Commit 1d42509e authored Dec 11, 2019 by Valentin Schneider Committed by Ingo Molnar Dec 25, 2019

sched/fair: Make EAS wakeup placement consider uclamp restrictions



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.

Tested-By: Dietmar Eggemann <dietmar.eggemann@arm.com>
Suggested-by: Quentin Perret <qperret@google.com>
Signed-off-by: Valentin Schneider <valentin.schneider@arm.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Reviewed-by: Vincent Guittot <vincent.guittot@linaro.org>
Reviewed-by: Dietmar Eggemann <dietmar.eggemann@arm.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Link: https://lkml.kernel.org/r/20191211113851.24241-6-valentin.schneider@arm.com


Signed-off-by: Ingo Molnar <mingo@kernel.org>

parent a7008c07

kernel/sched/fair.c

+10 −2

Original line number	Diff line number	Diff line
		@@ -6273,9 +6273,18 @@ 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;

		@@ -6290,7 +6299,6 @@ 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;

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