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

[Xuewen Yan]

On Thu, Mar 19, 2026 at 9:08 AM Tejun Heo <tj@xxxxxxxxxx> wrote:
>
> 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.

Agree, and for scx_switched_all(), there is no need to call cpu_util_cfs(),
Maybe we could modify it as follows?

--->
diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index bf948db905ed..8f5d0c7e8ea2 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -8198,7 +8198,18 @@ 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;
+
+       if (scx_enabled()) {
+               unsigned long util = scx_cpuperf_target(cpu);
+
+                if (!scx_switched_all())
+                       util += cpu_util_cfs(cpu);
+       } else {
+               util = cpu_util_cfs(cpu);
+       }
+
+       return effective_cpu_util(cpu, util, NULL, NULL);
 }
---


Thanks!