Re: [PATCH v2 01/12] sched/core: uclamp: extend sched_setattr to support utilization clamping

[Joel Fernandes <joel@joelfernandes.org>]

On Mon, Jul 16, 2018 at 09:28:55AM +0100, Patrick Bellasi wrote:
> The SCHED_DEADLINE scheduling class provides an advanced and formal
> model to define tasks requirements which can be translated into proper
> decisions for both task placements and frequencies selections.
> Other classes have a more simplified model which is essentially based on
> the relatively simple concept of POSIX priorities.
> 
> Such a simple priority based model however does not allow to exploit
> some of the most advanced features of the Linux scheduler like, for
> example, driving frequencies selection via the schedutil cpufreq
> governor. However, also for non SCHED_DEADLINE tasks, it's still
> interesting to define tasks properties which can be used to better
> support certain scheduler decisions.
> 
> Utilization clamping aims at exposing to user-space a new set of
> per-task attributes which can be used to provide the scheduler with some
> hints about the expected/required utilization for a task.
> This will allow to implement a more advanced per-task frequency control
> mechanism which is not based just on a "passive" measured task
> utilization but on a more "active" approach. For example, it could be
> possible to boost interactive tasks, thus getting better performance, or
> cap background tasks, thus being more energy efficient.
> Ultimately, such a mechanism can be considered similar to the cpufreq's
> powersave, performance and userspace governor but with a much fine
> grained and per-task control.
> 
> Let's introduce a new API to set utilization clamping values for a
> specified task by extending sched_setattr, a syscall which already
> allows to define task specific properties for different scheduling
> classes.
> Specifically, a new pair of attributes allows to specify a minimum and
> maximum utilization which the scheduler should consider for a task.
> 
> 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: Vincent Guittot <vincent.guittot@linaro.org>
> Cc: Viresh Kumar <viresh.kumar@linaro.org>
> Cc: Paul Turner <pjt@google.com>
> Cc: Todd Kjos <tkjos@google.com>
> Cc: Joel Fernandes <joelaf@google.com>
> Cc: Steve Muckle <smuckle@google.com>
> Cc: Juri Lelli <juri.lelli@redhat.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
> ---
>  include/linux/sched.h            | 16 ++++++++
>  include/uapi/linux/sched.h       |  4 +-
>  include/uapi/linux/sched/types.h | 64 +++++++++++++++++++++++++++-----
>  init/Kconfig                     | 19 ++++++++++
>  kernel/sched/core.c              | 39 +++++++++++++++++++
>  5 files changed, 132 insertions(+), 10 deletions(-)
> 
> diff --git a/include/linux/sched.h b/include/linux/sched.h
> index 43731fe51c97..fd8495723088 100644
> --- a/include/linux/sched.h
> +++ b/include/linux/sched.h
> @@ -279,6 +279,17 @@ struct vtime {
>  	u64			gtime;
>  };
>  
> +enum uclamp_id {
> +	/* No utilization clamp group assigned */
> +	UCLAMP_NONE = -1,
> +
> +	UCLAMP_MIN = 0, /* Minimum utilization */
> +	UCLAMP_MAX,     /* Maximum utilization */
> +
> +	/* Utilization clamping constraints count */
> +	UCLAMP_CNT
> +};
> +
>  struct sched_info {
>  #ifdef CONFIG_SCHED_INFO
>  	/* Cumulative counters: */
> @@ -649,6 +660,11 @@ struct task_struct {
>  #endif
>  	struct sched_dl_entity		dl;
>  
> +#ifdef CONFIG_UCLAMP_TASK
> +	/* Utlization clamp values for this task */
> +	int				uclamp[UCLAMP_CNT];
> +#endif
> +
>  #ifdef CONFIG_PREEMPT_NOTIFIERS
>  	/* List of struct preempt_notifier: */
>  	struct hlist_head		preempt_notifiers;
> diff --git a/include/uapi/linux/sched.h b/include/uapi/linux/sched.h
> index 22627f80063e..c27d6e81517b 100644
> --- a/include/uapi/linux/sched.h
> +++ b/include/uapi/linux/sched.h
> @@ -50,9 +50,11 @@
>  #define SCHED_FLAG_RESET_ON_FORK	0x01
>  #define SCHED_FLAG_RECLAIM		0x02
>  #define SCHED_FLAG_DL_OVERRUN		0x04
> +#define SCHED_FLAG_UTIL_CLAMP		0x08
>  #define SCHED_FLAG_ALL	(SCHED_FLAG_RESET_ON_FORK	| \
>  			 SCHED_FLAG_RECLAIM		| \
> -			 SCHED_FLAG_DL_OVERRUN)
> +			 SCHED_FLAG_DL_OVERRUN		| \
> +			 SCHED_FLAG_UTIL_CLAMP)
>  
>  #endif /* _UAPI_LINUX_SCHED_H */
> diff --git a/include/uapi/linux/sched/types.h b/include/uapi/linux/sched/types.h
> index 10fbb8031930..7421cd25354d 100644
> --- a/include/uapi/linux/sched/types.h
> +++ b/include/uapi/linux/sched/types.h
> @@ -21,8 +21,33 @@ struct sched_param {
>   * the tasks may be useful for a wide variety of application fields, e.g.,
>   * multimedia, streaming, automation and control, and many others.
>   *
> - * This variant (sched_attr) is meant at describing a so-called
> - * sporadic time-constrained task. In such model a task is specified by:
> + * This variant (sched_attr) allows to define additional attributes to
> + * improve the scheduler knowledge about task requirements.
> + *
> + * Scheduling Class Attributes
> + * ===========================
> + *
> + * A subset of sched_attr attributes specifies the
> + * scheduling policy and relative POSIX attributes:
> + *
> + *  @size		size of the structure, for fwd/bwd compat.
> + *
> + *  @sched_policy	task's scheduling policy
> + *  @sched_nice		task's nice value      (SCHED_NORMAL/BATCH)
> + *  @sched_priority	task's static priority (SCHED_FIFO/RR)
> + *
> + * Certain more advanced scheduling features can be controlled by a
> + * predefined set of flags via the attribute:
> + *
> + *  @sched_flags	for customizing the scheduler behaviour
> + *
> + * Sporadic Time-Constrained Tasks Attributes
> + * ==========================================
> + *
> + * A subset of sched_attr attributes allows to describe a so-called
> + * sporadic time-constrained task.
> + *
> + * In such model a task is specified by:
>   *  - the activation period or minimum instance inter-arrival time;
>   *  - the maximum (or average, depending on the actual scheduling
>   *    discipline) computation time of all instances, a.k.a. runtime;
> @@ -34,14 +59,8 @@ struct sched_param {
>   * than the runtime and must be completed by time instant t equal to
>   * the instance activation time + the deadline.
>   *
> - * This is reflected by the actual fields of the sched_attr structure:
> + * This is reflected by the following fields of the sched_attr structure:
>   *
> - *  @size		size of the structure, for fwd/bwd compat.
> - *
> - *  @sched_policy	task's scheduling policy
> - *  @sched_flags	for customizing the scheduler behaviour
> - *  @sched_nice		task's nice value      (SCHED_NORMAL/BATCH)
> - *  @sched_priority	task's static priority (SCHED_FIFO/RR)
>   *  @sched_deadline	representative of the task's deadline
>   *  @sched_runtime	representative of the task's runtime
>   *  @sched_period	representative of the task's period
> @@ -53,6 +72,28 @@ struct sched_param {
>   * As of now, the SCHED_DEADLINE policy (sched_dl scheduling class) is the
>   * only user of this new interface. More information about the algorithm
>   * available in the scheduling class file or in Documentation/.
> + *
> + * Task Utilization Attributes
> + * ===========================
> + *
> + * A subset of sched_attr attributes allows to specify the utilization which
> + * should be expected by a task. These attributes allows to inform the
> + * scheduler about the utilization boundaries within which is safe to schedule
> + * the task. These utilization boundaries are valuable information to support
> + * scheduler decisions on both task placement and frequencies selection.
> + *
> + *  @sched_util_min	represents the minimum utilization
> + *  @sched_util_max	represents the maximum utilization
> + *
> + * Utilization is a value in the range [0..SCHED_CAPACITY_SCALE] which
> + * represents the percentage of CPU time used by a task when running at the
> + * maximum frequency on the highest capacity CPU of the system. Thus, for
> + * example, a 20% utilization task is a task running for 2ms every 10ms.
> + *
> + * A task with a min utilization value bigger then 0 is more likely to be
> + * scheduled on a CPU which can provide that bandwidth.
> + * A task with a max utilization value smaller then 1024 is more likely to be
> + * scheduled on a CPU which do not provide more then the required bandwidth.
>   */
>  struct sched_attr {
>  	__u32 size;
> @@ -70,6 +111,11 @@ struct sched_attr {
>  	__u64 sched_runtime;
>  	__u64 sched_deadline;
>  	__u64 sched_period;
> +
> +	/* Utilization hints */
> +	__u32 sched_util_min;
> +	__u32 sched_util_max;
> +
>  };
>  
>  #endif /* _UAPI_LINUX_SCHED_TYPES_H */
> diff --git a/init/Kconfig b/init/Kconfig
> index 041f3a022122..1d45a6877d6f 100644
> --- a/init/Kconfig
> +++ b/init/Kconfig
> @@ -583,6 +583,25 @@ config HAVE_UNSTABLE_SCHED_CLOCK
>  config GENERIC_SCHED_CLOCK
>  	bool
>  
> +menu "Scheduler features"
> +
> +config UCLAMP_TASK
> +	bool "Enable utilization clamping for RT/FAIR tasks"
> +	depends on CPU_FREQ_GOV_SCHEDUTIL

Does it make sense to depend on this? One could turn off schedutil and then
uclamp can't be used for any other purpose (big.LITTLE task placement etc)?

thanks,

- Joel