Re: [RFC PATCH 01/19] sched/fair: Simplify set_cpu_sd_state_*() with guards

[Shrikanth Hegde <sshegde@linux.ibm.com>]

On 9/4/25 9:44 AM, K Prateek Nayak wrote:
> Simplify set_cpu_sd_state_{busy,idle}() using guard(rcu).
> 
> Reviewed-by: Gautham R. Shenoy <gautham.shenoy@amd.com>
> Signed-off-by: K Prateek Nayak <kprateek.nayak@amd.com>
> ---
>   kernel/sched/fair.c | 20 ++++++++------------
>   1 file changed, 8 insertions(+), 12 deletions(-)
> 
> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index df8dc389af8e..61b59fd75ced 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -12449,16 +12449,14 @@ static void set_cpu_sd_state_busy(int cpu)
>   {
>   	struct sched_domain *sd;
>   
> -	rcu_read_lock();
> -	sd = rcu_dereference(per_cpu(sd_llc, cpu));
> +	guard(rcu)();
>   
> +	sd = rcu_dereference(per_cpu(sd_llc, cpu));
>   	if (!sd || !sd->nohz_idle)
> -		goto unlock;
> -	sd->nohz_idle = 0;
> +		return;
>   
> +	sd->nohz_idle = 0;
>   	atomic_inc(&sd->shared->nr_busy_cpus);
> -unlock:
> -	rcu_read_unlock();
>   }
>   
>   void nohz_balance_exit_idle(struct rq *rq)
> @@ -12479,16 +12477,14 @@ static void set_cpu_sd_state_idle(int cpu)
>   {
>   	struct sched_domain *sd;
>   
> -	rcu_read_lock();
> -	sd = rcu_dereference(per_cpu(sd_llc, cpu));
> +	guard(rcu)();
>   
> +	sd = rcu_dereference(per_cpu(sd_llc, cpu));
>   	if (!sd || sd->nohz_idle)
> -		goto unlock;
> -	sd->nohz_idle = 1;
> +		return;
>   
> +	sd->nohz_idle = 1;
>   	atomic_dec(&sd->shared->nr_busy_cpus);
> -unlock:
> -	rcu_read_unlock();
>   }
>   
>   /*

This looks good.
Reviewed-by: Shrikanth Hegde <sshegde@linux.ibm.com>

---

we have both sd_llc->shared and sd_llc_shared usage spread
across the code, is it possible to remove sd_llc_shared and use sd_llc->shared
instead?

Likely sd_llc is cache hot and access to shared should be fast too.
In turn this could free up some per cpu area.

Any thoughts?