Re: [PATCH] sched/fair: Skip sched-domain traversal when wake affine is disabled

[Kayra Cizmeci <kayracizmeci@gmail.com>]

Hi!

On Mon, Nov 09, 2026 at 05:35:22PM +0530, Srikar Dronamraju wrote:
> In select_task_rq_fair(), TTWU wakeups for which wake_wide() returns
> true, or where the waking CPU is not in the task's allowed CPU mask,
> want_affine is set to false.
> 
> For these wakeups, the scheduler does not perform wake-affine placement.
> The existing code already treats this as a fast-path by exiting the
> sched-domain walk before locating an SD_BALANCE_WAKE domain, ultimately
> falling through to select_idle_sibling().

Not before locating. If it doesn't locates SD_BALANCE_WAKE in the first
check it just leaves.

AFAICT SD_BALANCE_WAKE is not set anywhere tho.

So if you wanna do this, maybe you can do something like:

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index d4cc0caee626..b40b2eb5e577 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -10016,6 +10016,9 @@ select_task_rq_fair(struct task_struct *p, int prev_cpu, int wake_flags)
 		}
 
 		want_affine = !wake_wide(p) && cpumask_test_cpu(cpu, p->cpus_ptr);
+
+		if (!want_affine)
+			return select_idle_sibling(p, prev_cpu, new_cpu);
 	}
 
 	for_each_domain(cpu, tmp) {

> 
> However, select_task_rq_fair() still enters the sched-domain traversal
> only to break out almost immediately. Avoid this unnecessary work by
> jumping directly to the existing fast path when want_affine is false.
> 
> As part of this cleanup:
> 
>   - Move want_affine into the WF_TTWU block where it is used.
>   - Skip the sched-domain traversal entirely when want_affine is false.
>   - Remove the now redundant want_affine test from the
>     SD_WAKE_AFFINE check.
>   - Remove the corresponding !want_affine loop termination logic.
> 
> This preserves existing wakeup placement behavior while avoiding
> unnecessary sched-domain iteration for TTWU wakeups that cannot make
> use of wake-affine placement.
> 
> Signed-off-by: Srikar Dronamraju <srikar@linux.ibm.com>
> ---
>  kernel/sched/fair.c | 9 +++++----
>  1 file changed, 5 insertions(+), 4 deletions(-)
> 
> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index 3ebec186f982..58ff55e093cb 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -8832,7 +8832,6 @@ select_task_rq_fair(struct task_struct *p, int prev_cpu, int wake_flags)
>  	struct sched_domain *tmp, *sd = NULL;
>  	int cpu = smp_processor_id();
>  	int new_cpu = prev_cpu;
> -	int want_affine = 0;
>  	/* SD_flags and WF_flags share the first nibble */
>  	int sd_flag = wake_flags & 0xF;
>  
> @@ -8841,6 +8840,8 @@ select_task_rq_fair(struct task_struct *p, int prev_cpu, int wake_flags)
>  	 */
>  	lockdep_assert_held(&p->pi_lock);
>  	if (wake_flags & WF_TTWU) {
> +		int want_affine;
> +
>  		record_wakee(p);
>  
>  		if ((wake_flags & WF_CURRENT_CPU) &&
> @@ -8855,6 +8856,8 @@ select_task_rq_fair(struct task_struct *p, int prev_cpu, int wake_flags)
>  		}
>  
>  		want_affine = !wake_wide(p) && cpumask_test_cpu(cpu, p->cpus_ptr);
> +		if (!want_affine)
> +			goto fast_path;
>  	}
>  
>  	for_each_domain(cpu, tmp) {
> @@ -8862,7 +8865,7 @@ select_task_rq_fair(struct task_struct *p, int prev_cpu, int wake_flags)
>  		 * If both 'cpu' and 'prev_cpu' are part of this domain,
>  		 * cpu is a valid SD_WAKE_AFFINE target.
>  		 */
> -		if (want_affine && (tmp->flags & SD_WAKE_AFFINE) &&
> +		if ((tmp->flags & SD_WAKE_AFFINE) &&
>  		    cpumask_test_cpu(prev_cpu, sched_domain_span(tmp))) {
>  			if (cpu != prev_cpu)
>  				new_cpu = wake_affine(tmp, p, cpu, prev_cpu, sync);
> @@ -8878,8 +8881,6 @@ select_task_rq_fair(struct task_struct *p, int prev_cpu, int wake_flags)
>  		 */
>  		if (tmp->flags & sd_flag)
>  			sd = tmp;
> -		else if (!want_affine)
> -			break;
>  	}


This just changes the behavior.

Old Code WF_FORK 1, WF_TTWU 0:

want_affine = 0, so we skip the first if block in the loop then leave
once we find a domain without our flag. And go with slow path.


New Code:

want_affine is moved away and early exit is removed. So we enter the if
block and try to find a domain with prev_cpu. Because WF_TTWU 
is 0, we just return new_cpu. Or if we can't fimd the domain with
prev_cpu, slow path.


Cheers,

Kayra