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Date: Tue, 26 Sep 2023 16:20:18 +0530
From: Shrikanth Hegde <sshegde@...ux.vnet.ibm.com>
To: pierre.gondois@....com
Cc: dietmar.eggemann@....com, vschneid@...hat.com,
linux-kernel@...r.kernel.org, ionela.voinescu@....com,
quentin.perret@....com, srikar@...ux.vnet.ibm.com,
mgorman@...hsingularity.net, mingo@...nel.org, yu.c.chen@...el.com,
tim.c.chen@...ux.intel.com, pauld@...hat.com, mingo@...hat.com,
peterz@...radead.org, vincent.guittot@...aro.org
Subject: Re: [PATCH v4] sched/topology: change behaviour of sysctl
sched_energy_aware based on the platform
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@...ux.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
>
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