[PATCH v6 0/2] sched: EAS changes for EM complexity and sysctl

[PATCH v6 0/2] sched: EAS changes for EM complexity and sysctl

[Shrikanth Hegde]

In Brief:
[PATCH v6 1/2] sched/topology: Remove EM_MAX_COMPLEXITY limit
Since the EAS complexity was greatly reduced, bigger platforms can
handle EAS. To reflect this improvement, remove the EAS complexity check.

[PATCH v6 2/2] sched/topology: change behaviour of sysctl sched_energy_aware
based on the platform
Depending on the platform sysctl will either enable/disable EAS and NOP
in case if EAS is not supported.

Patchset contains these two patches. Second patch depends on the first
patch to be applied first.

v5->v6:
Valentin pointed out that sd_asym_cpucapacity is under rcu and to avoid
writing to sysctl if EAS is not possible. Did these two.
v4->v5:
sched_is_eas_possible missed handling of case when EM complexity was high.
Dietmar suggested that there was work done already which removes these
checks. Since it makes sched_is_eas_possible cleaner, picked up that
patch along with v4 and made it as a patchset.
Instead of using first CPU in cpu_active_mask, doing a simple loop across
all CPU in cpu_active_mask to check if there is any asymmetric CPU
capacities since it was breaking EAS capabilities over CPUSET islands.
v3->v4:
valentin suggested it would be better to consider simpler approach that
was mentioned in v2. It is a standard approach to keep the knob visible
but change how read and write are handled. Did that and Refactored the
code to use a common function in build_perf_domains and in sysctl handler.
v2->v3:
Chen Yu and Pierre Gondois both pointed out that if platform becomes
capable of EAS later, this patch was not allowing that to happen.
Addressed that by using a variable to indicate the sysctl change
and re-worded the commit message with desired behaviour,
v1->v2:
Chen Yu had pointed out that this will not destroy the perf domains on
architectures where EAS is supported by changing the sysctl.
[v1] Link: https://lore.kernel.org/lkml/20230829065040.920629-1-sshegde@linux.vnet.ibm.com/
[v2] Link: https://lore.kernel.org/lkml/20230901065249.137242-1-sshegde@linux.vnet.ibm.com/
[v3] Link: https://lore.kernel.org/lkml/20230913114807.665094-1-sshegde@linux.vnet.ibm.com/
[v4] Link: https://lore.kernel.org/lkml/20230926100046.405188-1-sshegde@linux.vnet.ibm.com/
[v5] Link: https://lore.kernel.org/lkml/20230929155209.667764-1-sshegde@linux.vnet.ibm.com/

Pierre Gondois (1):
  sched/topology: Remove EM_MAX_COMPLEXITY limit

Shrikanth Hegde (1):
  sched/topology: change behaviour of sysctl sched_energy_aware based on
    the platform

 Documentation/admin-guide/sysctl/kernel.rst |   3 +-
 Documentation/scheduler/sched-energy.rst    |  29 +---
 kernel/sched/topology.c                     | 151 ++++++++++----------
 3 files changed, 82 insertions(+), 101 deletions(-)

--
2.31.1

[PATCH v6 1/2] sched/topology: Remove EM_MAX_COMPLEXITY limit

[Shrikanth Hegde]

From: Pierre Gondois <Pierre.Gondois@arm.com>

The Energy Aware Scheduler (EAS) estimates the energy consumption
of placing a task on different CPUs. The goal is to minimize this
energy consumption. Estimating the energy of different task placements
is increasingly complex with the size of the platform. To avoid having
a slow wake-up path, EAS is only enabled if this complexity is low
enough.

The current complexity limit was set in:
commit b68a4c0dba3b1 ("sched/topology: Disable EAS on inappropriate
platforms").
base on the first implementation of EAS, which was re-computing
the power of the whole platform for each task placement scenario, cf:
commit 390031e4c309 ("sched/fair: Introduce an energy estimation helper
function").
but the complexity of EAS was reduced in:
commit eb92692b2544d ("sched/fair: Speed-up energy-aware wake-ups")
and find_energy_efficient_cpu() (feec) algorithm was updated in:
commit 3e8c6c9aac42 ("sched/fair: Remove task_util from effective
utilization in feec()")

find_energy_efficient_cpu() (feec) is now doing:
feec()
\_ for_each_pd(pd) [0]
  // get max_spare_cap_cpu and compute_prev_delta
  \_ for_each_cpu(pd) [1]

  \_ eenv_pd_busy_time(pd) [2]
	\_ for_each_cpu(pd)

  // compute_energy(pd) without the task
  \_ eenv_pd_max_util(pd, -1) [3.0]
    \_ for_each_cpu(pd)
  \_ em_cpu_energy(pd, -1)
    \_ for_each_ps(pd)

  // compute_energy(pd) with the task on prev_cpu
  \_ eenv_pd_max_util(pd, prev_cpu) [3.1]
    \_ for_each_cpu(pd)
  \_ em_cpu_energy(pd, prev_cpu)
    \_ for_each_ps(pd)

  // compute_energy(pd) with the task on max_spare_cap_cpu
  \_ eenv_pd_max_util(pd, max_spare_cap_cpu) [3.2]
    \_ for_each_cpu(pd)
  \_ em_cpu_energy(pd, max_spare_cap_cpu)
    \_ for_each_ps(pd)

[3.1] happens only once since prev_cpu is unique. With the same
definitions for nr_pd, nr_cpus and nr_ps, the complexity is of:
nr_pd * (2 * [nr_cpus in pd] + 2 * ([nr_cpus in pd] + [nr_ps in pd]))
+ ([nr_cpus in pd] + [nr_ps in pd])

 [0]  * (     [1] + [2]      +       [3.0] + [3.2]                  )
+ [3.1]

= nr_pd * (4 * [nr_cpus in pd] + 2 * [nr_ps in pd])
+ [nr_cpus in prev pd] + nr_ps

The complexity limit was set to 2048 in:
commit b68a4c0dba3b1 ("sched/topology: Disable EAS on inappropriate
platforms")
to make "EAS usable up to 16 CPUs with per-CPU DVFS and less than 8
performance states each". For the same platform, the complexity would
actually be of:
16 * (4 + 2 * 7) + 1 + 7 = 296

Since the EAS complexity was greatly reduced, bigger platforms can
handle EAS. For instance, a platform with 112 CPUs with 7 performance
states each would not reach it:
112 * (4 + 2 * 7) + 1 + 7 = 2024

To reflect this improvement, remove the EAS complexity check.
Note that a limit on the number of CPUs still holds against
EM_MAX_NUM_CPUS to avoid overflows during the energy estimation.

Signed-off-by: Pierre Gondois <Pierre.Gondois@arm.com>
Reviewed-by: Lukasz Luba <lukasz.luba@arm.com>
Reviewed-by: Dietmar Eggemann <dietmar.eggemann@arm.com>
---
 Documentation/scheduler/sched-energy.rst | 29 ++----------------
 kernel/sched/topology.c                  | 39 ++----------------------
 2 files changed, 6 insertions(+), 62 deletions(-)

diff --git a/Documentation/scheduler/sched-energy.rst b/Documentation/scheduler/sched-energy.rst
index fc853c8cc346..70e2921ef725 100644
--- a/Documentation/scheduler/sched-energy.rst
+++ b/Documentation/scheduler/sched-energy.rst
@@ -359,32 +359,9 @@ in milli-Watts or in an 'abstract scale'.
 6.3 - Energy Model complexity
 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

-The task wake-up path is very latency-sensitive. When the EM of a platform is
-too complex (too many CPUs, too many performance domains, too many performance
-states, ...), the cost of using it in the wake-up path can become prohibitive.
-The energy-aware wake-up algorithm has a complexity of:
-
-	C = Nd * (Nc + Ns)
-
-with: Nd the number of performance domains; Nc the number of CPUs; and Ns the
-total number of OPPs (ex: for two perf. domains with 4 OPPs each, Ns = 8).
-
-A complexity check is performed at the root domain level, when scheduling
-domains are built. EAS will not start on a root domain if its C happens to be
-higher than the completely arbitrary EM_MAX_COMPLEXITY threshold (2048 at the
-time of writing).
-
-If you really want to use EAS but the complexity of your platform's Energy
-Model is too high to be used with a single root domain, you're left with only
-two possible options:
-
-    1. split your system into separate, smaller, root domains using exclusive
-       cpusets and enable EAS locally on each of them. This option has the
-       benefit to work out of the box but the drawback of preventing load
-       balance between root domains, which can result in an unbalanced system
-       overall;
-    2. submit patches to reduce the complexity of the EAS wake-up algorithm,
-       hence enabling it to cope with larger EMs in reasonable time.
+EAS does not impose any complexity limit on the number of PDs/OPPs/CPUs but
+restricts the number of CPUs to EM_MAX_NUM_CPUS to prevent overflows during
+the energy estimation.


 6.4 - Schedutil governor
diff --git a/kernel/sched/topology.c b/kernel/sched/topology.c
index a7b50bba7829..e0b9920e7e3e 100644
--- a/kernel/sched/topology.c
+++ b/kernel/sched/topology.c
@@ -348,32 +348,13 @@ static void sched_energy_set(bool has_eas)
  *    1. an Energy Model (EM) is available;
  *    2. the SD_ASYM_CPUCAPACITY flag is set in the sched_domain hierarchy.
  *    3. no SMT is detected.
- *    4. the EM complexity is low enough to keep scheduling overheads low;
- *    5. schedutil is driving the frequency of all CPUs of the rd;
- *    6. frequency invariance support is present;
- *
- * The complexity of the Energy Model is defined as:
- *
- *              C = nr_pd * (nr_cpus + nr_ps)
- *
- * with parameters defined as:
- *  - nr_pd:    the number of performance domains
- *  - nr_cpus:  the number of CPUs
- *  - nr_ps:    the sum of the number of performance states of all performance
- *              domains (for example, on a system with 2 performance domains,
- *              with 10 performance states each, nr_ps = 2 * 10 = 20).
- *
- * It is generally not a good idea to use such a model in the wake-up path on
- * very complex platforms because of the associated scheduling overheads. The
- * arbitrary constraint below prevents that. It makes EAS usable up to 16 CPUs
- * with per-CPU DVFS and less than 8 performance states each, for example.
+ *    4. schedutil is driving the frequency of all CPUs of the rd;
+ *    5. frequency invariance support is present;
  */
-#define EM_MAX_COMPLEXITY 2048
-
 extern struct cpufreq_governor schedutil_gov;
 static bool build_perf_domains(const struct cpumask *cpu_map)
 {
-	int i, nr_pd = 0, nr_ps = 0, nr_cpus = cpumask_weight(cpu_map);
+	int i;
 	struct perf_domain *pd = NULL, *tmp;
 	int cpu = cpumask_first(cpu_map);
 	struct root_domain *rd = cpu_rq(cpu)->rd;
@@ -431,20 +412,6 @@ static bool build_perf_domains(const struct cpumask *cpu_map)
 			goto free;
 		tmp->next = pd;
 		pd = tmp;
-
-		/*
-		 * Count performance domains and performance states for the
-		 * complexity check.
-		 */
-		nr_pd++;
-		nr_ps += em_pd_nr_perf_states(pd->em_pd);
-	}
-
-	/* Bail out if the Energy Model complexity is too high. */
-	if (nr_pd * (nr_ps + nr_cpus) > EM_MAX_COMPLEXITY) {
-		WARN(1, "rd %*pbl: Failed to start EAS, EM complexity is too high\n",
-						cpumask_pr_args(cpu_map));
-		goto free;
 	}

 	perf_domain_debug(cpu_map, pd);
--
2.31.1

Re: [PATCH v6 1/2] sched/topology: Remove EM_MAX_COMPLEXITY limit

[Ingo Molnar]

* Shrikanth Hegde <sshegde@linux.vnet.ibm.com> wrote:

> From: Pierre Gondois <Pierre.Gondois@arm.com>
> 
> The Energy Aware Scheduler (EAS) estimates the energy consumption
> of placing a task on different CPUs. The goal is to minimize this
> energy consumption. Estimating the energy of different task placements
> is increasingly complex with the size of the platform. To avoid having
> a slow wake-up path, EAS is only enabled if this complexity is low
> enough.

...

> To reflect this improvement, remove the EAS complexity check.
> Note that a limit on the number of CPUs still holds against
> EM_MAX_NUM_CPUS to avoid overflows during the energy estimation.
> 
> Signed-off-by: Pierre Gondois <Pierre.Gondois@arm.com>
> Reviewed-by: Lukasz Luba <lukasz.luba@arm.com>
> Reviewed-by: Dietmar Eggemann <dietmar.eggemann@arm.com>
> ---
>  Documentation/scheduler/sched-energy.rst | 29 ++----------------
>  kernel/sched/topology.c                  | 39 ++----------------------
>  2 files changed, 6 insertions(+), 62 deletions(-)

I've applied this patch to tip:sched/core, for a v6.7 merge, thanks!

	Ingo

[tip: sched/core] sched/topology: Remove the EM_MAX_COMPLEXITY limit

[tip-bot2 for Pierre Gondois]

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

Commit-ID:     5b77261c5510f1e6f4d359e97dd3e39ee7259c3d
Gitweb:        https://git.kernel.org/tip/5b77261c5510f1e6f4d359e97dd3e39ee7259c3d
Author:        Pierre Gondois <Pierre.Gondois@arm.com>
AuthorDate:    Mon, 09 Oct 2023 11:30:36 +05:30
Committer:     Ingo Molnar <mingo@kernel.org>
CommitterDate: Mon, 09 Oct 2023 13:07:27 +02:00

sched/topology: Remove the EM_MAX_COMPLEXITY limit

The Energy Aware Scheduler (EAS) estimates the energy consumption
of placing a task on different CPUs. The goal is to minimize this
energy consumption. Estimating the energy of different task placements
is increasingly complex with the size of the platform.

To avoid having a slow wake-up path, EAS is only enabled if this
complexity is low enough.

The current complexity limit was set in:

  b68a4c0dba3b1 ("sched/topology: Disable EAS on inappropriate platforms")

... based on the first implementation of EAS, which was re-computing
the power of the whole platform for each task placement scenario, see:

  390031e4c309 ("sched/fair: Introduce an energy estimation helper function")

... but the complexity of EAS was reduced in:

  eb92692b2544d ("sched/fair: Speed-up energy-aware wake-ups")

... and find_energy_efficient_cpu() (feec) algorithm was updated in:

  3e8c6c9aac42 ("sched/fair: Remove task_util from effective utilization in feec()")

find_energy_efficient_cpu() (feec) is now doing:

	feec()
	\_ for_each_pd(pd) [0]
	  // get max_spare_cap_cpu and compute_prev_delta
	  \_ for_each_cpu(pd) [1]

	  \_ eenv_pd_busy_time(pd) [2]
		\_ for_each_cpu(pd)

	  // compute_energy(pd) without the task
	  \_ eenv_pd_max_util(pd, -1) [3.0]
	    \_ for_each_cpu(pd)
	  \_ em_cpu_energy(pd, -1)
	    \_ for_each_ps(pd)

	  // compute_energy(pd) with the task on prev_cpu
	  \_ eenv_pd_max_util(pd, prev_cpu) [3.1]
	    \_ for_each_cpu(pd)
	  \_ em_cpu_energy(pd, prev_cpu)
	    \_ for_each_ps(pd)

	  // compute_energy(pd) with the task on max_spare_cap_cpu
	  \_ eenv_pd_max_util(pd, max_spare_cap_cpu) [3.2]
	    \_ for_each_cpu(pd)
	  \_ em_cpu_energy(pd, max_spare_cap_cpu)
	    \_ for_each_ps(pd)

	[3.1] happens only once since prev_cpu is unique. With the same
	      definitions for nr_pd, nr_cpus and nr_ps, the complexity is of:

		nr_pd * (2 * [nr_cpus in pd] + 2 * ([nr_cpus in pd] + [nr_ps in pd]))
		+ ([nr_cpus in pd] + [nr_ps in pd])

		 [0]  * (     [1] + [2]      +       [3.0] + [3.2]                  )
		+ [3.1]

		= nr_pd * (4 * [nr_cpus in pd] + 2 * [nr_ps in pd])
		+ [nr_cpus in prev pd] + nr_ps

The complexity limit was set to 2048 in:

  b68a4c0dba3b1 ("sched/topology: Disable EAS on inappropriate platforms")

... to make "EAS usable up to 16 CPUs with per-CPU DVFS and less than 8
performance states each". For the same platform, the complexity would
actually be of:

  16 * (4 + 2 * 7) + 1 + 7 = 296

Since the EAS complexity was greatly reduced since the limit was
introduced, bigger platforms can handle EAS.

For instance, a platform with 112 CPUs with 7 performance states
each would not reach it:

  112 * (4 + 2 * 7) + 1 + 7 = 2024

To reflect this improvement in the underlying EAS code, remove
the EAS complexity check.

Note that a limit on the number of CPUs still holds against
EM_MAX_NUM_CPUS to avoid overflows during the energy estimation.

[ mingo: Updates to the changelog. ]

Signed-off-by: Pierre Gondois <Pierre.Gondois@arm.com>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Reviewed-by: Lukasz Luba <lukasz.luba@arm.com>
Reviewed-by: Dietmar Eggemann <dietmar.eggemann@arm.com>
Link: https://lore.kernel.org/r/20231009060037.170765-2-sshegde@linux.vnet.ibm.com
---
 Documentation/scheduler/sched-energy.rst | 29 +----------------
 kernel/sched/topology.c                  | 39 +----------------------
 2 files changed, 6 insertions(+), 62 deletions(-)

diff --git a/Documentation/scheduler/sched-energy.rst b/Documentation/scheduler/sched-energy.rst
index fc853c8..70e2921 100644
--- a/Documentation/scheduler/sched-energy.rst
+++ b/Documentation/scheduler/sched-energy.rst
@@ -359,32 +359,9 @@ in milli-Watts or in an 'abstract scale'.
 6.3 - Energy Model complexity
 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
 
-The task wake-up path is very latency-sensitive. When the EM of a platform is
-too complex (too many CPUs, too many performance domains, too many performance
-states, ...), the cost of using it in the wake-up path can become prohibitive.
-The energy-aware wake-up algorithm has a complexity of:
-
-	C = Nd * (Nc + Ns)
-
-with: Nd the number of performance domains; Nc the number of CPUs; and Ns the
-total number of OPPs (ex: for two perf. domains with 4 OPPs each, Ns = 8).
-
-A complexity check is performed at the root domain level, when scheduling
-domains are built. EAS will not start on a root domain if its C happens to be
-higher than the completely arbitrary EM_MAX_COMPLEXITY threshold (2048 at the
-time of writing).
-
-If you really want to use EAS but the complexity of your platform's Energy
-Model is too high to be used with a single root domain, you're left with only
-two possible options:
-
-    1. split your system into separate, smaller, root domains using exclusive
-       cpusets and enable EAS locally on each of them. This option has the
-       benefit to work out of the box but the drawback of preventing load
-       balance between root domains, which can result in an unbalanced system
-       overall;
-    2. submit patches to reduce the complexity of the EAS wake-up algorithm,
-       hence enabling it to cope with larger EMs in reasonable time.
+EAS does not impose any complexity limit on the number of PDs/OPPs/CPUs but
+restricts the number of CPUs to EM_MAX_NUM_CPUS to prevent overflows during
+the energy estimation.
 
 
 6.4 - Schedutil governor
diff --git a/kernel/sched/topology.c b/kernel/sched/topology.c
index 1cc5959..fcda3f0 100644
--- a/kernel/sched/topology.c
+++ b/kernel/sched/topology.c
@@ -348,32 +348,13 @@ static void sched_energy_set(bool has_eas)
  *    1. an Energy Model (EM) is available;
  *    2. the SD_ASYM_CPUCAPACITY flag is set in the sched_domain hierarchy.
  *    3. no SMT is detected.
- *    4. the EM complexity is low enough to keep scheduling overheads low;
- *    5. schedutil is driving the frequency of all CPUs of the rd;
- *    6. frequency invariance support is present;
- *
- * The complexity of the Energy Model is defined as:
- *
- *              C = nr_pd * (nr_cpus + nr_ps)
- *
- * with parameters defined as:
- *  - nr_pd:    the number of performance domains
- *  - nr_cpus:  the number of CPUs
- *  - nr_ps:    the sum of the number of performance states of all performance
- *              domains (for example, on a system with 2 performance domains,
- *              with 10 performance states each, nr_ps = 2 * 10 = 20).
- *
- * It is generally not a good idea to use such a model in the wake-up path on
- * very complex platforms because of the associated scheduling overheads. The
- * arbitrary constraint below prevents that. It makes EAS usable up to 16 CPUs
- * with per-CPU DVFS and less than 8 performance states each, for example.
+ *    4. schedutil is driving the frequency of all CPUs of the rd;
+ *    5. frequency invariance support is present;
  */
-#define EM_MAX_COMPLEXITY 2048
-
 extern struct cpufreq_governor schedutil_gov;
 static bool build_perf_domains(const struct cpumask *cpu_map)
 {
-	int i, nr_pd = 0, nr_ps = 0, nr_cpus = cpumask_weight(cpu_map);
+	int i;
 	struct perf_domain *pd = NULL, *tmp;
 	int cpu = cpumask_first(cpu_map);
 	struct root_domain *rd = cpu_rq(cpu)->rd;
@@ -431,20 +412,6 @@ static bool build_perf_domains(const struct cpumask *cpu_map)
 			goto free;
 		tmp->next = pd;
 		pd = tmp;
-
-		/*
-		 * Count performance domains and performance states for the
-		 * complexity check.
-		 */
-		nr_pd++;
-		nr_ps += em_pd_nr_perf_states(pd->em_pd);
-	}
-
-	/* Bail out if the Energy Model complexity is too high. */
-	if (nr_pd * (nr_ps + nr_cpus) > EM_MAX_COMPLEXITY) {
-		WARN(1, "rd %*pbl: Failed to start EAS, EM complexity is too high\n",
-						cpumask_pr_args(cpu_map));
-		goto free;
 	}
 
 	perf_domain_debug(cpu_map, pd);

[PATCH v6 2/2] sched/topology: change behaviour of sysctl sched_energy_aware based on the platform

[Shrikanth Hegde]

sysctl sched_energy_aware is available for the admin to disable/enable
energy aware scheduling(EAS). EAS is enabled only if few conditions are
met by the platform. They are, asymmetric CPU capacity, no SMT,
schedutil CPUfreq governor, frequency invariant load tracking etc.
A platform may boot without EAS capability, but could gain such
capability at runtime. For example, changing/registering the CPUfreq
governor to schedutil.

At present, though platform doesn't support EAS, this sysctl returns 1
and it ends up calling build_perf_domains on write to 1 and
NOP when writing to 0. That is confusing and un-necessary.

Desired behavior would be to have this sysctl to enable/disable the EAS
on supported platform. On non-supported platform write to the sysctl
would return not supported error and read of the sysctl would return
empty. So
sched_energy_aware returns empty - EAS is not possible at this moment
This will include EAS capable platforms which have at least one EAS
condition false during startup, e.g. not using the schedutil CPUfreq governor
sched_energy_aware returns 0 - EAS is supported but disabled by admin.
sched_energy_aware returns 1 - EAS is supported and enabled.

User can find out the reason why EAS is not possible by checking
info messages. sched_is_eas_possible returns true if the platform
can do EAS at this moment.

Tested-by: Pierre Gondois <pierre.gondois@arm.com>
Signed-off-by: Shrikanth Hegde <sshegde@linux.vnet.ibm.com>
---
 Documentation/admin-guide/sysctl/kernel.rst |   3 +-
 kernel/sched/topology.c                     | 112 +++++++++++++-------
 2 files changed, 76 insertions(+), 39 deletions(-)

diff --git a/Documentation/admin-guide/sysctl/kernel.rst b/Documentation/admin-guide/sysctl/kernel.rst
index cf33de56da27..d89ac2bd8dc4 100644
--- a/Documentation/admin-guide/sysctl/kernel.rst
+++ b/Documentation/admin-guide/sysctl/kernel.rst
@@ -1182,7 +1182,8 @@ automatically on platforms where it can run (that is,
 platforms with asymmetric CPU topologies and having an Energy
 Model available). If your platform happens to meet the
 requirements for EAS but you do not want to use it, change
-this value to 0.
+this value to 0. On Non-EAS platforms, write operation fails and
+read doesn't return anything.

 task_delayacct
 ===============
diff --git a/kernel/sched/topology.c b/kernel/sched/topology.c
index e0b9920e7e3e..dcf8c3558b01 100644
--- a/kernel/sched/topology.c
+++ b/kernel/sched/topology.c
@@ -212,6 +212,70 @@ static unsigned int sysctl_sched_energy_aware = 1;
 static DEFINE_MUTEX(sched_energy_mutex);
 static bool sched_energy_update;

+extern struct cpufreq_governor schedutil_gov;
+static bool sched_is_eas_possible(const struct cpumask *cpu_mask)
+{
+	bool any_asym_capacity = false;
+	struct cpufreq_policy *policy;
+	struct cpufreq_governor *gov;
+	int i;
+
+	/* EAS is enabled for asymmetric CPU capacity topologies. */
+	for_each_cpu(i, cpu_mask) {
+		if (rcu_access_pointer(per_cpu(sd_asym_cpucapacity, i))) {
+			any_asym_capacity = true;
+			break;
+		}
+	}
+	if (!any_asym_capacity) {
+		if (sched_debug()) {
+			pr_info("rd %*pbl: Checking EAS, CPUs do not have asymmetric capacities\n",
+				cpumask_pr_args(cpu_mask));
+		}
+		return false;
+	}
+
+	/* EAS definitely does *not* handle SMT */
+	if (sched_smt_active()) {
+		if (sched_debug()) {
+			pr_info("rd %*pbl: Checking EAS, SMT is not supported\n",
+				cpumask_pr_args(cpu_mask));
+		}
+		return false;
+	}
+
+	if (!arch_scale_freq_invariant()) {
+		if (sched_debug()) {
+			pr_info("rd %*pbl: Checking EAS: frequency-invariant load tracking not yet supported",
+				cpumask_pr_args(cpu_mask));
+		}
+		return false;
+	}
+
+	/* Do not attempt EAS if schedutil is not being used. */
+	for_each_cpu(i, cpu_mask) {
+		policy = cpufreq_cpu_get(i);
+		if (!policy) {
+			if (sched_debug()) {
+				pr_info("rd %*pbl: Checking EAS, cpufreq policy not set for CPU: %d",
+					cpumask_pr_args(cpu_mask), i);
+			}
+			return false;
+		}
+		gov = policy->governor;
+		cpufreq_cpu_put(policy);
+		if (gov != &schedutil_gov) {
+			if (sched_debug()) {
+				pr_info("rd %*pbl: Checking EAS, schedutil is mandatory\n",
+					cpumask_pr_args(cpu_mask));
+			}
+			return false;
+		}
+	}
+
+	return true;
+}
+
 void rebuild_sched_domains_energy(void)
 {
 	mutex_lock(&sched_energy_mutex);
@@ -230,6 +294,15 @@ static int sched_energy_aware_handler(struct ctl_table *table, int write,
 	if (write && !capable(CAP_SYS_ADMIN))
 		return -EPERM;

+	if (!sched_is_eas_possible(cpu_active_mask)) {
+		if (write) {
+			return -EOPNOTSUPP;
+		} else {
+			*lenp = 0;
+			return 0;
+		}
+	}
+
 	ret = proc_dointvec_minmax(table, write, buffer, lenp, ppos);
 	if (!ret && write) {
 		state = static_branch_unlikely(&sched_energy_present);
@@ -351,61 +424,24 @@ static void sched_energy_set(bool has_eas)
  *    4. schedutil is driving the frequency of all CPUs of the rd;
  *    5. frequency invariance support is present;
  */
-extern struct cpufreq_governor schedutil_gov;
 static bool build_perf_domains(const struct cpumask *cpu_map)
 {
 	int i;
 	struct perf_domain *pd = NULL, *tmp;
 	int cpu = cpumask_first(cpu_map);
 	struct root_domain *rd = cpu_rq(cpu)->rd;
-	struct cpufreq_policy *policy;
-	struct cpufreq_governor *gov;

 	if (!sysctl_sched_energy_aware)
 		goto free;

-	/* EAS is enabled for asymmetric CPU capacity topologies. */
-	if (!per_cpu(sd_asym_cpucapacity, cpu)) {
-		if (sched_debug()) {
-			pr_info("rd %*pbl: CPUs do not have asymmetric capacities\n",
-					cpumask_pr_args(cpu_map));
-		}
-		goto free;
-	}
-
-	/* EAS definitely does *not* handle SMT */
-	if (sched_smt_active()) {
-		pr_warn("rd %*pbl: Disabling EAS, SMT is not supported\n",
-			cpumask_pr_args(cpu_map));
-		goto free;
-	}
-
-	if (!arch_scale_freq_invariant()) {
-		if (sched_debug()) {
-			pr_warn("rd %*pbl: Disabling EAS: frequency-invariant load tracking not yet supported",
-				cpumask_pr_args(cpu_map));
-		}
+	if (!sched_is_eas_possible(cpu_map))
 		goto free;
-	}

 	for_each_cpu(i, cpu_map) {
 		/* Skip already covered CPUs. */
 		if (find_pd(pd, i))
 			continue;

-		/* Do not attempt EAS if schedutil is not being used. */
-		policy = cpufreq_cpu_get(i);
-		if (!policy)
-			goto free;
-		gov = policy->governor;
-		cpufreq_cpu_put(policy);
-		if (gov != &schedutil_gov) {
-			if (rd->pd)
-				pr_warn("rd %*pbl: Disabling EAS, schedutil is mandatory\n",
-						cpumask_pr_args(cpu_map));
-			goto free;
-		}
-
 		/* Create the new pd and add it to the local list. */
 		tmp = pd_init(i);
 		if (!tmp)
--
2.31.1

Re: [PATCH v6 2/2] sched/topology: change behaviour of sysctl sched_energy_aware based on the platform

[Valentin Schneider]

On 09/10/23 11:30, Shrikanth Hegde wrote:
> sysctl sched_energy_aware is available for the admin to disable/enable
> energy aware scheduling(EAS). EAS is enabled only if few conditions are
> met by the platform. They are, asymmetric CPU capacity, no SMT,
> schedutil CPUfreq governor, frequency invariant load tracking etc.
> A platform may boot without EAS capability, but could gain such
> capability at runtime. For example, changing/registering the CPUfreq
> governor to schedutil.
>
> At present, though platform doesn't support EAS, this sysctl returns 1
> and it ends up calling build_perf_domains on write to 1 and
> NOP when writing to 0. That is confusing and un-necessary.
>
> Desired behavior would be to have this sysctl to enable/disable the EAS
> on supported platform. On non-supported platform write to the sysctl
> would return not supported error and read of the sysctl would return
> empty. So
> sched_energy_aware returns empty - EAS is not possible at this moment
> This will include EAS capable platforms which have at least one EAS
> condition false during startup, e.g. not using the schedutil CPUfreq governor
> sched_energy_aware returns 0 - EAS is supported but disabled by admin.
> sched_energy_aware returns 1 - EAS is supported and enabled.
>
> User can find out the reason why EAS is not possible by checking
> info messages. sched_is_eas_possible returns true if the platform
> can do EAS at this moment.
>
> Tested-by: Pierre Gondois <pierre.gondois@arm.com>
> Signed-off-by: Shrikanth Hegde <sshegde@linux.vnet.ibm.com>

Reviewed-by: Valentin Schneider <vschneid@redhat.com>

[tip: sched/core] sched/topology: Move the declaration of 'schedutil_gov' to kernel/sched/sched.h

[tip-bot2 for Ingo Molnar]

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

Commit-ID:     f2273f4e19e29f7d0be6a2393f18369cd1b496c8
Gitweb:        https://git.kernel.org/tip/f2273f4e19e29f7d0be6a2393f18369cd1b496c8
Author:        Ingo Molnar <mingo@kernel.org>
AuthorDate:    Mon, 09 Oct 2023 17:31:26 +02:00
Committer:     Ingo Molnar <mingo@kernel.org>
CommitterDate: Mon, 09 Oct 2023 17:33:10 +02:00

sched/topology: Move the declaration of 'schedutil_gov' to kernel/sched/sched.h

Move it out of the .c file into the shared scheduler-internal header file,
to gain type-checking.

Signed-off-by: Ingo Molnar <mingo@kernel.org>
Cc: Shrikanth Hegde <sshegde@linux.vnet.ibm.com>
Cc: Valentin Schneider <vschneid@redhat.com>
Link: https://lore.kernel.org/r/20231009060037.170765-3-sshegde@linux.vnet.ibm.com
---
 kernel/sched/sched.h    | 2 ++
 kernel/sched/topology.c | 1 -
 2 files changed, 2 insertions(+), 1 deletion(-)

diff --git a/kernel/sched/sched.h b/kernel/sched/sched.h
index 7e7fedc..faf9031 100644
--- a/kernel/sched/sched.h
+++ b/kernel/sched/sched.h
@@ -3203,6 +3203,8 @@ static inline bool sched_energy_enabled(void)
 	return static_branch_unlikely(&sched_energy_present);
 }
 
+extern struct cpufreq_governor schedutil_gov;
+
 #else /* ! (CONFIG_ENERGY_MODEL && CONFIG_CPU_FREQ_GOV_SCHEDUTIL) */
 
 #define perf_domain_span(pd) NULL
diff --git a/kernel/sched/topology.c b/kernel/sched/topology.c
index 4cbbdac..d950861 100644
--- a/kernel/sched/topology.c
+++ b/kernel/sched/topology.c
@@ -212,7 +212,6 @@ static unsigned int sysctl_sched_energy_aware = 1;
 static DEFINE_MUTEX(sched_energy_mutex);
 static bool sched_energy_update;
 
-extern struct cpufreq_governor schedutil_gov;
 static bool sched_is_eas_possible(const struct cpumask *cpu_mask)
 {
 	bool any_asym_capacity = false;

[tip: sched/core] sched/topology: Change behaviour of the 'sched_energy_aware' sysctl, based on the platform

[tip-bot2 for Shrikanth Hegde]

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

Commit-ID:     8f833c82cdab7b4049bcfe88311d35fa5f24e422
Gitweb:        https://git.kernel.org/tip/8f833c82cdab7b4049bcfe88311d35fa5f24e422
Author:        Shrikanth Hegde <sshegde@linux.vnet.ibm.com>
AuthorDate:    Mon, 09 Oct 2023 11:30:37 +05:30
Committer:     Ingo Molnar <mingo@kernel.org>
CommitterDate: Mon, 09 Oct 2023 17:24:44 +02:00

sched/topology: Change behaviour of the 'sched_energy_aware' sysctl, based on the platform

The 'sched_energy_aware' sysctl is available for the admin to disable/enable
energy aware scheduling(EAS). EAS is enabled only if few conditions are
met by the platform. They are, asymmetric CPU capacity, no SMT,
schedutil CPUfreq governor, frequency invariant load tracking etc.
A platform may boot without EAS capability, but could gain such
capability at runtime. For example, changing/registering the cpufreq
governor to schedutil.

At present, though platform doesn't support EAS, this sysctl returns 1
and it ends up calling build_perf_domains on write to 1 and
NOP when writing to 0. That is confusing and un-necessary.

Desired behavior would be to have this sysctl to enable/disable the EAS
on supported platform. On non-supported platform write to the sysctl
would return not supported error and read of the sysctl would return
empty. So sched_energy_aware returns empty - EAS is not possible at this moment
This will include EAS capable platforms which have at least one EAS
condition false during startup, e.g. not using the schedutil cpufreq governor
sched_energy_aware returns 0 - EAS is supported but disabled by admin.
sched_energy_aware returns 1 - EAS is supported and enabled.

User can find out the reason why EAS is not possible by checking
info messages. sched_is_eas_possible returns true if the platform
can do EAS at this moment.

Signed-off-by: Shrikanth Hegde <sshegde@linux.vnet.ibm.com>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Tested-by: Pierre Gondois <pierre.gondois@arm.com>
Reviewed-by: Valentin Schneider <vschneid@redhat.com>
Link: https://lore.kernel.org/r/20231009060037.170765-3-sshegde@linux.vnet.ibm.com
---
 Documentation/admin-guide/sysctl/kernel.rst |   3 +-
 kernel/sched/topology.c                     | 112 ++++++++++++-------
 2 files changed, 76 insertions(+), 39 deletions(-)

diff --git a/Documentation/admin-guide/sysctl/kernel.rst b/Documentation/admin-guide/sysctl/kernel.rst
index cf33de5..d89ac2b 100644
--- a/Documentation/admin-guide/sysctl/kernel.rst
+++ b/Documentation/admin-guide/sysctl/kernel.rst
@@ -1182,7 +1182,8 @@ automatically on platforms where it can run (that is,
 platforms with asymmetric CPU topologies and having an Energy
 Model available). If your platform happens to meet the
 requirements for EAS but you do not want to use it, change
-this value to 0.
+this value to 0. On Non-EAS platforms, write operation fails and
+read doesn't return anything.
 
 task_delayacct
 ===============
diff --git a/kernel/sched/topology.c b/kernel/sched/topology.c
index fcda3f0..4cbbdac 100644
--- a/kernel/sched/topology.c
+++ b/kernel/sched/topology.c
@@ -212,6 +212,70 @@ static unsigned int sysctl_sched_energy_aware = 1;
 static DEFINE_MUTEX(sched_energy_mutex);
 static bool sched_energy_update;
 
+extern struct cpufreq_governor schedutil_gov;
+static bool sched_is_eas_possible(const struct cpumask *cpu_mask)
+{
+	bool any_asym_capacity = false;
+	struct cpufreq_policy *policy;
+	struct cpufreq_governor *gov;
+	int i;
+
+	/* EAS is enabled for asymmetric CPU capacity topologies. */
+	for_each_cpu(i, cpu_mask) {
+		if (rcu_access_pointer(per_cpu(sd_asym_cpucapacity, i))) {
+			any_asym_capacity = true;
+			break;
+		}
+	}
+	if (!any_asym_capacity) {
+		if (sched_debug()) {
+			pr_info("rd %*pbl: Checking EAS, CPUs do not have asymmetric capacities\n",
+				cpumask_pr_args(cpu_mask));
+		}
+		return false;
+	}
+
+	/* EAS definitely does *not* handle SMT */
+	if (sched_smt_active()) {
+		if (sched_debug()) {
+			pr_info("rd %*pbl: Checking EAS, SMT is not supported\n",
+				cpumask_pr_args(cpu_mask));
+		}
+		return false;
+	}
+
+	if (!arch_scale_freq_invariant()) {
+		if (sched_debug()) {
+			pr_info("rd %*pbl: Checking EAS: frequency-invariant load tracking not yet supported",
+				cpumask_pr_args(cpu_mask));
+		}
+		return false;
+	}
+
+	/* Do not attempt EAS if schedutil is not being used. */
+	for_each_cpu(i, cpu_mask) {
+		policy = cpufreq_cpu_get(i);
+		if (!policy) {
+			if (sched_debug()) {
+				pr_info("rd %*pbl: Checking EAS, cpufreq policy not set for CPU: %d",
+					cpumask_pr_args(cpu_mask), i);
+			}
+			return false;
+		}
+		gov = policy->governor;
+		cpufreq_cpu_put(policy);
+		if (gov != &schedutil_gov) {
+			if (sched_debug()) {
+				pr_info("rd %*pbl: Checking EAS, schedutil is mandatory\n",
+					cpumask_pr_args(cpu_mask));
+			}
+			return false;
+		}
+	}
+
+	return true;
+}
+
 void rebuild_sched_domains_energy(void)
 {
 	mutex_lock(&sched_energy_mutex);
@@ -230,6 +294,15 @@ static int sched_energy_aware_handler(struct ctl_table *table, int write,
 	if (write && !capable(CAP_SYS_ADMIN))
 		return -EPERM;
 
+	if (!sched_is_eas_possible(cpu_active_mask)) {
+		if (write) {
+			return -EOPNOTSUPP;
+		} else {
+			*lenp = 0;
+			return 0;
+		}
+	}
+
 	ret = proc_dointvec_minmax(table, write, buffer, lenp, ppos);
 	if (!ret && write) {
 		state = static_branch_unlikely(&sched_energy_present);
@@ -351,61 +424,24 @@ static void sched_energy_set(bool has_eas)
  *    4. schedutil is driving the frequency of all CPUs of the rd;
  *    5. frequency invariance support is present;
  */
-extern struct cpufreq_governor schedutil_gov;
 static bool build_perf_domains(const struct cpumask *cpu_map)
 {
 	int i;
 	struct perf_domain *pd = NULL, *tmp;
 	int cpu = cpumask_first(cpu_map);
 	struct root_domain *rd = cpu_rq(cpu)->rd;
-	struct cpufreq_policy *policy;
-	struct cpufreq_governor *gov;
 
 	if (!sysctl_sched_energy_aware)
 		goto free;
 
-	/* EAS is enabled for asymmetric CPU capacity topologies. */
-	if (!per_cpu(sd_asym_cpucapacity, cpu)) {
-		if (sched_debug()) {
-			pr_info("rd %*pbl: CPUs do not have asymmetric capacities\n",
-					cpumask_pr_args(cpu_map));
-		}
-		goto free;
-	}
-
-	/* EAS definitely does *not* handle SMT */
-	if (sched_smt_active()) {
-		pr_warn("rd %*pbl: Disabling EAS, SMT is not supported\n",
-			cpumask_pr_args(cpu_map));
-		goto free;
-	}
-
-	if (!arch_scale_freq_invariant()) {
-		if (sched_debug()) {
-			pr_warn("rd %*pbl: Disabling EAS: frequency-invariant load tracking not yet supported",
-				cpumask_pr_args(cpu_map));
-		}
+	if (!sched_is_eas_possible(cpu_map))
 		goto free;
-	}
 
 	for_each_cpu(i, cpu_map) {
 		/* Skip already covered CPUs. */
 		if (find_pd(pd, i))
 			continue;
 
-		/* Do not attempt EAS if schedutil is not being used. */
-		policy = cpufreq_cpu_get(i);
-		if (!policy)
-			goto free;
-		gov = policy->governor;
-		cpufreq_cpu_put(policy);
-		if (gov != &schedutil_gov) {
-			if (rd->pd)
-				pr_warn("rd %*pbl: Disabling EAS, schedutil is mandatory\n",
-						cpumask_pr_args(cpu_map));
-			goto free;
-		}
-
 		/* Create the new pd and add it to the local list. */
 		tmp = pd_init(i);
 		if (!tmp)