[PATCH v2 0/1] sched/fair: feec() improvement

[PATCH v2 0/1] sched/fair: feec() improvement

[Pierre Gondois]

v2:
- Dropped 'sched/fair: Use IRQ scaling for all sched classes'
- Added Reviewed-by from Dietmar.

This patch-set aims to improve find_energy_efficient_cpu()
by removing max_spare_cap_cpu from the candidate CPUs if
prev_cpu is already the CPU with the maximum spared capacity.

Pierre Gondois (1):
  sched/fair: Check if prev_cpu has highest spare cap in feec()

 kernel/sched/fair.c | 13 +++++++------
 1 file changed, 7 insertions(+), 6 deletions(-)

-- 
2.25.1

[PATCH v2 1/1] sched/fair: Check if prev_cpu has highest spare cap in feec()

[Pierre Gondois]

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.

Reviewed-by: Dietmar Eggemann <dietmar.eggemann@arm.com>
Signed-off-by: Pierre Gondois <pierre.gondois@arm.com>
---
 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 914096c5b1ae..bcae7bdd5582 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -6900,7 +6900,7 @@ static int find_energy_efficient_cpu(struct task_struct *p, int prev_cpu)
 	for (; pd; pd = pd->next) {
 		unsigned long cpu_cap, cpu_thermal_cap, util;
 		unsigned long cur_delta, max_spare_cap = 0;
-		bool compute_prev_delta = false;
+		unsigned long prev_spare_cap = 0;
 		int max_spare_cap_cpu = -1;
 		unsigned long base_energy;
 
@@ -6944,18 +6944,19 @@ 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);
@@ -6963,7 +6964,7 @@ 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 */
@@ -6974,7 +6975,7 @@ 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 */
-- 
2.25.1

Re: [PATCH v2 1/1] sched/fair: Check if prev_cpu has highest spare cap in feec()

[Vincent Guittot]

On Thu, 6 Oct 2022 at 10:11, Pierre Gondois <pierre.gondois@arm.com> wrote:
>
> 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.
>
> Reviewed-by: Dietmar Eggemann <dietmar.eggemann@arm.com>

nit: the reviewed tag should be after the signed-off

> Signed-off-by: Pierre Gondois <pierre.gondois@arm.com>

Reviewed-by: Vincent Guittot <vincent.guittot@linaro.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 914096c5b1ae..bcae7bdd5582 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -6900,7 +6900,7 @@ static int find_energy_efficient_cpu(struct task_struct *p, int prev_cpu)
>         for (; pd; pd = pd->next) {
>                 unsigned long cpu_cap, cpu_thermal_cap, util;
>                 unsigned long cur_delta, max_spare_cap = 0;
> -               bool compute_prev_delta = false;
> +               unsigned long prev_spare_cap = 0;
>                 int max_spare_cap_cpu = -1;
>                 unsigned long base_energy;
>
> @@ -6944,18 +6944,19 @@ 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);
> @@ -6963,7 +6964,7 @@ 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 */
> @@ -6974,7 +6975,7 @@ 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 */
> --
> 2.25.1
>

[tip: sched/core] sched/fair: Check if prev_cpu has highest spare cap in feec()

[tip-bot2 for Pierre Gondois]

The following commit has been merged into the sched/core branch of tip:

Commit-ID:     ad841e569f5c88e3332b32a000f251f33ff32187
Gitweb:        https://git.kernel.org/tip/ad841e569f5c88e3332b32a000f251f33ff32187
Author:        Pierre Gondois <pierre.gondois@arm.com>
AuthorDate:    Thu, 06 Oct 2022 10:10:52 +02:00
Committer:     Peter Zijlstra <peterz@infradead.org>
CommitterDate: Thu, 27 Oct 2022 11:01:20 +02:00

sched/fair: Check if prev_cpu has highest spare cap in feec()

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
---
 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 919d016..4cc56c9 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -7221,7 +7221,7 @@ 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;
 
@@ -7283,18 +7283,19 @@ 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);
@@ -7302,7 +7303,7 @@ 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 */
@@ -7313,7 +7314,7 @@ 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 */