Re: [PATCH] sched/core: forced idle accounting

[Peter Zijlstra]

On Mon, Oct 11, 2021 at 05:12:43PM -0700, Josh Don wrote:

> > > +             if (WARN_ON_ONCE(!nr_running)) {
> > > +                     /* can't be forced idle without a running task */
> > > +             } else {
> > > +                     delta *= nr_forced_idle;
> > > +                     delta /= nr_running;
> > > +             }
> >
> > Now the comment sayeth:
> >
> > > +     /*
> > > +      * For larger SMT configurations, we need to scale the charged
> > > +      * forced idle amount since there can be more than one forced idle
> > > +      * sibling and more than one running cookied task.
> > > +      */
> >
> > But why?
> 
> We scale by the number of cpus actually forced idle, since we don't
> want to falsely over or under charge forced idle time (defined
> strictly as time where we have a runnable task but idle the cpu). The
> more important scaling here though is the division over the number of
> running entities. This is done so that the aggregate amount of forced
> idle over some group of threads makes sense. Ie if we have a cpu with
> SMT8, and a group of 7 threads sharing a cookie, we don't want to
> accrue 7 units of forced idle time per unit time while the 8th SMT is
> forced idle.

So why not simply compute the strict per-cpu force-idle time and let
userspace sort out the rest?