Re: [PATCH 2/4] sched/core: get this cpu once in ttwu_queue_cond

[Mukesh Kumar Chaurasiya]

On Tue, Mar 24, 2026 at 01:06:28AM +0530, Shrikanth Hegde wrote:
> Calling smp_processor_id() on:
> - In CONFIG_DEBUG_PREEMPT=y, if preemption/irq is disabled, then it does
> not print any warning.
> - In CONFIG_DEBUG_PREEMPT=n, it doesn't do anything apart from getting
> __smp_processor_id
> 
> So with both CONFIG_DEBUG_PREEMPT=y/n, in preemption disabled section
> it is better to cache the value. It could save a few cycles. Though
> tiny, repeated could add up to a small value.
> 
> ttwu_queue_cond is called with interrupt disabled. So preemption is
> disabled. Hence cache the value once instead.
> 
> Signed-off-by: Shrikanth Hegde <sshegde@xxxxxxxxxxxxx>
> ---
>  kernel/sched/core.c | 6 ++++--
>  1 file changed, 4 insertions(+), 2 deletions(-)
> 
> diff --git a/kernel/sched/core.c b/kernel/sched/core.c
> index 49af425f6a73..14996c7a0a31 100644
> --- a/kernel/sched/core.c
> +++ b/kernel/sched/core.c
> @@ -3900,6 +3900,8 @@ bool cpus_share_resources(int this_cpu, int that_cpu)
>  
>  static inline bool ttwu_queue_cond(struct task_struct *p, int cpu)
>  {
> +	int this_cpu = smp_processor_id();
> +
>  	/* See SCX_OPS_ALLOW_QUEUED_WAKEUP. */
>  	if (!scx_allow_ttwu_queue(p))
>  		return false;
> @@ -3924,10 +3926,10 @@ static inline bool ttwu_queue_cond(struct task_struct *p, int cpu)
>  	 * If the CPU does not share cache, then queue the task on the
>  	 * remote rqs wakelist to avoid accessing remote data.
>  	 */
> -	if (!cpus_share_cache(smp_processor_id(), cpu))
> +	if (!cpus_share_cache(this_cpu, cpu))
>  		return true;
>  
> -	if (cpu == smp_processor_id())
> +	if (cpu == this_cpu)
>  		return false;
>  
>  	/*
> -- 
> 2.47.3
> 
LGTM

Reviewed-by: Mukesh Kumar Chaurasiya (IBM) <mkchauras@xxxxxxxxx>