[PATCH v3 5/5] sched/fair: Make EAS wakeup placement consider uclamp restrictions

[Valentin Schneider]

task_fits_capacity() has just been made uclamp-aware, and
find_energy_efficient_cpu() needs to go through the same treatment.

Things are somewhat different here however - using the task max clamp isn't
sufficient. Consider the following setup:

  The target runqueue, rq:
    rq.cpu_capacity_orig = 512
    rq.cfs.avg.util_avg = 200
    rq.uclamp.max = 768 // the max p.uclamp.max of all enqueued p's is 768

  The waking task, p (not yet enqueued on rq):
    p.util_est = 600
    p.uclamp.max = 100

Now, consider the following code which doesn't use the rq clamps:

  util = uclamp_task_util(p);
  // Does the task fit in the spare CPU capacity?
  cpu = cpu_of(rq);
  fits_capacity(util, cpu_capacity(cpu) - cpu_util(cpu))

This would lead to:

  util = 100;
  fits_capacity(100, 512 - 200)

fits_capacity() would return true. However, enqueuing p on that CPU *will*
cause it to become overutilized since rq clamp values are max-aggregated,
so we'd remain with

  rq.uclamp.max = 768

which comes from the other tasks already enqueued on rq. Thus, we could
select a high enough frequency to reach beyond 0.8 * 512 utilization
(== overutilized) after enqueuing p on rq. What find_energy_efficient_cpu()
needs here is uclamp_rq_util_with() which lets us peek at the future
utilization landscape, including rq-wide uclamp values.

Make find_energy_efficient_cpu() use uclamp_rq_util_with() for its
fits_capacity() check. This is in line with what compute_energy() ends up
using for estimating utilization.

Suggested-by: Quentin Perret <qperret@xxxxxxxxxx>
Reviewed-by: Vincent Guittot <vincent.guittot@xxxxxxxxxx>
Signed-off-by: Valentin Schneider <valentin.schneider@xxxxxxx>
---
 kernel/sched/fair.c | 12 ++++++++++--
 1 file changed, 10 insertions(+), 2 deletions(-)

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index a9c93c5427bf..956d9578935a 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -6282,9 +6282,18 @@ static int find_energy_efficient_cpu(struct task_struct *p, int prev_cpu)
 			if (!cpumask_test_cpu(cpu, p->cpus_ptr))
 				continue;
 
-			/* Skip CPUs that will be overutilized. */
 			util = cpu_util_next(cpu, p, cpu);
 			cpu_cap = capacity_of(cpu);
+			spare_cap = cpu_cap - util;
+
+			/*
+			 * Skip CPUs that cannot satisfy the capacity request.
+			 * IOW, placing the task there would make the CPU
+			 * overutilized. Take uclamp into account to see how
+			 * much capacity we can get out of the CPU; this is
+			 * aligned with schedutil_cpu_util().
+			 */
+			util = uclamp_rq_util_with(cpu_rq(cpu), util, p);
 			if (!fits_capacity(util, cpu_cap))
 				continue;
 
@@ -6299,7 +6308,6 @@ static int find_energy_efficient_cpu(struct task_struct *p, int prev_cpu)
 			 * Find the CPU with the maximum spare capacity in
 			 * the performance domain
 			 */
-			spare_cap = cpu_cap - util;
 			if (spare_cap > max_spare_cap) {
 				max_spare_cap = spare_cap;
 				max_spare_cap_cpu = cpu;
-- 
2.24.0