Re: [PATCH] sched/fair: Let sync wakeups target the waker's core

[Chen Yu]

Hi Madadi,

On Thu, Aug 06, 2026 at 10:20:38AM +0530, Madadi Vineeth Reddy wrote:

[ ... ]

> >> -static int select_idle_sibling(struct task_struct *p, int prev, int target)
> >> +static int select_idle_sibling(struct task_struct *p, int prev, int target, bool sync_core)
> >>  {
> >>  	bool has_idle_core = false;
> >>  	struct sched_domain *sd;
> >> @@ -9035,6 +9056,12 @@ static int select_idle_sibling(struct task_struct *p, int prev, int target)
> >>  			if ((unsigned int)i < nr_cpumask_bits)
> >>  				return i;
> >>  		}
> >> +
> >> +		if (sync_core) {
> >> +			i = select_idle_sync_core(p, sd, target);
> >> +			if ((unsigned int)i < nr_cpumask_bits)
> >> +				return i;
> >> +		}
> >>  	}
> >>  
> >>  	i = select_idle_cpu(p, sd, has_idle_core, target);
> >> @@ -9733,8 +9760,16 @@ select_task_rq_fair(struct task_struct *p, int prev_cpu, int wake_flags)
> >>  		return sched_balance_find_dst_cpu(sd, p, cpu, prev_cpu, sd_flag);
> >>  
> >>  	/* Fast path */
> >> -	if (wake_flags & WF_TTWU)
> >> -		return select_idle_sibling(p, prev_cpu, new_cpu);
> >> +	if (wake_flags & WF_TTWU) {
> >> +		bool sync_core = false;
> >> +		if (want_affine && sync && new_cpu == cpu) {
> >> +			struct rq *rq = cpu_rq(cpu);
> >> +
> >> +			sync_core = (rq->nr_running - cfs_h_nr_delayed(rq)) == 1;

If I understand correctly, the goal is to choose an idle SMT sibling as the waker
CPU, if:

1. the wakeup has WF_SYNC, and
2. the waker's SMT sibling CPUs are all idle, and
3. the waker is about to release the CPU.
In this way, we can "stack" the wakee on a core that is about to become idle to
get better cache locality.

Condition 3 above might not always hold true, because WF_SYNC is not restricted to
task context. softirq may also call wake_up_interruptible_sync_poll() with WF_SYNC,
and in that case, current is whatever task the softirq happened to interrupt.

Given that, would it be reasonable to add in_task() check to gate the softirq case?


=======================================================================================
BTW, in your git log:
"WF_SYNC tells the scheduler the waker is about to block ... when the waker's runqueue
holds a single runnable task it returns the waker's CPU, select_idle_sibling() then
discards that decision, because available_idle_cpu() is false for a CPU that is still
running the waker"

Thanks for this description. I realized that WF_SYNC is not what I previously thought:
stacking the wakee on the same CPU as the waker - that's not exactly right.
Now my understanding is that, WF_SYNC is actually asking the wakee to find an idle CPU
in the waker's LLC domain within select_idle_sibling(), humm, not sure if I missed anything:

sd = rcu_dereference_all(per_cpu(sd_llc, target));
======================================================================================

thanks,
Chenyu