i.e nohz.nr_cpus > 0
> -	 * Skip the read if time is not due.
> -	 *
> -	 * If none are in tickless mode, there maybe a narrow window
> -	 * (28 jiffies, HZ=1000) where flags maybe set and kick_ilb called.
> -	 * But idle load balancing is not done as find_new_ilb fails.
> -	 * That's very rare. So read nohz.nr_cpus only if time is due.
> -	 */
> -	if (time_before(now, nohz.next_balance))
> -		goto out;
>  
> -	/*
> -	 * None are in tickless mode and hence no need for NOHZ idle load
> -	 * balancing
> -	 */
> -	if (unlikely(cpumask_empty(nohz.idle_cpus_mask)))
> -		return;
> -
> -	if (rq->nr_running >= 2) {
> -		flags = NOHZ_STATS_KICK | NOHZ_BALANCE_KICK;
> -		goto out;
> -	}
> -
> -	rcu_read_lock();
> +	guard(rcu)();

and since this is only called from:

  sched_tick() /* IRQs disabled */
    sched_balance_trigger()
      nohz_balancer_kick()

with IRQs disabled, we can get rid of that rcu_read_lock() entirely.

>  
>  	sd = rcu_dereference_all(rq->sd);
>  	if (sd) {
-- 
Thanks and Regards,
Prateek