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Subject[tip: sched/core] sched/fair: Make EAS wakeup placement consider uclamp restrictions
The following commit has been merged into the sched/core branch of tip:

Commit-ID: 1d42509e475cdc8542aa5b3e03a7e845244f4f57
Gitweb: https://git.kernel.org/tip/1d42509e475cdc8542aa5b3e03a7e845244f4f57
Author: Valentin Schneider <valentin.schneider@arm.com>
AuthorDate: Wed, 11 Dec 2019 11:38:51
Committer: Ingo Molnar <mingo@kernel.org>
CommitterDate: Wed, 25 Dec 2019 10:42:09 +01:00

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>
---
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 26c59bc..2d170b5 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -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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