Re: [RFC PATCH] sched: Add scx_cpuperf_target in sched_cpu_util()

[Tejun Heo]

On Wed, Mar 18, 2026 at 01:47:18PM +0100, Peter Zijlstra wrote:
> On Wed, Mar 18, 2026 at 08:17:55PM +0800, Xuewen Yan wrote:
> > diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> > index bf948db905ed..20adb6fede2a 100644
> > --- a/kernel/sched/fair.c
> > +++ b/kernel/sched/fair.c
> > @@ -8198,7 +8198,12 @@ unsigned long effective_cpu_util(int cpu, unsigned long util_cfs,
> >  
> >  unsigned long sched_cpu_util(int cpu)
> >  {
> > -	return effective_cpu_util(cpu, cpu_util_cfs(cpu), NULL, NULL);
> > +	unsigned long util = scx_cpuperf_target(cpu);
> > +
> > +	if (!scx_switched_all())
> > +		util += cpu_util_cfs(cpu);
> > +
> > +	return effective_cpu_util(cpu, util, NULL, NULL);
> >  }
> 
> This puts the common case of no ext muck into the slow path of that
> static_branch.
> 
> This wants to be something like:
> 
> unsigned long sched_cpu_util(int cpu)
> {
> 	unsigned long util = cpu_util_cfs(cpu);
> 
> 	if (scx_enabled()) {
> 		unsigned long scx_util = scx_cpuperf_target(cpu);
> 
> 		if (!scx_switched_all())
> 			scx_util += util;
> 
> 		util = scx_util;
> 	}
> 
> 	return effective_cpu_util(cpu, util, NULL, NULL);
> }

scx_switched_all() is an unlikely static branch just like scx_enabled() and
scx_cpuperf_target() has scx_enabled() in it too, so the difference for the
fair path between the two versions is two noop run-throughs vs. one. Either
way is fine but it is more code for likely no discernible gain.

Thanks.

-- 
tejun