Messages in this thread Patch in this message | | | From | Valentin Schneider <> | Subject | [PATCH v3 5/5] sched/fair: Make EAS wakeup placement consider uclamp restrictions | Date | Wed, 11 Dec 2019 11:38:51 +0000 |
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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@google.com> Reviewed-by: Vincent Guittot <vincent.guittot@linaro.org> Signed-off-by: Valentin Schneider <valentin.schneider@arm.com> --- 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
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