Re: [PATCH v2 1/3] sched/uclamp: Set max_spare_cap_cpu even if max_spare_cap is 0

[Qais Yousef <qyousef@layalina.io>]

On 02/07/23 10:45, Vincent Guittot wrote:
> On Sun, 5 Feb 2023 at 23:43, Qais Yousef <qyousef@layalina.io> wrote:
> >
> > When uclamp_max is being used, the util of the task could be higher than
> > the spare capacity of the CPU, but due to uclamp_max value we force fit
> > it there.
> >
> > The way the condition for checking for max_spare_cap in
> > find_energy_efficient_cpu() was constructed; it ignored any CPU that has
> > its spare_cap less than or _equal_ to max_spare_cap. Since we initialize
> > max_spare_cap to 0; this lead to never setting max_spare_cap_cpu and
> > hence ending up never performing compute_energy() for this cluster and
> > missing an opportunity for a better energy efficient placement to honour
> > uclamp_max setting.
> >
> >         max_spare_cap = 0;
> >         cpu_cap = capacity_of(cpu) - task_util(p);  // 0 if task_util(p) is high
> >
> >         ...
> >
> >         util_fits_cpu(...);             // will return true if uclamp_max forces it to fit
> >
> >         ...
> >
> >         // this logic will fail to update max_spare_cap_cpu if cpu_cap is 0
> >         if (cpu_cap > max_spare_cap) {
> >                 max_spare_cap = cpu_cap;
> >                 max_spare_cap_cpu = cpu;
> >         }
> >
> > prev_spare_cap suffers from a similar problem.
> >
> > Fix the logic by converting the variables into long and treating -1
> > value as 'not populated' instead of 0 which is a viable and correct
> > spare capacity value.
> >
> > Fixes: 1d42509e475c ("sched/fair: Make EAS wakeup placement consider uclamp restrictions")
> > Signed-off-by: Qais Yousef (Google) <qyousef@layalina.io>
> > ---
> >  kernel/sched/fair.c | 9 ++++-----
> >  1 file changed, 4 insertions(+), 5 deletions(-)
> >
> > diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> > index c6c8e7f52935..7a21ee74139f 100644
> > --- a/kernel/sched/fair.c
> > +++ b/kernel/sched/fair.c
> > @@ -7382,11 +7382,10 @@ static int find_energy_efficient_cpu(struct task_struct *p, int prev_cpu)
> >         for (; pd; pd = pd->next) {
> >                 unsigned long util_min = p_util_min, util_max = p_util_max;
> >                 unsigned long cpu_cap, cpu_thermal_cap, util;
> > -               unsigned long cur_delta, max_spare_cap = 0;
> > +               long prev_spare_cap = -1, max_spare_cap = -1;
> >                 unsigned long rq_util_min, rq_util_max;
> > -               unsigned long prev_spare_cap = 0;
> > +               unsigned long cur_delta, base_energy;
> >                 int max_spare_cap_cpu = -1;
> > -               unsigned long base_energy;
> >                 int fits, max_fits = -1;
> >
> >                 cpumask_and(cpus, perf_domain_span(pd), cpu_online_mask);
> > @@ -7461,7 +7460,7 @@ static int find_energy_efficient_cpu(struct task_struct *p, int prev_cpu)
> >                         }
> >                 }
> >
> > -               if (max_spare_cap_cpu < 0 && prev_spare_cap == 0)
> > +               if (max_spare_cap_cpu < 0 && prev_spare_cap < 0)
> >                         continue;
> >
> >                 eenv_pd_busy_time(&eenv, cpus, p);
> > @@ -7469,7 +7468,7 @@ static int find_energy_efficient_cpu(struct task_struct *p, int prev_cpu)
> >                 base_energy = compute_energy(&eenv, pd, cpus, p, -1);
> >
> >                 /* Evaluate the energy impact of using prev_cpu. */
> > -               if (prev_spare_cap > 0) {
> > +               if (prev_spare_cap > -1) {
> >                         prev_delta = compute_energy(&eenv, pd, cpus, p,
> >                                                     prev_cpu);
> >                         /* CPU utilization has changed */
> 
> I think that you also need the change below to make sure that the
> signed comparison will be used. I have quickly checked the assembly
> code for aarch64 and your patch keeps using unsigned comparison (b.ls)
>    ((fits == max_fits) && (cpu_cap > max_spare_cap))) {
> ffff8000080e4c94: f94067e0 ldr x0, [sp, #200]
> ffff8000080e4c98: eb00003f cmp x1, x0
> ffff8000080e4c9c: 54ff98a9 b.ls ffff8000080e3fb0
> <select_task_rq_fair+0x570>  // b.plast
> 
> Whereas the change below make it to use the signed version (b.le)
>    ((fits == max_fits) && ((long)cpu_cap > max_spare_cap))) {
> ffff8000080e4c94: f94067e0 ldr x0, [sp, #200]
> ffff8000080e4c98: eb00003f cmp x1, x0
> ffff8000080e4c9c: 54ff98ad b.le ffff8000080e3fb0 <select_task_rq_fair+0x570>
> 
> -- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -7522,7 +7522,7 @@ static int find_energy_efficient_cpu(struct
> task_struct *p, int prev_cpu)
>                                 prev_spare_cap = cpu_cap;
>                                 prev_fits = fits;
>                         } else if ((fits > max_fits) ||
> -                                  ((fits == max_fits) && (cpu_cap >
> max_spare_cap))) {
> +                                  ((fits == max_fits) &&
> ((long)cpu_cap > max_spare_cap))) {
>                                 /*
>                                  * Find the CPU with the maximum spare capacity
>                                  * among the remaining CPUs in the performance

Isn't it better to go back to v1 form then? The inconsistent type paired with
the cast isn't getting too ugly for me :(

I don't think we can convert cpu_cap to long without having to do more work as
it is used with 'util'.


Cheers

--
Qais Yousef