Re: [RFC PATCH 1/2] sched/fair: Introduce UTIL_FITS_CAPACITY feature

[Mathieu Desnoyers]

On 2023-10-19 09:28, Mathieu Desnoyers wrote:
> On 2023-10-19 07:35, Chen Yu wrote:
[...]
> > > +/*
> > > + * Returns true if adding the task utilization to the estimated
> > > + * utilization of the runnable tasks on @cpu does not exceed the
> > > + * capacity of @cpu.
> > > + *
> > > + * This considers only the utilization of _runnable_ tasks on the @cpu
> > > + * runqueue, excluding blocked and sleeping tasks. This is achieved by
> > > + * using the runqueue util_est.enqueued, and by estimating the capacity
> > > + * of @cpu based on arch_scale_cpu_capacity and 
> > > arch_scale_thermal_pressure
> > > + * rather than capacity_of() because capacity_of() considers
> > > + * blocked/sleeping tasks in other scheduler classes.
> > > + *
> > > + * The utilization vs capacity comparison is done without the margin
> > > + * provided by fits_capacity(), because fits_capacity() is used to
> > > + * validate whether the utilization of a task fits within the overall
> > > + * capacity of a cpu, whereas this function validates whether the task
> > > + * utilization fits within the _remaining_ capacity of the cpu, 
> > > which is
> > > + * more precise.
> > > + */
> > > +static inline bool task_fits_remaining_cpu_capacity(unsigned long 
> > > task_util,
> > > +                            int cpu)
> > > +{
> > > +    unsigned long total_util, capacity;
> > > +
> > > +    if (!sched_util_fits_capacity_active())
> > > +        return false;
> > > +    total_util = READ_ONCE(cpu_rq(cpu)->cfs.avg.util_est.enqueued) + 
> > > task_util;
> > > +    capacity = arch_scale_cpu_capacity(cpu) - 
> > > arch_scale_thermal_pressure(cpu);
> >
> > scale_rt_capacity(cpu) could provide the remaining cpu capacity after 
> > substracted by
> > the side activity(rt tasks/thermal pressure/irq time), maybe it would 
> > be more accurate?
>
> AFAIU, scale_rt_capacity(cpu) works similarly to capacity_of(cpu) and 
> considers blocked and sleeping tasks in the rq->avg_rt.util_avg and 
> rq->avg_dl.util_avg. I'm not sure sure about rq->avg_irq.util_avg and 
> thermal_load_avg().
>
> This goes against what is needed here: we need a utilization that only 
> considers enqueued runnable tasks (exluding blocked and sleeping tasks). 
> Or am I missing something ?

I was wrong. Looking more closely at dl and rt sched classes, unlike the 
fair sched class, they don't appear to take into account 
sleeping/blocked tasks in their util_avg. They just accumulate the rq 
util_sum and derive a rq util_avg from it. Likewise for thermal and irq.

So both capacity_of(cpu) and scale_rt_capacity(cpu) would appear to do 
what we need here, but AFAIU capacity_of(cpu) is based on a metric which 
is only updated once per jiffy or so.

Let me try using scale_rt_capacity(cpu) then.

Thanks!

Mathieu

--
Mathieu Desnoyers
EfficiOS Inc.
https://www.efficios.com