Date:   Tue, 14 Apr 2020 20:21:52 +0200
From:   Patrick Bellasi <patrick.bellasi@...bug.net>
To:     Qais Yousef <qais.yousef@....com>
Cc:     Ingo Molnar <mingo@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Jonathan Corbet <corbet@....net>,
        Juri Lelli <juri.lelli@...hat.com>,
        Vincent Guittot <vincent.guittot@...aro.org>,
        Dietmar Eggemann <dietmar.eggemann@....com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Ben Segall <bsegall@...gle.com>, Mel Gorman <mgorman@...e.de>,
        Luis Chamberlain <mcgrof@...nel.org>,
        Kees Cook <keescook@...omium.org>,
        Iurii Zaikin <yzaikin@...gle.com>,
        Quentin Perret <qperret@...gle.com>,
        Valentin Schneider <valentin.schneider@....com>,
        Pavan Kondeti <pkondeti@...eaurora.org>,
        linux-doc@...r.kernel.org, linux-kernel@...r.kernel.org,
        linux-fsdevel@...r.kernel.org
Subject: Re: [PATCH 1/2] sched/uclamp: Add a new sysctl to control RT default
 boost value

Hi Qais!

On 03-Apr 13:30, Qais Yousef wrote:

[...]

> diff --git a/include/linux/sched/sysctl.h b/include/linux/sched/sysctl.h
> index d4f6215ee03f..91204480fabc 100644
> --- a/include/linux/sched/sysctl.h
> +++ b/include/linux/sched/sysctl.h
> @@ -59,6 +59,7 @@ extern int sysctl_sched_rt_runtime;
>  #ifdef CONFIG_UCLAMP_TASK
>  extern unsigned int sysctl_sched_uclamp_util_min;
>  extern unsigned int sysctl_sched_uclamp_util_max;
> +extern unsigned int sysctl_sched_rt_default_uclamp_util_min;

nit-pick: I would prefer to keep the same prefix of the already
exising knobs, i.e. sysctl_sched_uclamp_util_min_rt

The same change for consistency should be applied to all the following
symbols related to "uclamp_util_min_rt".

NOTE: I would not use "default" as I think that what we are doing is
exactly force setting a user_defined value for all RT tasks. More on
that later...

>  #endif
>  
>  #ifdef CONFIG_CFS_BANDWIDTH
> diff --git a/kernel/sched/core.c b/kernel/sched/core.c
> index 1a9983da4408..a726b26a5056 100644
> --- a/kernel/sched/core.c
> +++ b/kernel/sched/core.c
> @@ -797,6 +797,27 @@ unsigned int sysctl_sched_uclamp_util_min = SCHED_CAPACITY_SCALE;
>  /* Max allowed maximum utilization */
>  unsigned int sysctl_sched_uclamp_util_max = SCHED_CAPACITY_SCALE;
>  
> +/*
> + * By default RT tasks run at the maximum performance point/capacity of the
> + * system. Uclamp enforces this by always setting UCLAMP_MIN of RT tasks to
> + * SCHED_CAPACITY_SCALE.
> + *
> + * This knob allows admins to change the default behavior when uclamp is being
> + * used. In battery powered devices, particularly, running at the maximum
> + * capacity and frequency will increase energy consumption and shorten the
> + * battery life.
> + *
> + * This knob only affects the default value RT has when a new RT task is
> + * forked or has just changed policy to RT, given the user hasn't modified the
> + * uclamp.min value of the task via sched_setattr().
> + *
> + * This knob will not override the system default sched_util_clamp_min defined
> + * above.
> + *
> + * Any modification is applied lazily on the next RT task wakeup.
> + */
> +unsigned int sysctl_sched_rt_default_uclamp_util_min = SCHED_CAPACITY_SCALE;
> +
>  /* All clamps are required to be less or equal than these values */
>  static struct uclamp_se uclamp_default[UCLAMP_CNT];
>  
> @@ -924,6 +945,14 @@ uclamp_eff_get(struct task_struct *p, enum uclamp_id clamp_id)
>  	return uc_req;
>  }
>  
> +static void uclamp_rt_sync_default_util_min(struct task_struct *p)
> +{
> +	struct uclamp_se *uc_se = &p->uclamp_req[UCLAMP_MIN];

Don't we have to filter for RT tasks only here?

> +
> +	if (!uc_se->user_defined)
> +		uclamp_se_set(uc_se, sysctl_sched_rt_default_uclamp_util_min, false);

Here you are actually setting a user-requested value, why not marking
it as that, i.e. by using true for the last parameter?

Moreover, by keeping user_defined=false I think you are not getting
what you want for RT tasks running in a nested cgroup.

Let say a subgroup is still with the util_min=1024 inherited from the
system defaults, in uclamp_tg_restrict() we will still return the max
value and not what you requested for. Isn't it?

IOW, what about:

---8<---
static void uclamp_sync_util_min_rt(struct task_struct *p)
{
	struct uclamp_se *uc_se = &p->uclamp_req[UCLAMP_MIN];

  if (likely(uc_se->user_defined || !rt_task(p)))
    return;

  uclamp_se_set(uc_se, sysctl_sched_uclamp_util_min_rt, true);
}
---8<---


> +}
> +
>  unsigned long uclamp_eff_value(struct task_struct *p, enum uclamp_id clamp_id)
>  {
>  	struct uclamp_se uc_eff;
> @@ -1030,6 +1059,12 @@ static inline void uclamp_rq_inc(struct rq *rq, struct task_struct *p)
>  	if (unlikely(!p->sched_class->uclamp_enabled))
>  		return;
>  
> +	/*
> +	 * When sysctl_sched_rt_default_uclamp_util_min value is changed by the
> +	 * user, we apply any new value on the next wakeup, which is here.
> +	 */
> +	uclamp_rt_sync_default_util_min(p);
> +
>  	for_each_clamp_id(clamp_id)
>  		uclamp_rq_inc_id(rq, p, clamp_id);
>  
> @@ -1121,12 +1156,13 @@ int sysctl_sched_uclamp_handler(struct ctl_table *table, int write,
>  				loff_t *ppos)
>  {
>  	bool update_root_tg = false;
> -	int old_min, old_max;
> +	int old_min, old_max, old_rt_min;
>  	int result;
>  
>  	mutex_lock(&uclamp_mutex);
>  	old_min = sysctl_sched_uclamp_util_min;
>  	old_max = sysctl_sched_uclamp_util_max;
> +	old_rt_min = sysctl_sched_rt_default_uclamp_util_min;

Perpahs it's just my OCD but, is not "old_min_rt" reading better?

>  
>  	result = proc_dointvec(table, write, buffer, lenp, ppos);
>  	if (result)
> @@ -1134,12 +1170,23 @@ int sysctl_sched_uclamp_handler(struct ctl_table *table, int write,
>  	if (!write)
>  		goto done;
>  
> +	/*
> +	 * The new value will be applied to all RT tasks the next time they
> +	 * wakeup, assuming the task is using the system default and not a user
> +	 * specified value. In the latter we shall leave the value as the user
> +	 * requested.
> +	 */

Should not this comment go before the next block?

>  	if (sysctl_sched_uclamp_util_min > sysctl_sched_uclamp_util_max ||
>  	    sysctl_sched_uclamp_util_max > SCHED_CAPACITY_SCALE) {
>  		result = -EINVAL;
>  		goto undo;
>  	}
>  
> +	if (sysctl_sched_rt_default_uclamp_util_min > SCHED_CAPACITY_SCALE) {
> +		result = -EINVAL;
> +		goto undo;
> +	}
> +
>  	if (old_min != sysctl_sched_uclamp_util_min) {
>  		uclamp_se_set(&uclamp_default[UCLAMP_MIN],
>  			      sysctl_sched_uclamp_util_min, false);
> @@ -1165,6 +1212,7 @@ int sysctl_sched_uclamp_handler(struct ctl_table *table, int write,
>  undo:
>  	sysctl_sched_uclamp_util_min = old_min;
>  	sysctl_sched_uclamp_util_max = old_max;
> +	sysctl_sched_rt_default_uclamp_util_min = old_rt_min;
>  done:
>  	mutex_unlock(&uclamp_mutex);
>  
> @@ -1207,9 +1255,13 @@ static void __setscheduler_uclamp(struct task_struct *p,
>  		if (uc_se->user_defined)
>  			continue;
>  
> -		/* By default, RT tasks always get 100% boost */
> +		/*
> +		 * By default, RT tasks always get 100% boost, which the admins
> +		 * are allowed to change via
> +		 * sysctl_sched_rt_default_uclamp_util_min knob.
> +		 */
>  		if (unlikely(rt_task(p) && clamp_id == UCLAMP_MIN))
> -			clamp_value = uclamp_none(UCLAMP_MAX);
> +			clamp_value = sysctl_sched_rt_default_uclamp_util_min;
>
>  		uclamp_se_set(uc_se, clamp_value, false);
>  	}
> @@ -1241,9 +1293,13 @@ static void uclamp_fork(struct task_struct *p)
>  	for_each_clamp_id(clamp_id) {
>  		unsigned int clamp_value = uclamp_none(clamp_id);
>  
> -		/* By default, RT tasks always get 100% boost */
> +		/*
> +		 * By default, RT tasks always get 100% boost, which the admins
> +		 * are allowed to change via
> +		 * sysctl_sched_rt_default_uclamp_util_min knob.
> +		 */
>  		if (unlikely(rt_task(p) && clamp_id == UCLAMP_MIN))
> -			clamp_value = uclamp_none(UCLAMP_MAX);
> +			clamp_value = sysctl_sched_rt_default_uclamp_util_min;
>  

This is not required, look at this Quentin's patch:

   Message-ID: <20200414161320.251897-1-qperret@...gle.com>
   https://lore.kernel.org/lkml/20200414161320.251897-1-qperret@google.com/

>  		uclamp_se_set(&p->uclamp_req[clamp_id], clamp_value, false);
>  	}
> diff --git a/kernel/sysctl.c b/kernel/sysctl.c
> index ad5b88a53c5a..0272ae8c6147 100644
> --- a/kernel/sysctl.c
> +++ b/kernel/sysctl.c
> @@ -465,6 +465,13 @@ static struct ctl_table kern_table[] = {
>  		.mode		= 0644,
>  		.proc_handler	= sysctl_sched_uclamp_handler,
>  	},
> +	{
> +		.procname	= "sched_rt_default_util_clamp_min",
> +		.data		= &sysctl_sched_rt_default_uclamp_util_min,
> +		.maxlen		= sizeof(unsigned int),
> +		.mode		= 0644,
> +		.proc_handler	= sysctl_sched_uclamp_handler,
> +	},
>  #endif
>  #ifdef CONFIG_SCHED_AUTOGROUP
>  	{

Best,
Patrick

-- 
#include <best/regards.h>

Patrick Bellasi

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