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

[Vincent Guittot]

On Sun, 5 Feb 2023 at 23:43, Qais Yousef <qyousef@xxxxxxxxxxx> 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@xxxxxxxxxxx>
> ---
>  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

> --
> 2.25.1
>