Re: [PATCH v7 12/15] sched/core: uclamp: Propagate parent clamps

[Suren Baghdasaryan <surenb@google.com>]

On Fri, Feb 8, 2019 at 2:06 AM Patrick Bellasi <patrick.bellasi@arm.com> wrote:
>
> In order to properly support hierarchical resources control, the cgroup
> delegation model requires that attribute writes from a child group never
> fail but still are (potentially) constrained based on parent's assigned
> resources. This requires to properly propagate and aggregate parent
> attributes down to its descendants.
>
> Let's implement this mechanism by adding a new "effective" clamp value
> for each task group. The effective clamp value is defined as the smaller
> value between the clamp value of a group and the effective clamp value
> of its parent. This is the actual clamp value enforced on tasks in a
> task group.

In patch 10 in this series you mentioned "b) do not enforce any
constraints and/or dependencies between the parent and its child
nodes"

This patch seems to change that behavior. If so, should it be documented?

> Since it can be interesting for userspace, e.g. system management
> software, to know exactly what the currently propagated/enforced
> configuration is, the effective clamp values are exposed to user-space
> by means of a new pair of read-only attributes
> cpu.util.{min,max}.effective.
>
> Signed-off-by: Patrick Bellasi <patrick.bellasi@arm.com>
> Cc: Ingo Molnar <mingo@redhat.com>
> Cc: Peter Zijlstra <peterz@infradead.org>
> Cc: Tejun Heo <tj@kernel.org>
>
> ---
> Changes in v7:
>  Others:
>  - ensure clamp values are not tunable at root cgroup level
> ---
>  Documentation/admin-guide/cgroup-v2.rst |  19 ++++
>  kernel/sched/core.c                     | 118 +++++++++++++++++++++++-
>  2 files changed, 133 insertions(+), 4 deletions(-)
>
> diff --git a/Documentation/admin-guide/cgroup-v2.rst b/Documentation/admin-guide/cgroup-v2.rst
> index 47710a77f4fa..7aad2435e961 100644
> --- a/Documentation/admin-guide/cgroup-v2.rst
> +++ b/Documentation/admin-guide/cgroup-v2.rst
> @@ -990,6 +990,16 @@ All time durations are in microseconds.
>          values similar to the sched_setattr(2). This minimum utilization
>          value is used to clamp the task specific minimum utilization clamp.
>
> +  cpu.util.min.effective
> +        A read-only single value file which exists on non-root cgroups and
> +        reports minimum utilization clamp value currently enforced on a task
> +        group.
> +
> +        The actual minimum utilization in the range [0, 1024].
> +
> +        This value can be lower then cpu.util.min in case a parent cgroup
> +        allows only smaller minimum utilization values.
> +
>    cpu.util.max
>          A read-write single value file which exists on non-root cgroups.
>          The default is "1024". i.e. no utilization capping
> @@ -1000,6 +1010,15 @@ All time durations are in microseconds.
>          values similar to the sched_setattr(2). This maximum utilization
>          value is used to clamp the task specific maximum utilization clamp.
>
> +  cpu.util.max.effective
> +        A read-only single value file which exists on non-root cgroups and
> +        reports maximum utilization clamp value currently enforced on a task
> +        group.
> +
> +        The actual maximum utilization in the range [0, 1024].
> +
> +        This value can be lower then cpu.util.max in case a parent cgroup
> +        is enforcing a more restrictive clamping on max utilization.
>
>
>  Memory
> diff --git a/kernel/sched/core.c b/kernel/sched/core.c
> index 122ab069ade5..1e54517acd58 100644
> --- a/kernel/sched/core.c
> +++ b/kernel/sched/core.c
> @@ -720,6 +720,18 @@ static void set_load_weight(struct task_struct *p, bool update_load)
>  }
>
>  #ifdef CONFIG_UCLAMP_TASK
> +/*
> + * Serializes updates of utilization clamp values
> + *
> + * The (slow-path) user-space triggers utilization clamp value updates which
> + * can require updates on (fast-path) scheduler's data structures used to
> + * support enqueue/dequeue operations.
> + * While the per-CPU rq lock protects fast-path update operations, user-space
> + * requests are serialized using a mutex to reduce the risk of conflicting
> + * updates or API abuses.
> + */
> +static DEFINE_MUTEX(uclamp_mutex);
> +
>  /* Max allowed minimum utilization */
>  unsigned int sysctl_sched_uclamp_util_min = SCHED_CAPACITY_SCALE;
>
> @@ -1127,6 +1139,8 @@ static void __init init_uclamp(void)
>         unsigned int value;
>         int cpu;
>
> +       mutex_init(&uclamp_mutex);
> +
>         for_each_possible_cpu(cpu) {
>                 memset(&cpu_rq(cpu)->uclamp, 0, sizeof(struct uclamp_rq));
>                 cpu_rq(cpu)->uclamp_flags = 0;
> @@ -6758,6 +6772,10 @@ static inline int alloc_uclamp_sched_group(struct task_group *tg,
>                         parent->uclamp[clamp_id].value;
>                 tg->uclamp[clamp_id].bucket_id =
>                         parent->uclamp[clamp_id].bucket_id;
> +               tg->uclamp[clamp_id].effective.value =
> +                       parent->uclamp[clamp_id].effective.value;
> +               tg->uclamp[clamp_id].effective.bucket_id =
> +                       parent->uclamp[clamp_id].effective.bucket_id;
>         }
>  #endif
>
> @@ -7011,6 +7029,53 @@ static void cpu_cgroup_attach(struct cgroup_taskset *tset)
>  }
>
>  #ifdef CONFIG_UCLAMP_TASK_GROUP
> +static void cpu_util_update_hier(struct cgroup_subsys_state *css,

s/cpu_util_update_hier/cpu_util_update_heir ?

> +                                unsigned int clamp_id, unsigned int bucket_id,
> +                                unsigned int value)
> +{
> +       struct cgroup_subsys_state *top_css = css;
> +       struct uclamp_se *uc_se, *uc_parent;
> +
> +       css_for_each_descendant_pre(css, top_css) {
> +               /*
> +                * The first visited task group is top_css, which clamp value
> +                * is the one passed as parameter. For descendent task
> +                * groups we consider their current value.
> +                */
> +               uc_se = &css_tg(css)->uclamp[clamp_id];
> +               if (css != top_css) {
> +                       value = uc_se->value;
> +                       bucket_id = uc_se->effective.bucket_id;
> +               }
> +               uc_parent = NULL;
> +               if (css_tg(css)->parent)
> +                       uc_parent = &css_tg(css)->parent->uclamp[clamp_id];
> +
> +               /*
> +                * Skip the whole subtrees if the current effective clamp is
> +                * already matching the TG's clamp value.
> +                * In this case, all the subtrees already have top_value, or a
> +                * more restrictive value, as effective clamp.
> +                */
> +               if (uc_se->effective.value == value &&
> +                   uc_parent && uc_parent->effective.value >= value) {
> +                       css = css_rightmost_descendant(css);
> +                       continue;
> +               }
> +
> +               /* Propagate the most restrictive effective value */
> +               if (uc_parent && uc_parent->effective.value < value) {
> +                       value = uc_parent->effective.value;
> +                       bucket_id = uc_parent->effective.bucket_id;
> +               }
> +               if (uc_se->effective.value == value)
> +                       continue;
> +
> +               uc_se->effective.value = value;
> +               uc_se->effective.bucket_id = bucket_id;
> +       }
> +}
> +
>  static int cpu_util_min_write_u64(struct cgroup_subsys_state *css,
>                                   struct cftype *cftype, u64 min_value)
>  {
> @@ -7020,6 +7085,7 @@ static int cpu_util_min_write_u64(struct cgroup_subsys_state *css,
>         if (min_value > SCHED_CAPACITY_SCALE)
>                 return -ERANGE;
>
> +       mutex_lock(&uclamp_mutex);
>         rcu_read_lock();
>
>         tg = css_tg(css);
> @@ -7038,8 +7104,13 @@ static int cpu_util_min_write_u64(struct cgroup_subsys_state *css,
>         tg->uclamp[UCLAMP_MIN].value = min_value;
>         tg->uclamp[UCLAMP_MIN].bucket_id = uclamp_bucket_id(min_value);
>
> +       /* Update effective clamps to track the most restrictive value */
> +       cpu_util_update_hier(css, UCLAMP_MIN, tg->uclamp[UCLAMP_MIN].bucket_id,
> +                            min_value);
> +
>  out:
>         rcu_read_unlock();
> +       mutex_unlock(&uclamp_mutex);
>
>         return ret;
>  }
> @@ -7053,6 +7124,7 @@ static int cpu_util_max_write_u64(struct cgroup_subsys_state *css,
>         if (max_value > SCHED_CAPACITY_SCALE)
>                 return -ERANGE;
>
> +       mutex_lock(&uclamp_mutex);
>         rcu_read_lock();
>
>         tg = css_tg(css);
> @@ -7071,21 +7143,29 @@ static int cpu_util_max_write_u64(struct cgroup_subsys_state *css,
>         tg->uclamp[UCLAMP_MAX].value = max_value;
>         tg->uclamp[UCLAMP_MAX].bucket_id = uclamp_bucket_id(max_value);
>
> +       /* Update effective clamps to track the most restrictive value */
> +       cpu_util_update_hier(css, UCLAMP_MAX, tg->uclamp[UCLAMP_MAX].bucket_id,
> +                            max_value);
> +
>  out:
>         rcu_read_unlock();
> +       mutex_unlock(&uclamp_mutex);
>
>         return ret;
>  }
>
>  static inline u64 cpu_uclamp_read(struct cgroup_subsys_state *css,
> -                                 enum uclamp_id clamp_id)
> +                                 enum uclamp_id clamp_id,
> +                                 bool effective)
>  {
>         struct task_group *tg;
>         u64 util_clamp;
>
>         rcu_read_lock();
>         tg = css_tg(css);
> -       util_clamp = tg->uclamp[clamp_id].value;
> +       util_clamp = effective
> +               ? tg->uclamp[clamp_id].effective.value
> +               : tg->uclamp[clamp_id].value;
>         rcu_read_unlock();
>
>         return util_clamp;
> @@ -7094,13 +7174,25 @@ static inline u64 cpu_uclamp_read(struct cgroup_subsys_state *css,
>  static u64 cpu_util_min_read_u64(struct cgroup_subsys_state *css,
>                                  struct cftype *cft)
>  {
> -       return cpu_uclamp_read(css, UCLAMP_MIN);
> +       return cpu_uclamp_read(css, UCLAMP_MIN, false);
>  }
>
>  static u64 cpu_util_max_read_u64(struct cgroup_subsys_state *css,
>                                  struct cftype *cft)
>  {
> -       return cpu_uclamp_read(css, UCLAMP_MAX);
> +       return cpu_uclamp_read(css, UCLAMP_MAX, false);
> +}
> +
> +static u64 cpu_util_min_effective_read_u64(struct cgroup_subsys_state *css,
> +                                          struct cftype *cft)
> +{
> +       return cpu_uclamp_read(css, UCLAMP_MIN, true);
> +}
> +
> +static u64 cpu_util_max_effective_read_u64(struct cgroup_subsys_state *css,
> +                                          struct cftype *cft)
> +{
> +       return cpu_uclamp_read(css, UCLAMP_MAX, true);
>  }
>  #endif /* CONFIG_UCLAMP_TASK_GROUP */
>
> @@ -7448,11 +7540,19 @@ static struct cftype cpu_legacy_files[] = {
>                 .read_u64 = cpu_util_min_read_u64,
>                 .write_u64 = cpu_util_min_write_u64,
>         },
> +       {
> +               .name = "util.min.effective",
> +               .read_u64 = cpu_util_min_effective_read_u64,
> +       },
>         {
>                 .name = "util.max",
>                 .read_u64 = cpu_util_max_read_u64,
>                 .write_u64 = cpu_util_max_write_u64,
>         },
> +       {
> +               .name = "util.max.effective",
> +               .read_u64 = cpu_util_max_effective_read_u64,
> +       },
>  #endif
>         { }     /* Terminate */
>  };
> @@ -7628,12 +7728,22 @@ static struct cftype cpu_files[] = {
>                 .read_u64 = cpu_util_min_read_u64,
>                 .write_u64 = cpu_util_min_write_u64,
>         },
> +       {
> +               .name = "util.min.effective",
> +               .flags = CFTYPE_NOT_ON_ROOT,
> +               .read_u64 = cpu_util_min_effective_read_u64,
> +       },
>         {
>                 .name = "util.max",
>                 .flags = CFTYPE_NOT_ON_ROOT,
>                 .read_u64 = cpu_util_max_read_u64,
>                 .write_u64 = cpu_util_max_write_u64,
>         },
> +       {
> +               .name = "util.max.effective",
> +               .flags = CFTYPE_NOT_ON_ROOT,
> +               .read_u64 = cpu_util_max_effective_read_u64,
> +       },
>  #endif
>         { }     /* terminate */
>  };
> --
> 2.20.1
>