From: Vincent Donnefort vincent.donnefort@arm.com
[ Upstream commit 0372e1cf70c28de6babcba38ef97b6ae3400b101 ]
find_energy_efficient_cpu() (feec()) computes for each perf_domain (pd) an energy delta as follows:
feec(task) for_each_pd base_energy = compute_energy(task, -1, pd) -> for_each_cpu(pd) -> cpu_util_next(cpu, task, -1)
energy_delta = compute_energy(task, dst_cpu, pd) -> for_each_cpu(pd) -> cpu_util_next(cpu, task, dst_cpu) energy_delta -= base_energy
Then it picks the best CPU as being the one that minimizes energy_delta.
cpu_util_next() estimates the CPU utilization that would happen if the task was placed on dst_cpu as follows:
max(cpu_util + task_util, cpu_util_est + _task_util_est)
The task contribution to the energy delta can then be either:
(1) _task_util_est, on a mostly idle CPU, where cpu_util is close to 0 and _task_util_est > cpu_util. (2) task_util, on a mostly busy CPU, where cpu_util > _task_util_est.
(cpu_util_est doesn't appear here. It is 0 when a CPU is idle and otherwise must be small enough so that feec() takes the CPU as a potential target for the task placement)
This is problematic for feec(), as cpu_util_next() might give an unfair advantage to a CPU which is mostly busy (2) compared to one which is mostly idle (1). _task_util_est being always bigger than task_util in feec() (as the task is waking up), the task contribution to the energy might look smaller on certain CPUs (2) and this breaks the energy comparison.
This issue is, moreover, not sporadic. By starving idle CPUs, it keeps their cpu_util < _task_util_est (1) while others will maintain cpu_util > _task_util_est (2).
Fix this problem by always using max(task_util, _task_util_est) as a task contribution to the energy (ENERGY_UTIL). The new estimated CPU utilization for the energy would then be:
max(cpu_util, cpu_util_est) + max(task_util, _task_util_est)
compute_energy() still needs to know which OPP would be selected if the task would be migrated in the perf_domain (FREQUENCY_UTIL). Hence, cpu_util_next() is still used to estimate the maximum util within the pd.
Signed-off-by: Vincent Donnefort vincent.donnefort@arm.com Signed-off-by: Peter Zijlstra (Intel) peterz@infradead.org Signed-off-by: Ingo Molnar mingo@kernel.org Reviewed-by: Quentin Perret qperret@google.com Reviewed-by: Dietmar Eggemann dietmar.eggemann@arm.com Link: https://lkml.kernel.org/r/20210225083612.1113823-2-vincent.donnefort@arm.com Signed-off-by: Sasha Levin sashal@kernel.org --- kernel/sched/fair.c | 24 ++++++++++++++++++++---- 1 file changed, 20 insertions(+), 4 deletions(-)
diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c index 794c2cb945f8..e3c2dcb1b015 100644 --- a/kernel/sched/fair.c +++ b/kernel/sched/fair.c @@ -6518,8 +6518,24 @@ compute_energy(struct task_struct *p, int dst_cpu, struct perf_domain *pd) * its pd list and will not be accounted by compute_energy(). */ for_each_cpu_and(cpu, pd_mask, cpu_online_mask) { - unsigned long cpu_util, util_cfs = cpu_util_next(cpu, p, dst_cpu); - struct task_struct *tsk = cpu == dst_cpu ? p : NULL; + unsigned long util_freq = cpu_util_next(cpu, p, dst_cpu); + unsigned long cpu_util, util_running = util_freq; + struct task_struct *tsk = NULL; + + /* + * When @p is placed on @cpu: + * + * util_running = max(cpu_util, cpu_util_est) + + * max(task_util, _task_util_est) + * + * while cpu_util_next is: max(cpu_util + task_util, + * cpu_util_est + _task_util_est) + */ + if (cpu == dst_cpu) { + tsk = p; + util_running = + cpu_util_next(cpu, p, -1) + task_util_est(p); + }
/* * Busy time computation: utilization clamping is not @@ -6527,7 +6543,7 @@ compute_energy(struct task_struct *p, int dst_cpu, struct perf_domain *pd) * is already enough to scale the EM reported power * consumption at the (eventually clamped) cpu_capacity. */ - sum_util += effective_cpu_util(cpu, util_cfs, cpu_cap, + sum_util += effective_cpu_util(cpu, util_running, cpu_cap, ENERGY_UTIL, NULL);
/* @@ -6537,7 +6553,7 @@ compute_energy(struct task_struct *p, int dst_cpu, struct perf_domain *pd) * NOTE: in case RT tasks are running, by default the * FREQUENCY_UTIL's utilization can be max OPP. */ - cpu_util = effective_cpu_util(cpu, util_cfs, cpu_cap, + cpu_util = effective_cpu_util(cpu, util_freq, cpu_cap, FREQUENCY_UTIL, tsk); max_util = max(max_util, cpu_util); }