Re: [PATCH v4] sched/topology: change behaviour of sysctl sched_energy_aware based on the platform

[Shrikanth Hegde <sshegde@linux.vnet.ibm.com>]

On 9/26/23 3:30 PM, 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,
> valid cpufreq policy, 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 policy.
> 
> At present, though platform doesn't support EAS, this sysctl returns 1
> and it ends up calling rebuild of sched domain 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
> 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.
> 


On Power10 system which has SMT and symmetric CPU capacity operations              
would be as below.                                                              
# cat sched_energy_aware                                                        
# echo 0 > sched_energy_aware                                                   
-bash: echo: write error: Operation not supported                               
# echo 1 > sched_energy_aware                                                      
-bash: echo: write error: Operation not supported                               
dmesg | tail                                                                    
[ 1608.233159] rd 0-95: Checking EAS, CPUs do not have asymmetric capacities     
[ 1612.026148] rd 0-95: Checking EAS, CPUs do not have asymmetric capacities       
[ 1616.122406] rd 0-95: Checking EAS, CPUs do not have asymmetric capacities  


Pierre, 
Could you please help testing this on your platform which supports EAS. 
That would be helpful. 

> sched_is_eas_possible return if the platform can do EAS at this moment.
> It takes most of the cases into account except one where EM complexity is
> too high as the code was bit tricky to separate that.
> 
> 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/
> 
> Signed-off-by: Shrikanth Hegde <sshegde@linux.vnet.ibm.com>
> ---
>  Documentation/admin-guide/sysctl/kernel.rst |   3 +-
>  kernel/sched/topology.c                     | 107 +++++++++++++-------
>  2 files changed, 71 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 a7b50bba7829..839ddc80a5ac 100644
> --- a/kernel/sched/topology.c
> +++ b/kernel/sched/topology.c
> @@ -212,6 +212,64 @@ 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)
> +{
> +	int cpu = cpumask_first(cpu_mask);
> +	struct cpufreq_policy *policy;
> +	struct cpufreq_governor *gov;
> +	int i;
> +
> +	/* EAS is enabled for asymmetric CPU capacity topologies. */
> +	if (!per_cpu(sd_asym_cpucapacity, cpu)) {
> +		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);
> @@ -231,6 +289,14 @@ static int sched_energy_aware_handler(struct ctl_table *table, int write,
>  		return -EPERM;
> 
>  	ret = proc_dointvec_minmax(table, write, buffer, lenp, ppos);
> +	if (!sched_is_eas_possible(cpu_active_mask)) {
> +		if (write) {
> +			return -EOPNOTSUPP;
> +		} else {
> +			*lenp = 0;
> +			return 0;
> +		}
> +	}
>  	if (!ret && write) {
>  		state = static_branch_unlikely(&sched_energy_present);
>  		if (state != sysctl_sched_energy_aware)
> @@ -370,61 +436,24 @@ static void sched_energy_set(bool has_eas)
>   */
>  #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);
>  	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)
> @@ -458,6 +487,8 @@ static bool build_perf_domains(const struct cpumask *cpu_map)
>  	return !!pd;
> 
>  free:
> +	if (sched_debug())
> +		pr_warn("rd %*pbl: Disabling EAS", cpumask_pr_args(cpu_map));
>  	free_pd(pd);
>  	tmp = rd->pd;
>  	rcu_assign_pointer(rd->pd, NULL);
> --
> 2.31.1
>