Re: [PATCH] sched/fair: Prefer waker CPU for reciprocal sync wakeups

[Shubhang]

Hi Christian,

Thanks for the review.

On Wed, 22 Jul 2026, Christian Loehle wrote:

> On 7/22/26 01:04, Shubhang Kaushik (Ampere) wrote:
> > Pipe and futex ping-pong workloads can be dominated by handoff cost. In
> > such cases, placing the wakee on an idle CPU can be slower than keeping
> > the pair on the same runqueue.
> >
> > Use the existing last_wakee and wake_wide() state to identify narrow
> > reciprocal WF_SYNC wakeups:
> >
> > A wakes B
> > B wakes A
> > A wakes B
> > ...
>
> But the futex ping-pong doesn't set WF_SYNC in the first place or am I missing
> something?

Yes, the futex runs were only regression checks here. I'll drop the futex 
wording from the description and keep the motivation to pipe sync 
handoffs.

> > @@ -9548,6 +9570,7 @@ select_task_rq_fair(struct task_struct *p, int prev_cpu, int wake_flags)
> >  	int cpu = smp_processor_id();
> >  	int new_cpu = prev_cpu;
> >  	int want_affine = 0;
> > +	int wide = 0;
> >  	/* SD_flags and WF_flags share the first nibble */
> >  	int sd_flag = wake_flags & 0xF;
> >
> > @@ -9557,6 +9580,7 @@ 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) {
> >  		record_wakee(p);
> > +		wide = wake_wide(p);
> >
> >  		if ((wake_flags & WF_CURRENT_CPU) &&
> >  		    cpumask_test_cpu(cpu, p->cpus_ptr))
> > @@ -9569,7 +9593,12 @@ select_task_rq_fair(struct task_struct *p, int prev_cpu, int wake_flags)
> >  			new_cpu = prev_cpu;
> >  		}
> >
> > -		want_affine = !wake_wide(p) && cpumask_test_cpu(cpu, p->cpus_ptr);
> > +		if (sync && !wide &&
> > +		    READ_ONCE(p->last_wakee) == current &&
> > +		    prefer_sync_pair_cpu(p, cpu))
> > +			return cpu;
>
> This must move further below (to check for SD_WAKE_AFFINE) otherwise it ignores isolcpus.

Agreed. The early return bypasses the existing SD_WAKE_AFFINE check,
including the isolcpus handling that comes through the sched-domain
setup. I'll move it into the wake-affine block so it only applies after
that check.

Regards,
Shubhang Kaushik