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

[Peter Zijlstra]

On Thu, Mar 19, 2026 at 12:02:29PM +0100, Uros Bizjak wrote:
> On Thu, Mar 19, 2026 at 11:27 AM Peter Zijlstra <peterz@xxxxxxxxxxxxx> wrote:
> >
> > On Thu, Mar 19, 2026 at 11:01:03AM +0100, Uros Bizjak wrote:
> > > On Thu, Mar 19, 2026 at 10:02 AM Peter Zijlstra <peterz@xxxxxxxxxxxxx> wrote:
> > >
> > > > That fastpath is definitely better; the slowpath is worse, but that is
> > > > in part because the compilers are stupid and cannot eliminate
> > > > static_branch().
> > >
> > > asm gotos are implicitly volatile because they are control flow
> > > primitives. The compiler will *not* remove them.
> >
> > Yes, but I want ponies ;-)
> >
> >   if (static_branch_unlikely(&foo)) {
> >         if (static_branch_unlikely(&foo)) {
> >                 /* A */
> >         } else {
> >                 /* B */
> >         }
> >         /* C */
> >   }
> >
> > Is a very common occurence. And we all know this really should be:
> >
> >   if (static_branch_unlikely(&foo)) {
> >           /* A */
> >           /* C */
> >   }
> >
> > So how can we make this happen? IMO marking those functions __const
> > should tell the compiler that yes, it can elimintate them.
> 
> Huh, __const promises that the function does not access global memory
> and that the function does not have side effects other than returning
> a value. asm volatile inside the __const function creates a side
> effect, so removing function calls would be considered a
> misoptimization. Probably this could lead to undefined behavior in
> terms of what the compiler expects from a __const function.

So since the whole function reduces to a single branch or nop, it does
not in fact access memory or have side effects, right?

(and there is still __pure, for if we somehow consider the key governing
the text patching to be an 'access' in this case)

> > You should not try and protect the user. If they use __const
> > incorrectly, they get to keep the pieces and all that.
> 
> I'm afraid that here the user wants the "__const with only partial
> properties of __const" function, where the compiler does not know what
> the user really wants.

Well, clearly I'm not a compiler person :-) I just want to be able to
tell the compiler that it can collapse these branches. Do you have
another option that would be less offensive to compiler folks such as
yourself?