Re: [PATCH v6 08/16] sched/cpufreq: uclamp: Add utilization clamping for FAIR tasks

["Rafael J. Wysocki" <rjw@rjwysocki.net>]

On Tuesday, January 15, 2019 11:15:05 AM CET Patrick Bellasi wrote:
> Each time a frequency update is required via schedutil, a frequency is
> selected to (possibly) satisfy the utilization reported by each
> scheduling class. However, when utilization clamping is in use, the
> frequency selection should consider userspace utilization clamping
> hints.  This will allow, for example, to:
> 
>  - boost tasks which are directly affecting the user experience
>    by running them at least at a minimum "requested" frequency
> 
>  - cap low priority tasks not directly affecting the user experience
>    by running them only up to a maximum "allowed" frequency
> 
> These constraints are meant to support a per-task based tuning of the
> frequency selection thus supporting a fine grained definition of
> performance boosting vs energy saving strategies in kernel space.
> 
> Add support to clamp the utilization and IOWait boost of RUNNABLE FAIR
> tasks within the boundaries defined by their aggregated utilization
> clamp constraints.
> Based on the max(min_util, max_util) of each task, max-aggregated the
> CPU clamp value in a way to give the boosted tasks the performance they
> need when they happen to be co-scheduled with other capped tasks.
> 
> Signed-off-by: Patrick Bellasi <patrick.bellasi@arm.com>
> Cc: Ingo Molnar <mingo@redhat.com>
> Cc: Peter Zijlstra <peterz@infradead.org>
> Cc: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
> 
> ---
> Changes in v6:
>  Message-ID: <20181107113849.GC14309@e110439-lin>
>  - sanity check util_max >= util_min
>  Others:
>  - wholesale s/group/bucket/
>  - wholesale s/_{get,put}/_{inc,dec}/ to match refcount APIs
> ---
>  kernel/sched/cpufreq_schedutil.c | 27 ++++++++++++++++++++++++---
>  kernel/sched/sched.h             | 23 +++++++++++++++++++++++
>  2 files changed, 47 insertions(+), 3 deletions(-)
> 
> diff --git a/kernel/sched/cpufreq_schedutil.c b/kernel/sched/cpufreq_schedutil.c
> index 033ec7c45f13..520ee2b785e7 100644
> --- a/kernel/sched/cpufreq_schedutil.c
> +++ b/kernel/sched/cpufreq_schedutil.c
> @@ -218,8 +218,15 @@ unsigned long schedutil_freq_util(int cpu, unsigned long util_cfs,
>  	 * CFS tasks and we use the same metric to track the effective
>  	 * utilization (PELT windows are synchronized) we can directly add them
>  	 * to obtain the CPU's actual utilization.
> +	 *
> +	 * CFS utilization can be boosted or capped, depending on utilization
> +	 * clamp constraints requested by currently RUNNABLE tasks.
> +	 * When there are no CFS RUNNABLE tasks, clamps are released and
> +	 * frequency will be gracefully reduced with the utilization decay.
>  	 */
> -	util = util_cfs;
> +	util = (type == ENERGY_UTIL)
> +		? util_cfs
> +		: uclamp_util(rq, util_cfs);
>  	util += cpu_util_rt(rq);
>  
>  	dl_util = cpu_util_dl(rq);
> @@ -327,6 +334,7 @@ static void sugov_iowait_boost(struct sugov_cpu *sg_cpu, u64 time,
>  			       unsigned int flags)
>  {
>  	bool set_iowait_boost = flags & SCHED_CPUFREQ_IOWAIT;
> +	unsigned int max_boost;
>  
>  	/* Reset boost if the CPU appears to have been idle enough */
>  	if (sg_cpu->iowait_boost &&
> @@ -342,11 +350,24 @@ static void sugov_iowait_boost(struct sugov_cpu *sg_cpu, u64 time,
>  		return;
>  	sg_cpu->iowait_boost_pending = true;
>  
> +	/*
> +	 * Boost FAIR tasks only up to the CPU clamped utilization.
> +	 *
> +	 * Since DL tasks have a much more advanced bandwidth control, it's
> +	 * safe to assume that IO boost does not apply to those tasks.
> +	 * Instead, since RT tasks are not utilization clamped, we don't want
> +	 * to apply clamping on IO boost while there is blocked RT
> +	 * utilization.
> +	 */
> +	max_boost = sg_cpu->iowait_boost_max;
> +	if (!cpu_util_rt(cpu_rq(sg_cpu->cpu)))
> +		max_boost = uclamp_util(cpu_rq(sg_cpu->cpu), max_boost);
> +
>  	/* Double the boost at each request */
>  	if (sg_cpu->iowait_boost) {
>  		sg_cpu->iowait_boost <<= 1;
> -		if (sg_cpu->iowait_boost > sg_cpu->iowait_boost_max)
> -			sg_cpu->iowait_boost = sg_cpu->iowait_boost_max;
> +		if (sg_cpu->iowait_boost > max_boost)
> +			sg_cpu->iowait_boost = max_boost;
>  		return;
>  	}
>  
> diff --git a/kernel/sched/sched.h b/kernel/sched/sched.h
> index b7f3ee8ba164..95d62a2a0b44 100644
> --- a/kernel/sched/sched.h
> +++ b/kernel/sched/sched.h
> @@ -2267,6 +2267,29 @@ static inline unsigned int uclamp_none(int clamp_id)
>  	return SCHED_CAPACITY_SCALE;
>  }
>  
> +#ifdef CONFIG_UCLAMP_TASK
> +static inline unsigned int uclamp_util(struct rq *rq, unsigned int util)
> +{
> +	unsigned int min_util = READ_ONCE(rq->uclamp[UCLAMP_MIN].value);
> +	unsigned int max_util = READ_ONCE(rq->uclamp[UCLAMP_MAX].value);
> +
> +	/*
> +	 * Since CPU's {min,max}_util clamps are MAX aggregated considering
> +	 * RUNNABLE tasks with _different_ clamps, we can end up with an
> +	 * invertion, which we can fix at usage time.
> +	 */
> +	if (unlikely(min_util >= max_util))
> +		return min_util;
> +
> +	return clamp(util, min_util, max_util);
> +}
> +#else /* CONFIG_UCLAMP_TASK */
> +static inline unsigned int uclamp_util(struct rq *rq, unsigned int util)
> +{
> +	return util;
> +}
> +#endif /* CONFIG_UCLAMP_TASK */
> +
>  #ifdef arch_scale_freq_capacity
>  # ifndef arch_scale_freq_invariant
>  #  define arch_scale_freq_invariant()	true
> 

IMO it would be better to combine this patch with the next one.

At least some things in it I was about to ask about would go away
then. :-)

Besides, I don't really see a reason for the split here.