Re: [PATCH v4 08/16] sched/core: uclamp: propagate parent clamps

[Suren Baghdasaryan <surenb@google.com>]

On Tue, Aug 28, 2018 at 6:53 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 represent also the clamp value which is actually
> used to clamp tasks in each task group.
>
> Since it can be interesting for tasks in a cgroup to know exactly what
> is the currently propagated/enforced configuration, 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>
> Cc: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
> Cc: Viresh Kumar <viresh.kumar@linaro.org>
> Cc: Suren Baghdasaryan <surenb@google.com>
> Cc: Todd Kjos <tkjos@google.com>
> Cc: Joel Fernandes <joelaf@google.com>
> Cc: Juri Lelli <juri.lelli@redhat.com>
> Cc: Quentin Perret <quentin.perret@arm.com>
> Cc: Dietmar Eggemann <dietmar.eggemann@arm.com>
> Cc: Morten Rasmussen <morten.rasmussen@arm.com>
> Cc: linux-kernel@vger.kernel.org
> Cc: linux-pm@vger.kernel.org
>
> ---
> Changes in v4:
>  Message-ID: <20180816140731.GD2960@e110439-lin>
>  - add ".effective" attributes to the default hierarchy
>  Others:
>  - small documentation fixes
>  - rebased on v4.19-rc1
>
> Changes in v3:
>  Message-ID: <20180409222417.GK3126663@devbig577.frc2.facebook.com>
>  - new patch in v3, to implement a suggestion from v1 review
> ---
>  Documentation/admin-guide/cgroup-v2.rst |  25 +++++-
>  include/linux/sched.h                   |   8 ++
>  kernel/sched/core.c                     | 112 +++++++++++++++++++++++-
>  3 files changed, 139 insertions(+), 6 deletions(-)
>
> diff --git a/Documentation/admin-guide/cgroup-v2.rst b/Documentation/admin-guide/cgroup-v2.rst
> index 80ef7bdc517b..72272f58d304 100644
> --- a/Documentation/admin-guide/cgroup-v2.rst
> +++ b/Documentation/admin-guide/cgroup-v2.rst
> @@ -976,22 +976,43 @@ All time durations are in microseconds.
>          A read-write single value file which exists on non-root cgroups.
>          The default is "0", i.e. no bandwidth boosting.
>
> -        The minimum utilization in the range [0, 1023].
> +        The requested minimum utilization in the range [0, 1023].
>
>          This interface allows reading and setting minimum utilization clamp
>          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, 1023].
> +
> +        This value can be lower then cpu.util.min in case a parent cgroup
> +        is enforcing a more restrictive clamping on minimum utilization.

IMHO if cpu.util.min=0 means "no restrictions" on UCLAMP_MIN then
calling parent's lower cpu.util.min value "more restrictive clamping"
is confusing. I would suggest to rephrase this to smth like "...in
case a parent cgroup requires lower cpu.util.min clamping."

> +
>    cpu.util.max
>          A read-write single value file which exists on non-root cgroups.
>          The default is "1023". i.e. no bandwidth clamping
>
> -        The maximum utilization in the range [0, 1023].
> +        The requested maximum utilization in the range [0, 1023].
>
>          This interface allows reading and setting maximum utilization clamp
>          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, 1023].
> +
> +        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/include/linux/sched.h b/include/linux/sched.h
> index dc39b67a366a..2da130d17e70 100644
> --- a/include/linux/sched.h
> +++ b/include/linux/sched.h
> @@ -591,6 +591,14 @@ struct sched_dl_entity {
>  struct uclamp_se {
>         unsigned int value;
>         unsigned int group_id;
> +       /*
> +        * Effective task (group) clamp value.
> +        * For task groups is the value (eventually) enforced by a parent task
> +        * group.
> +        */
> +       struct {
> +               unsigned int value;
> +       } effective;
>  };
>
>  union rcu_special {
> diff --git a/kernel/sched/core.c b/kernel/sched/core.c
> index dcbf22abd0bf..b2d438b6484b 100644
> --- a/kernel/sched/core.c
> +++ b/kernel/sched/core.c
> @@ -1254,6 +1254,8 @@ static inline int alloc_uclamp_sched_group(struct task_group *tg,
>
>         for (clamp_id = 0; clamp_id < UCLAMP_CNT; ++clamp_id) {
>                 uc_se = &tg->uclamp[clamp_id];
> +               uc_se->effective.value =
> +                       parent->uclamp[clamp_id].effective.value;
>                 uc_se->value = parent->uclamp[clamp_id].value;
>                 uc_se->group_id = parent->uclamp[clamp_id].group_id;
>         }
> @@ -1415,6 +1417,7 @@ static void __init init_uclamp(void)
>  #ifdef CONFIG_UCLAMP_TASK_GROUP
>                 /* Init root TG's clamp group */
>                 uc_se = &root_task_group.uclamp[clamp_id];
> +               uc_se->effective.value = uclamp_none(clamp_id);
>                 uc_se->value = uclamp_none(clamp_id);
>                 uc_se->group_id = 0;
>  #endif
> @@ -7226,6 +7229,68 @@ static void cpu_cgroup_attach(struct cgroup_taskset *tset)
>  }
>
>  #ifdef CONFIG_UCLAMP_TASK_GROUP
> +/**
> + * cpu_util_update_hier: propagete effective clamp down the hierarchy

typo: propagate

> + * @css: the task group to update
> + * @clamp_id: the clamp index to update
> + * @value: the new task group clamp value
> + *
> + * The effective clamp for a TG is expected to track the most restrictive
> + * value between the TG's clamp value and it's parent effective clamp value.
> + * This method achieve that:
> + * 1. updating the current TG effective value
> + * 2. walking all the descendant task group that needs an update
> + *
> + * A TG's effective clamp needs to be updated when its current value is not
> + * matching the TG's clamp value. In this case indeed either:
> + * a) the parent has got a more relaxed clamp value
> + *    thus potentially we can relax the effective value for this group
> + * b) the parent has got a more strict clamp value
> + *    thus potentially we have to restrict the effective value of this group
> + *
> + * Restriction and relaxation of current TG's effective clamp values needs to
> + * be propagated down to all the descendants. When a subgroup is found which
> + * has already its effective clamp value matching its clamp value, then we can
> + * safely skip all its descendants which are granted to be already in sync.
> + */
> +static void cpu_util_update_hier(struct cgroup_subsys_state *css,
> +                                int clamp_id, 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;
> +               /*
> +                * Skip the whole subtrees if the current effective clamp is
> +                * alredy matching the TG's clamp value.

typo: already

> +                * In this case, all the subtrees already have top_value, or a
> +                * more restrictive, as effective clamp.
> +                */
> +               uc_parent = &css_tg(css)->parent->uclamp[clamp_id];
> +               if (uc_se->effective.value == value &&
> +                   uc_parent->effective.value >= value) {
> +                       css = css_rightmost_descendant(css);
> +                       continue;
> +               }
> +
> +               /* Propagate the most restrictive effective value */
> +               if (uc_parent->effective.value < value)
> +                       value = uc_parent->effective.value;
> +               if (uc_se->effective.value == value)
> +                       continue;
> +
> +               uc_se->effective.value = value;
> +       }
> +}
> +
>  static int cpu_util_min_write_u64(struct cgroup_subsys_state *css,
>                                   struct cftype *cftype, u64 min_value)
>  {
> @@ -7245,6 +7310,9 @@ static int cpu_util_min_write_u64(struct cgroup_subsys_state *css,
>         if (tg->uclamp[UCLAMP_MAX].value < min_value)
>                 goto out;
>
> +       /* Update effective clamps to track the most restrictive value */
> +       cpu_util_update_hier(css, UCLAMP_MIN, min_value);
> +
>  out:
>         rcu_read_unlock();
>
> @@ -7270,6 +7338,9 @@ static int cpu_util_max_write_u64(struct cgroup_subsys_state *css,
>         if (tg->uclamp[UCLAMP_MIN].value > max_value)
>                 goto out;
>
> +       /* Update effective clamps to track the most restrictive value */
> +       cpu_util_update_hier(css, UCLAMP_MAX, max_value);
> +
>  out:
>         rcu_read_unlock();
>
> @@ -7277,14 +7348,17 @@ static int cpu_util_max_write_u64(struct cgroup_subsys_state *css,
>  }
>
>  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;
> @@ -7293,13 +7367,25 @@ 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 */
>
> @@ -7647,11 +7733,19 @@ 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 */
>  };
> @@ -7827,12 +7921,22 @@ 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 */
>  };
> --
> 2.18.0
>