Re: [PATCH v12 3/6] sched/core: uclamp: Propagate system defaults to root group

From: Michal Koutný
Date: Thu Jul 25 2019 - 14:05:48 EST

On Thu, Jul 18, 2019 at 07:17:45PM +0100, Patrick Bellasi <patrick.bellasi@xxxxxxx> wrote:
> The clamp values are not tunable at the level of the root task group.
> That's for two main reasons:
> - the root group represents "system resources" which are always
> entirely available from the cgroup standpoint.
> - when tuning/restricting "system resources" makes sense, tuning must
> be done using a system wide API which should also be available when
> control groups are not.
> When a system wide restriction is available, cgroups should be aware of
> its value in order to know exactly how much "system resources" are
> available for the subgroups.
IIUC, the global default would apply in uclamp_eff_get(), so this
propagation isn't strictly necessary in order to apply to tasks (that's
how it works under !CONFIG_UCLAMP_TASK_GROUP).
The reason is that effective value (which isn't exposed currently) in a
group takes into account this global restriction, right?

> @@ -1043,12 +1063,17 @@ int sysctl_sched_uclamp_handler(struct ctl_table *table, int write,
> [...]
> + if (update_root_tg)
> + uclamp_update_root_tg();
> +
> /*
> * Updating all the RUNNABLE task is expensive, keep it simple and do
> * just a lazy update at each next enqueue time.
Since uclamp_update_root_tg() traverses down to
uclamp_update_active_tasks() is this comment half true now?

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