sched/fair: Consider capacity inversion in util_fits_cpu()

We do consider thermal pressure in util_fits_cpu() for uclamp_min only.
With the exception of the biggest cores which by definition are the max
performance point of the system and all tasks by definition should fit.

Even under thermal pressure, the capacity of the biggest CPU is the
highest in the system and should still fit every task. Except when it
reaches capacity inversion point, then this is no longer true.

We can handle this by using the inverted capacity as capacity_orig in
util_fits_cpu(). Which not only addresses the problem above, but also
ensure uclamp_max now considers the inverted capacity. Force fitting
a task when a CPU is in this adverse state will contribute to making the
thermal throttling last longer.

Signed-off-by: Qais Yousef <[email protected]>
Signed-off-by: Peter Zijlstra (Intel) <[email protected]>
Link: https://lore.kernel.org/r/[email protected]

Author: Qais Yousef

kernel/sched/fair.c

@@ -4465,12 +4465,16 @@ static inline int util_fits_cpu(unsigned long util,
 	 * For uclamp_max, we can tolerate a drop in performance level as the
 	 * goal is to cap the task. So it's okay if it's getting less.
 	 *
-	 * In case of capacity inversion, which is not handled yet, we should
-	 * honour the inverted capacity for both uclamp_min and uclamp_max all
-	 * the time.
+	 * In case of capacity inversion we should honour the inverted capacity
+	 * for both uclamp_min and uclamp_max all the time.
 	 */
-	capacity_orig = capacity_orig_of(cpu);
-	capacity_orig_thermal = capacity_orig - arch_scale_thermal_pressure(cpu);
+	capacity_orig = cpu_in_capacity_inversion(cpu);
+	if (capacity_orig) {
+		capacity_orig_thermal = capacity_orig;
+	} else {
+		capacity_orig = capacity_orig_of(cpu);
+		capacity_orig_thermal = capacity_orig - arch_scale_thermal_pressure(cpu);
+	}
 
 	/*
 	 * We want to force a task to fit a cpu as implied by uclamp_max.