LinuxLists.cc - [PATCH v4] sched/fair: unlink misfit task from cpu overutilized

2023-01-19 18:50:56

Subject: [PATCH v4] sched/fair: unlink misfit task from cpu overutilized

By taking into account uclamp_min, the 1:1 relation between task misfit
and cpu overutilized is no more true as a task with a small util_avg may
not fit a high capacity cpu because of uclamp_min constraint.

Add a new state in util_fits_cpu() to reflect the case that task would fit
a CPU except for the uclamp_min hint which is a performance requirement.

Use -1 to reflect that a CPU doesn't fit only because of uclamp_min so we
can use this new value to take additional action to select the best CPU
that doesn't match uclamp_min hint.

Signed-off-by: Vincent Guittot <[email protected]>
---

Change since v3:
- Keep current condition for uclamp_max_fits in util_fits_cpu()
- Update some comments

kernel/sched/fair.c | 105 ++++++++++++++++++++++++++++++++++----------
1 file changed, 82 insertions(+), 23 deletions(-)

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index d4db72f8f84e..54e14da53274 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -4561,8 +4561,8 @@ static inline int util_fits_cpu(unsigned long util,
* handle the case uclamp_min > uclamp_max.
*/
uclamp_min = min(uclamp_min, uclamp_max);
- if (util < uclamp_min && capacity_orig != SCHED_CAPACITY_SCALE)
- fits = fits && (uclamp_min <= capacity_orig_thermal);
+ if (fits && (util < uclamp_min) && (uclamp_min > capacity_orig_thermal))
+ return -1;

return fits;
}
@@ -4572,7 +4572,11 @@ static inline int task_fits_cpu(struct task_struct *p, int cpu)
unsigned long uclamp_min = uclamp_eff_value(p, UCLAMP_MIN);
unsigned long uclamp_max = uclamp_eff_value(p, UCLAMP_MAX);
unsigned long util = task_util_est(p);
- return util_fits_cpu(util, uclamp_min, uclamp_max, cpu);
+ /*
+ * Return true only if the cpu fully fits the task requirements, which
+ * include the utilization but also the performance hints.
+ */
+ return (util_fits_cpu(util, uclamp_min, uclamp_max, cpu) > 0);
}

static inline void update_misfit_status(struct task_struct *p, struct rq *rq)
@@ -6138,6 +6142,7 @@ static inline bool cpu_overutilized(int cpu)
unsigned long rq_util_min = uclamp_rq_get(cpu_rq(cpu), UCLAMP_MIN);
unsigned long rq_util_max = uclamp_rq_get(cpu_rq(cpu), UCLAMP_MAX);

+ /* Return true only if the utilization doesn't fits CPU's capacity */
return !util_fits_cpu(cpu_util_cfs(cpu), rq_util_min, rq_util_max, cpu);
}

@@ -6931,6 +6936,7 @@ static int
select_idle_capacity(struct task_struct *p, struct sched_domain *sd, int target)
{
unsigned long task_util, util_min, util_max, best_cap = 0;
+ int fits, best_fits = 0;
int cpu, best_cpu = -1;
struct cpumask *cpus;

@@ -6946,12 +6952,28 @@ select_idle_capacity(struct task_struct *p, struct sched_domain *sd, int target)

if (!available_idle_cpu(cpu) && !sched_idle_cpu(cpu))
continue;
- if (util_fits_cpu(task_util, util_min, util_max, cpu))
+
+ fits = util_fits_cpu(task_util, util_min, util_max, cpu);
+
+ /* This CPU fits with all requirements */
+ if (fits > 0)
return cpu;
+ /*
+ * Only the min performance hint (i.e. uclamp_min) doesn't fit.
+ * Look for the CPU with best capacity.
+ */
+ else if (fits < 0)
+ cpu_cap = capacity_orig_of(cpu) - thermal_load_avg(cpu_rq(cpu));

- if (cpu_cap > best_cap) {
+ /*
+ * First, select CPU which fits better (-1 being better than 0).
+ * Then, select the one with best capacity at same level.
+ */
+ if ((fits < best_fits) ||
+ ((fits == best_fits) && (cpu_cap > best_cap))) {
best_cap = cpu_cap;
best_cpu = cpu;
+ best_fits = fits;
}
}

@@ -6964,7 +6986,11 @@ static inline bool asym_fits_cpu(unsigned long util,
int cpu)
{
if (sched_asym_cpucap_active())
- return util_fits_cpu(util, util_min, util_max, cpu);
+ /*
+ * Return true only if the cpu fully fits the task requirements
+ * which include the utilization and the performance hints.
+ */
+ return (util_fits_cpu(util, util_min, util_max, cpu) > 0);

return true;
}
@@ -7331,6 +7357,9 @@ static int find_energy_efficient_cpu(struct task_struct *p, int prev_cpu)
unsigned long p_util_max = uclamp_is_used() ? uclamp_eff_value(p, UCLAMP_MAX) : 1024;
struct root_domain *rd = this_rq()->rd;
int cpu, best_energy_cpu, target = -1;
+ int prev_fits = -1, best_fits = -1;
+ unsigned long best_thermal_cap = 0;
+ unsigned long prev_thermal_cap = 0;
struct sched_domain *sd;
struct perf_domain *pd;
struct energy_env eenv;
@@ -7366,6 +7395,7 @@ static int find_energy_efficient_cpu(struct task_struct *p, int prev_cpu)
unsigned long prev_spare_cap = 0;
int max_spare_cap_cpu = -1;
unsigned long base_energy;
+ int fits, max_fits = -1;

cpumask_and(cpus, perf_domain_span(pd), cpu_online_mask);

@@ -7418,7 +7448,9 @@ static int find_energy_efficient_cpu(struct task_struct *p, int prev_cpu)
util_max = max(rq_util_max, p_util_max);
}
}
- if (!util_fits_cpu(util, util_min, util_max, cpu))
+
+ fits = util_fits_cpu(util, util_min, util_max, cpu);
+ if (!fits)
continue;

lsub_positive(&cpu_cap, util);
@@ -7426,7 +7458,9 @@ static int find_energy_efficient_cpu(struct task_struct *p, int prev_cpu)
if (cpu == prev_cpu) {
/* Always use prev_cpu as a candidate. */
prev_spare_cap = cpu_cap;
- } else if (cpu_cap > max_spare_cap) {
+ prev_fits = fits;
+ } else if ((fits > max_fits) ||
+ ((fits == max_fits) && (cpu_cap > max_spare_cap))) {
/*
* Find the CPU with the maximum spare capacity
* among the remaining CPUs in the performance
@@ -7434,6 +7468,7 @@ static int find_energy_efficient_cpu(struct task_struct *p, int prev_cpu)
*/
max_spare_cap = cpu_cap;
max_spare_cap_cpu = cpu;
+ max_fits = fits;
}
}

@@ -7452,26 +7487,50 @@ static int find_energy_efficient_cpu(struct task_struct *p, int prev_cpu)
if (prev_delta < base_energy)
goto unlock;
prev_delta -= base_energy;
+ prev_thermal_cap = cpu_thermal_cap;
best_delta = min(best_delta, prev_delta);
}

/* Evaluate the energy impact of using max_spare_cap_cpu. */
if (max_spare_cap_cpu >= 0 && max_spare_cap > prev_spare_cap) {
+ /* Current best energy cpu fits better */
+ if (max_fits < best_fits)
+ continue;
+
+ /*
+ * Both don't fit performance hint (i.e. uclamp_min)
+ * but best energy cpu has better capacity.
+ */
+ if ((max_fits < 0) &&
+ (cpu_thermal_cap <= best_thermal_cap))
+ continue;
+
cur_delta = compute_energy(&eenv, pd, cpus, p,
max_spare_cap_cpu);
/* CPU utilization has changed */
if (cur_delta < base_energy)
goto unlock;
cur_delta -= base_energy;
- if (cur_delta < best_delta) {
- best_delta = cur_delta;
- best_energy_cpu = max_spare_cap_cpu;
- }
+
+ /*
+ * Both fit for the task but best energy cpu has lower
+ * energy impact.
+ */
+ if ((max_fits > 0) && (best_fits > 0) &&
+ (cur_delta >= best_delta))
+ continue;
+
+ best_delta = cur_delta;
+ best_energy_cpu = max_spare_cap_cpu;
+ best_fits = max_fits;
+ best_thermal_cap = cpu_thermal_cap;
}
}
rcu_read_unlock();

- if (best_delta < prev_delta)
+ if ((best_fits > prev_fits) ||
+ ((best_fits > 0) && (best_delta < prev_delta)) ||
+ ((best_fits < 0) && (best_thermal_cap > prev_thermal_cap)))
target = best_energy_cpu;

return target;
@@ -10265,24 +10324,23 @@ static struct sched_group *find_busiest_group(struct lb_env *env)
*/
update_sd_lb_stats(env, &sds);

- if (sched_energy_enabled()) {
- struct root_domain *rd = env->dst_rq->rd;
-
- if (rcu_dereference(rd->pd) && !READ_ONCE(rd->overutilized))
- goto out_balanced;
- }
-
- local = &sds.local_stat;
- busiest = &sds.busiest_stat;
-
/* There is no busy sibling group to pull tasks from */
if (!sds.busiest)
goto out_balanced;

+ busiest = &sds.busiest_stat;
+
/* Misfit tasks should be dealt with regardless of the avg load */
if (busiest->group_type == group_misfit_task)
goto force_balance;

+ if (sched_energy_enabled()) {
+ struct root_domain *rd = env->dst_rq->rd;
+
+ if (rcu_dereference(rd->pd) && !READ_ONCE(rd->overutilized))
+ goto out_balanced;
+ }
+
/* ASYM feature bypasses nice load balance check */
if (busiest->group_type == group_asym_packing)
goto force_balance;
@@ -10295,6 +10353,7 @@ static struct sched_group *find_busiest_group(struct lb_env *env)
if (busiest->group_type == group_imbalanced)
goto force_balance;

+ local = &sds.local_stat;
/*
* If the local group is busier than the selected busiest group
* don't try and pull any tasks.
--
2.34.1

2023-01-23 12:10:35

by Kajetan Puchalski

[permalink] [raw]

Subject: Re: [PATCH v4] sched/fair: unlink misfit task from cpu overutilized

2023-01-25 01:46:22

by Kajetan Puchalski

[permalink] [raw]

Subject: Re: [PATCH v4] sched/fair: unlink misfit task from cpu overutilized

2023-01-25 08:26:36

by Vincent Guittot

[permalink] [raw]

Subject: Re: [PATCH v4] sched/fair: unlink misfit task from cpu overutilized

2023-01-25 08:37:18

by Vincent Guittot

[permalink] [raw]

On 29/01/2023 17:21, Qais Yousef wrote:
> On 01/26/23 12:42, Dietmar Eggemann wrote:
>> On 19/01/2023 17:42, Vincent Guittot wrote:

[...]

>> We had already this discussion whether this patch can also remove
>> Capacity Inversion (CapInv).
>>
>> After studying the code again, I'm not so sure anymore.
>>
>> This patch:
>>
>> (1) adds a dedicated return value (-1) to util_fits_cpu() when:
>>
>> `util fits 80% capacity_of() && util < uclamp_min && uclamp_min >
>> capacity_orig_thermal (region c)`
>>
>> (2) Enhancements to the CPU selection in sic() and feec() to cater for
>> this new return value.
>
> -1 means that the task fits, but only uclamp_min hint fails. ie: the task util
> is small enough to run on this cpu, but it would like to run faster and this
> cpu can't satisfy this request at the moment.

Agreed.

>> IMHO this doesn't make the intention of CapInv in util_fits_cpu()
>> obsolete, which is using:
>>
>> in CapInv:
>>
>> capacity_orig = capacity_orig_of() - thermal_load_avg
>> capacity_orig_thermal = capacity_orig_of() - thermal_load_avg
>>
>> not in CapInv:
>>
>> capacity_orig = capacity_orig_of()
>> capacity_orig_thermal = capacity_orig_of() - th_pressure
>>
>> Maybe I still miss a bit of the story?
>
> Vincent approach is different to mine. I tried to hide all the complexity in
> util_fits_cpu() so all users don't care.
>
> But with Vincent changes, now the decision is delegated to the caller to decide
> what to do if thermal pressure is causing trouble.
>
> IOW, I expect this line only stay after Vincent patch
>
> capacity_orig_thermal = capacity_orig_of() - th_pressure

OK, makes sense (for now - rework of what capacity_orig_thermal should
really be still ahead of us).

Thanks!

-- Dietmar

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