[PATCH 0/3] Fix a couple of corner cases in feec() when using uclamp_max

[PATCH 0/3] Fix a couple of corner cases in feec() when using uclamp_max

[Qais Yousef]

Patch 1 addresses a bug because forcing a task on a small CPU to honour
uclamp_max hint means we can end up with spare_capacity = 0; but the logic is
constructed such that spare_capacity = 0 leads to ignoring this CPU as
a candidate to compute_energy().

Patch 2 addresses a bug due to an optimization in feec() that could lead to
ignoring tasks whose uclamp_max = 0 but task_util(0) != 0.

Patch 3 adds a new tracepoint in compute_energy() as it was helpful in
debugging these two problems.

This is based on tip/sched/core + Vincent's v4 of
Unlink util_fits_cpu()... patch [1]

[1] https://lore.kernel.org/lkml/20230119174244.2059628-1-vincent.guittot@linaro.org/

Qais Yousef (3):
  sched/uclamp: Set max_spare_cap_cpu even if max_spare_cap is 0
  sched/uclamp: Ignore (util == 0) optimization in feec() when
    p_util_max = 0
  sched/tp: Add new tracepoint to track compute energy computation

 include/trace/events/sched.h |  4 ++++
 kernel/sched/core.c          |  1 +
 kernel/sched/fair.c          | 23 +++++++++++++++--------
 3 files changed, 20 insertions(+), 8 deletions(-)

-- 
2.25.1

[PATCH 1/3] sched/uclamp: Set max_spare_cap_cpu even if max_spare_cap is 0

[Qais Yousef]

When uclamp_max is being used, the util of the task could be higher than
the spare capacity of the CPU, but due to uclamp_max value we force fit
it there.

The way the condition for checking for max_spare_cap in
find_energy_efficient_cpu() was constructed; it ignored any CPU that has
its spare_cap less than or _equal_ to max_spare_cap. Since we initialize
max_spare_cap to 0; this lead to never setting max_spare_cap_cpu and
hence ending up never performing compute_energy() for this cluster and
missing an opportunity for a better energy efficient placement to honour
uclamp_max setting.

	max_spare_cap = 0;
	cpu_cap = capacity_of(cpu) - task_util(p);  // 0 if task_util(p) is high

	...

	util_fits_cpu(...);		// will return true if uclamp_max forces it to fit

	...

	// this logic will fail to update max_spare_cap_cpu if cpu_cap is 0
	if (cpu_cap > max_spare_cap) {
		max_spare_cap = cpu_cap;
		max_spare_cap_cpu = cpu;
	}

prev_spare_cap suffers from a similar problem.

Fix the logic by treating -1UL value as 'not populated' instead of
0 which is a viable and correct spare capacity value.

Fixes: 1d42509e475c ("sched/fair: Make EAS wakeup placement consider uclamp restrictions")
Signed-off-by: Qais Yousef (Google) <qyousef@layalina.io>
---
 kernel/sched/fair.c | 14 ++++++++------
 1 file changed, 8 insertions(+), 6 deletions(-)

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index e29e9ea4cde8..ca2c389d3180 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -7390,9 +7390,9 @@ static int find_energy_efficient_cpu(struct task_struct *p, int prev_cpu)
 	for (; pd; pd = pd->next) {
 		unsigned long util_min = p_util_min, util_max = p_util_max;
 		unsigned long cpu_cap, cpu_thermal_cap, util;
-		unsigned long cur_delta, max_spare_cap = 0;
+		unsigned long cur_delta, max_spare_cap = -1UL;
 		unsigned long rq_util_min, rq_util_max;
-		unsigned long prev_spare_cap = 0;
+		unsigned long prev_spare_cap = -1UL;
 		int max_spare_cap_cpu = -1;
 		unsigned long base_energy;
 		int fits, max_fits = -1;
@@ -7457,7 +7457,8 @@ static int find_energy_efficient_cpu(struct task_struct *p, int prev_cpu)
 				prev_spare_cap = cpu_cap;
 				prev_fits = fits;
 			} else if ((fits > max_fits) ||
-				   ((fits == max_fits) && (cpu_cap > max_spare_cap))) {
+				   ((fits == max_fits) &&
+				   (cpu_cap > max_spare_cap || max_spare_cap == -1UL) {
 				/*
 				 * Find the CPU with the maximum spare capacity
 				 * among the remaining CPUs in the performance
@@ -7469,7 +7470,7 @@ static int find_energy_efficient_cpu(struct task_struct *p, int prev_cpu)
 			}
 		}
 
-		if (max_spare_cap_cpu < 0 && prev_spare_cap == 0)
+		if (max_spare_cap_cpu < 0 && prev_spare_cap == -1UL)
 			continue;
 
 		eenv_pd_busy_time(&eenv, cpus, p);
@@ -7477,7 +7478,7 @@ 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 (prev_spare_cap > 0) {
+		if (prev_spare_cap != -1UL) {
 			prev_delta = compute_energy(&eenv, pd, cpus, p,
 						    prev_cpu);
 			/* CPU utilization has changed */
@@ -7489,7 +7490,8 @@ 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 && max_spare_cap > prev_spare_cap) {
+		if (max_spare_cap_cpu >= 0 &&
+		    (max_spare_cap > prev_spare_cap || prev_spare_cap == -1UL)) {
 			/* Current best energy cpu fits better */
 			if (max_fits < best_fits)
 				continue;
-- 
2.25.1

[PATCH 2/3] sched/uclamp: Ignore (util == 0) optimization in feec() when p_util_max = 0

[Qais Yousef]

find_energy_efficient_cpu() bails out early if effective util of the
task is 0. When uclamp is being used, this could lead to wrong decisions
when uclamp_max is set to 0. Cater for that.

Fixes: d81304bc6193 ("sched/uclamp: Cater for uclamp in find_energy_efficient_cpu()'s early exit condition")
Signed-off-by: Qais Yousef (Google) <qyousef@layalina.io>
---
 kernel/sched/fair.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index ca2c389d3180..3521aad67aa0 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -7382,7 +7382,7 @@ static int find_energy_efficient_cpu(struct task_struct *p, int prev_cpu)
 	target = prev_cpu;
 
 	sync_entity_load_avg(&p->se);
-	if (!uclamp_task_util(p, p_util_min, p_util_max))
+	if (!uclamp_task_util(p, p_util_min, p_util_max) && p_util_max != 0)
 		goto unlock;
 
 	eenv_task_busy_time(&eenv, p, prev_cpu);
-- 
2.25.1

[PATCH 3/3] sched/tp: Add new tracepoint to track compute energy computation

[Qais Yousef]

It was useful to track feec() placement decision and debug the spare
capacity and optimization issues vs uclamp_max.

Signed-off-by: Qais Yousef (Google) <qyousef@layalina.io>
---
 include/trace/events/sched.h | 4 ++++
 kernel/sched/core.c          | 1 +
 kernel/sched/fair.c          | 7 ++++++-
 3 files changed, 11 insertions(+), 1 deletion(-)

diff --git a/include/trace/events/sched.h b/include/trace/events/sched.h
index fbb99a61f714..20cc884f72ff 100644
--- a/include/trace/events/sched.h
+++ b/include/trace/events/sched.h
@@ -735,6 +735,10 @@ DECLARE_TRACE(sched_update_nr_running_tp,
 	TP_PROTO(struct rq *rq, int change),
 	TP_ARGS(rq, change));
 
+DECLARE_TRACE(sched_compute_energy_tp,
+	TP_PROTO(struct task_struct *p, int dst_cpu, unsigned long energy),
+	TP_ARGS(p, dst_cpu, energy));
+
 #endif /* _TRACE_SCHED_H */
 
 /* This part must be outside protection */
diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index 03b8529db73f..79ad6b8ea93e 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -110,6 +110,7 @@ EXPORT_TRACEPOINT_SYMBOL_GPL(sched_overutilized_tp);
 EXPORT_TRACEPOINT_SYMBOL_GPL(sched_util_est_cfs_tp);
 EXPORT_TRACEPOINT_SYMBOL_GPL(sched_util_est_se_tp);
 EXPORT_TRACEPOINT_SYMBOL_GPL(sched_update_nr_running_tp);
+EXPORT_TRACEPOINT_SYMBOL_GPL(sched_compute_energy_tp);
 
 DEFINE_PER_CPU_SHARED_ALIGNED(struct rq, runqueues);
 
diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index 3521aad67aa0..3b55dbb0fcfe 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -7303,11 +7303,16 @@ compute_energy(struct energy_env *eenv, struct perf_domain *pd,
 {
 	unsigned long max_util = eenv_pd_max_util(eenv, pd_cpus, p, dst_cpu);
 	unsigned long busy_time = eenv->pd_busy_time;
+	unsigned long energy;
 
 	if (dst_cpu >= 0)
 		busy_time = min(eenv->pd_cap, busy_time + eenv->task_busy_time);
 
-	return em_cpu_energy(pd->em_pd, max_util, busy_time, eenv->cpu_cap);
+	energy = em_cpu_energy(pd->em_pd, max_util, busy_time, eenv->cpu_cap);
+
+	trace_sched_compute_energy_tp(p, dst_cpu, energy);
+
+	return energy;
 }
 
 /*
-- 
2.25.1

Re: [PATCH 1/3] sched/uclamp: Set max_spare_cap_cpu even if max_spare_cap is 0

[Qais Yousef]

On 01/29/23 16:14, Qais Yousef wrote:
> When uclamp_max is being used, the util of the task could be higher than
> the spare capacity of the CPU, but due to uclamp_max value we force fit
> it there.
> 
> The way the condition for checking for max_spare_cap in
> find_energy_efficient_cpu() was constructed; it ignored any CPU that has
> its spare_cap less than or _equal_ to max_spare_cap. Since we initialize
> max_spare_cap to 0; this lead to never setting max_spare_cap_cpu and
> hence ending up never performing compute_energy() for this cluster and
> missing an opportunity for a better energy efficient placement to honour
> uclamp_max setting.
> 
> 	max_spare_cap = 0;
> 	cpu_cap = capacity_of(cpu) - task_util(p);  // 0 if task_util(p) is high
> 
> 	...
> 
> 	util_fits_cpu(...);		// will return true if uclamp_max forces it to fit
> 
> 	...
> 
> 	// this logic will fail to update max_spare_cap_cpu if cpu_cap is 0
> 	if (cpu_cap > max_spare_cap) {
> 		max_spare_cap = cpu_cap;
> 		max_spare_cap_cpu = cpu;
> 	}
> 
> prev_spare_cap suffers from a similar problem.
> 
> Fix the logic by treating -1UL value as 'not populated' instead of
> 0 which is a viable and correct spare capacity value.
> 
> Fixes: 1d42509e475c ("sched/fair: Make EAS wakeup placement consider uclamp restrictions")
> Signed-off-by: Qais Yousef (Google) <qyousef@layalina.io>
> ---
>  kernel/sched/fair.c | 14 ++++++++------
>  1 file changed, 8 insertions(+), 6 deletions(-)
> 
> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index e29e9ea4cde8..ca2c389d3180 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -7390,9 +7390,9 @@ static int find_energy_efficient_cpu(struct task_struct *p, int prev_cpu)
>  	for (; pd; pd = pd->next) {
>  		unsigned long util_min = p_util_min, util_max = p_util_max;
>  		unsigned long cpu_cap, cpu_thermal_cap, util;
> -		unsigned long cur_delta, max_spare_cap = 0;
> +		unsigned long cur_delta, max_spare_cap = -1UL;
>  		unsigned long rq_util_min, rq_util_max;
> -		unsigned long prev_spare_cap = 0;
> +		unsigned long prev_spare_cap = -1UL;
>  		int max_spare_cap_cpu = -1;
>  		unsigned long base_energy;
>  		int fits, max_fits = -1;
> @@ -7457,7 +7457,8 @@ static int find_energy_efficient_cpu(struct task_struct *p, int prev_cpu)
>  				prev_spare_cap = cpu_cap;
>  				prev_fits = fits;
>  			} else if ((fits > max_fits) ||
> -				   ((fits == max_fits) && (cpu_cap > max_spare_cap))) {
> +				   ((fits == max_fits) &&
> +				   (cpu_cap > max_spare_cap || max_spare_cap == -1UL) {

Oops. Sorry I just realized I bodged this while rebasing and preparing the
patches for posting. There are missing termination parenthesis that will cause
compilation errors.

Apologies..


--
Qais Yousef

>  				/*
>  				 * Find the CPU with the maximum spare capacity
>  				 * among the remaining CPUs in the performance
> @@ -7469,7 +7470,7 @@ static int find_energy_efficient_cpu(struct task_struct *p, int prev_cpu)
>  			}
>  		}
>  
> -		if (max_spare_cap_cpu < 0 && prev_spare_cap == 0)
> +		if (max_spare_cap_cpu < 0 && prev_spare_cap == -1UL)
>  			continue;
>  
>  		eenv_pd_busy_time(&eenv, cpus, p);
> @@ -7477,7 +7478,7 @@ 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 (prev_spare_cap > 0) {
> +		if (prev_spare_cap != -1UL) {
>  			prev_delta = compute_energy(&eenv, pd, cpus, p,
>  						    prev_cpu);
>  			/* CPU utilization has changed */
> @@ -7489,7 +7490,8 @@ 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 && max_spare_cap > prev_spare_cap) {
> +		if (max_spare_cap_cpu >= 0 &&
> +		    (max_spare_cap > prev_spare_cap || prev_spare_cap == -1UL)) {
>  			/* Current best energy cpu fits better */
>  			if (max_fits < best_fits)
>  				continue;
> -- 
> 2.25.1
> 

Re: [PATCH 1/3] sched/uclamp: Set max_spare_cap_cpu even if max_spare_cap is 0

[Vincent Guittot]

On Sun, 29 Jan 2023 at 17:14, Qais Yousef <qyousef@layalina.io> wrote:
>
> When uclamp_max is being used, the util of the task could be higher than
> the spare capacity of the CPU, but due to uclamp_max value we force fit
> it there.
>
> The way the condition for checking for max_spare_cap in
> find_energy_efficient_cpu() was constructed; it ignored any CPU that has
> its spare_cap less than or _equal_ to max_spare_cap. Since we initialize
> max_spare_cap to 0; this lead to never setting max_spare_cap_cpu and
> hence ending up never performing compute_energy() for this cluster and
> missing an opportunity for a better energy efficient placement to honour
> uclamp_max setting.
>
>         max_spare_cap = 0;
>         cpu_cap = capacity_of(cpu) - task_util(p);  // 0 if task_util(p) is high
>
>         ...
>
>         util_fits_cpu(...);             // will return true if uclamp_max forces it to fit
>
>         ...
>
>         // this logic will fail to update max_spare_cap_cpu if cpu_cap is 0
>         if (cpu_cap > max_spare_cap) {
>                 max_spare_cap = cpu_cap;
>                 max_spare_cap_cpu = cpu;
>         }
>
> prev_spare_cap suffers from a similar problem.
>
> Fix the logic by treating -1UL value as 'not populated' instead of
> 0 which is a viable and correct spare capacity value.
>
> Fixes: 1d42509e475c ("sched/fair: Make EAS wakeup placement consider uclamp restrictions")
> Signed-off-by: Qais Yousef (Google) <qyousef@layalina.io>
> ---
>  kernel/sched/fair.c | 14 ++++++++------
>  1 file changed, 8 insertions(+), 6 deletions(-)
>
> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index e29e9ea4cde8..ca2c389d3180 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -7390,9 +7390,9 @@ static int find_energy_efficient_cpu(struct task_struct *p, int prev_cpu)
>         for (; pd; pd = pd->next) {
>                 unsigned long util_min = p_util_min, util_max = p_util_max;
>                 unsigned long cpu_cap, cpu_thermal_cap, util;
> -               unsigned long cur_delta, max_spare_cap = 0;
> +               unsigned long cur_delta, max_spare_cap = -1UL;
>                 unsigned long rq_util_min, rq_util_max;
> -               unsigned long prev_spare_cap = 0;
> +               unsigned long prev_spare_cap = -1UL;
>                 int max_spare_cap_cpu = -1;
>                 unsigned long base_energy;
>                 int fits, max_fits = -1;
> @@ -7457,7 +7457,8 @@ static int find_energy_efficient_cpu(struct task_struct *p, int prev_cpu)
>                                 prev_spare_cap = cpu_cap;
>                                 prev_fits = fits;
>                         } else if ((fits > max_fits) ||
> -                                  ((fits == max_fits) && (cpu_cap > max_spare_cap))) {
> +                                  ((fits == max_fits) &&
> +                                  (cpu_cap > max_spare_cap || max_spare_cap == -1UL) {

Can't we use a signed comparison to include the case of max_spare_cap
== -1 in cpu_cap > max_spare_cap ?

>                                 /*
>                                  * Find the CPU with the maximum spare capacity
>                                  * among the remaining CPUs in the performance
> @@ -7469,7 +7470,7 @@ static int find_energy_efficient_cpu(struct task_struct *p, int prev_cpu)
>                         }
>                 }
>
> -               if (max_spare_cap_cpu < 0 && prev_spare_cap == 0)
> +               if (max_spare_cap_cpu < 0 && prev_spare_cap == -1UL)
>                         continue;
>
>                 eenv_pd_busy_time(&eenv, cpus, p);
> @@ -7477,7 +7478,7 @@ 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 (prev_spare_cap > 0) {
> +               if (prev_spare_cap != -1UL) {
>                         prev_delta = compute_energy(&eenv, pd, cpus, p,
>                                                     prev_cpu);
>                         /* CPU utilization has changed */
> @@ -7489,7 +7490,8 @@ 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 && max_spare_cap > prev_spare_cap) {
> +               if (max_spare_cap_cpu >= 0 &&
> +                   (max_spare_cap > prev_spare_cap || prev_spare_cap == -1UL)) {
>                         /* Current best energy cpu fits better */
>                         if (max_fits < best_fits)
>                                 continue;
> --
> 2.25.1
>

Re: [PATCH 1/3] sched/uclamp: Set max_spare_cap_cpu even if max_spare_cap is 0

[Qais Yousef]

On 01/30/23 15:44, Vincent Guittot wrote:
> On Sun, 29 Jan 2023 at 17:14, Qais Yousef <qyousef@layalina.io> wrote:
> >
> > When uclamp_max is being used, the util of the task could be higher than
> > the spare capacity of the CPU, but due to uclamp_max value we force fit
> > it there.
> >
> > The way the condition for checking for max_spare_cap in
> > find_energy_efficient_cpu() was constructed; it ignored any CPU that has
> > its spare_cap less than or _equal_ to max_spare_cap. Since we initialize
> > max_spare_cap to 0; this lead to never setting max_spare_cap_cpu and
> > hence ending up never performing compute_energy() for this cluster and
> > missing an opportunity for a better energy efficient placement to honour
> > uclamp_max setting.
> >
> >         max_spare_cap = 0;
> >         cpu_cap = capacity_of(cpu) - task_util(p);  // 0 if task_util(p) is high
> >
> >         ...
> >
> >         util_fits_cpu(...);             // will return true if uclamp_max forces it to fit
> >
> >         ...
> >
> >         // this logic will fail to update max_spare_cap_cpu if cpu_cap is 0
> >         if (cpu_cap > max_spare_cap) {
> >                 max_spare_cap = cpu_cap;
> >                 max_spare_cap_cpu = cpu;
> >         }
> >
> > prev_spare_cap suffers from a similar problem.
> >
> > Fix the logic by treating -1UL value as 'not populated' instead of
> > 0 which is a viable and correct spare capacity value.
> >
> > Fixes: 1d42509e475c ("sched/fair: Make EAS wakeup placement consider uclamp restrictions")
> > Signed-off-by: Qais Yousef (Google) <qyousef@layalina.io>
> > ---
> >  kernel/sched/fair.c | 14 ++++++++------
> >  1 file changed, 8 insertions(+), 6 deletions(-)
> >
> > diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> > index e29e9ea4cde8..ca2c389d3180 100644
> > --- a/kernel/sched/fair.c
> > +++ b/kernel/sched/fair.c
> > @@ -7390,9 +7390,9 @@ static int find_energy_efficient_cpu(struct task_struct *p, int prev_cpu)
> >         for (; pd; pd = pd->next) {
> >                 unsigned long util_min = p_util_min, util_max = p_util_max;
> >                 unsigned long cpu_cap, cpu_thermal_cap, util;
> > -               unsigned long cur_delta, max_spare_cap = 0;
> > +               unsigned long cur_delta, max_spare_cap = -1UL;
> >                 unsigned long rq_util_min, rq_util_max;
> > -               unsigned long prev_spare_cap = 0;
> > +               unsigned long prev_spare_cap = -1UL;
> >                 int max_spare_cap_cpu = -1;
> >                 unsigned long base_energy;
> >                 int fits, max_fits = -1;
> > @@ -7457,7 +7457,8 @@ static int find_energy_efficient_cpu(struct task_struct *p, int prev_cpu)
> >                                 prev_spare_cap = cpu_cap;
> >                                 prev_fits = fits;
> >                         } else if ((fits > max_fits) ||
> > -                                  ((fits == max_fits) && (cpu_cap > max_spare_cap))) {
> > +                                  ((fits == max_fits) &&
> > +                                  (cpu_cap > max_spare_cap || max_spare_cap == -1UL) {
> 
> Can't we use a signed comparison to include the case of max_spare_cap
> == -1 in cpu_cap > max_spare_cap ?

By converting max_spare_cap to long, right?

My memory could be failing me, but I seem to remember we had mixed usage and
consolidated into unsigned long. That's why I didn't want to break the trend.

Anyway. If no one shouts against that, I don't mind going for that.


Thanks

--
Qais Yousef

> 
> >                                 /*
> >                                  * Find the CPU with the maximum spare capacity
> >                                  * among the remaining CPUs in the performance
> > @@ -7469,7 +7470,7 @@ static int find_energy_efficient_cpu(struct task_struct *p, int prev_cpu)
> >                         }
> >                 }
> >
> > -               if (max_spare_cap_cpu < 0 && prev_spare_cap == 0)
> > +               if (max_spare_cap_cpu < 0 && prev_spare_cap == -1UL)
> >                         continue;
> >
> >                 eenv_pd_busy_time(&eenv, cpus, p);
> > @@ -7477,7 +7478,7 @@ 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 (prev_spare_cap > 0) {
> > +               if (prev_spare_cap != -1UL) {
> >                         prev_delta = compute_energy(&eenv, pd, cpus, p,
> >                                                     prev_cpu);
> >                         /* CPU utilization has changed */
> > @@ -7489,7 +7490,8 @@ 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 && max_spare_cap > prev_spare_cap) {
> > +               if (max_spare_cap_cpu >= 0 &&
> > +                   (max_spare_cap > prev_spare_cap || prev_spare_cap == -1UL)) {
> >                         /* Current best energy cpu fits better */
> >                         if (max_fits < best_fits)
> >                                 continue;
> > --
> > 2.25.1
> >