From: Pierre Gondois pierre.gondois@arm.com
[ Upstream commit ad841e569f5c88e3332b32a000f251f33ff32187 ]
When evaluating the CPU candidates in the perf domain (pd) containing the previously used CPU (prev_cpu), find_energy_efficient_cpu() evaluates the energy of the pd: - without the task (base_energy) - with the task placed on prev_cpu (if the task fits) - with the task placed on the CPU with the highest spare capacity, prev_cpu being excluded from this set
If prev_cpu is already the CPU with the highest spare capacity, max_spare_cap_cpu will be the CPU with the second highest spare capacity.
On an Arm64 Juno-r2, with a workload of 10 tasks at a 10% duty cycle, when prev_cpu and max_spare_cap_cpu are both valid candidates, prev_spare_cap > max_spare_cap at ~82%. Thus the energy of the pd when placing the task on max_spare_cap_cpu is computed with no possible positive outcome 82% most of the time.
Do not consider max_spare_cap_cpu as a valid candidate if prev_spare_cap > max_spare_cap.
Signed-off-by: Pierre Gondois pierre.gondois@arm.com Signed-off-by: Peter Zijlstra (Intel) peterz@infradead.org Reviewed-by: Dietmar Eggemann dietmar.eggemann@arm.com Reviewed-by: Vincent Guittot vincent.guittot@linaro.org Link: https://lore.kernel.org/r/20221006081052.3862167-2-pierre.gondois@arm.com Stable-dep-of: e26fd28db828 ("sched/uclamp: Fix a uninitialized variable warnings") Signed-off-by: Sasha Levin sashal@kernel.org --- kernel/sched/fair.c | 13 +++++++------ 1 file changed, 7 insertions(+), 6 deletions(-)
diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c index 0f32acb05055..bb04ca795fc3 100644 --- a/kernel/sched/fair.c +++ b/kernel/sched/fair.c @@ -7217,7 +7217,7 @@ static int find_energy_efficient_cpu(struct task_struct *p, int prev_cpu) unsigned long cur_delta, max_spare_cap = 0; unsigned long rq_util_min, rq_util_max; unsigned long util_min, util_max; - bool compute_prev_delta = false; + unsigned long prev_spare_cap = 0; int max_spare_cap_cpu = -1; unsigned long base_energy;
@@ -7279,18 +7279,19 @@ static int find_energy_efficient_cpu(struct task_struct *p, int prev_cpu)
if (cpu == prev_cpu) { /* Always use prev_cpu as a candidate. */ - compute_prev_delta = true; + prev_spare_cap = cpu_cap; } else if (cpu_cap > max_spare_cap) { /* * Find the CPU with the maximum spare capacity - * in the performance domain. + * among the remaining CPUs in the performance + * domain. */ max_spare_cap = cpu_cap; max_spare_cap_cpu = cpu; } }
- if (max_spare_cap_cpu < 0 && !compute_prev_delta) + if (max_spare_cap_cpu < 0 && prev_spare_cap == 0) continue;
eenv_pd_busy_time(&eenv, cpus, p); @@ -7298,7 +7299,7 @@ static int find_energy_efficient_cpu(struct task_struct *p, int prev_cpu) base_energy = compute_energy(&eenv, pd, cpus, p, -1);
/* Evaluate the energy impact of using prev_cpu. */ - if (compute_prev_delta) { + if (prev_spare_cap > 0) { prev_delta = compute_energy(&eenv, pd, cpus, p, prev_cpu); /* CPU utilization has changed */ @@ -7309,7 +7310,7 @@ static int find_energy_efficient_cpu(struct task_struct *p, int prev_cpu) }
/* Evaluate the energy impact of using max_spare_cap_cpu. */ - if (max_spare_cap_cpu >= 0) { + if (max_spare_cap_cpu >= 0 && max_spare_cap > prev_spare_cap) { cur_delta = compute_energy(&eenv, pd, cpus, p, max_spare_cap_cpu); /* CPU utilization has changed */