Re: [PATCH 1/2] sched/uclamp: Add a new sysctl to control RT default boost value

[Patrick Bellasi <patrick.bellasi@matbug.net>]

On 14-Apr 20:21, Patrick Bellasi wrote:
> Hi Qais!

Hello againa!

> 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...

Had a second tought on that...

> 
> >  #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?

I think this is still a valid point.

> > +
> > +	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?

I think you don't want to set user_defined to ensure we keep updating
the value every time the task is enqueued, in case the "default"
should be updated at run-time.

> 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?

This is also not completely true since perhaps you assume that if an
RT task is running in a nested group with a non tuned uclamp_max then
that's probably what we want.

There is still a small concern due to the fact we don't distinguish
CFS and RT tasks when it comes to cgroup clamp values, which
potentially could still generate the same issue. Let say for example
you wanna allow CFS tasks to be boosted to max (util_min=1024) but
still want to run RT tasks only at lower OPPs.
Not sure if that could be a use case tho.
 
> 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);
                                                          ^^^^
                     This should remain false as in your patch.
> }
> ---8<---

Still, I was thinking that perhaps it would be better to massage the
code above into the generation of the effective value, in uclamp_eff_get().

Since you wanna (possibly) update the value at each enqueue time,
that's what conceptually is represented by the "effective clamp
value": a value that is computed by definition at enqueue time by
aggregating all the requests and constraints.

Poking with the effective value instead of the requested value will
fix also the ambiguity above, where we set a "requested values" with
user-defined=false.

> > +}
> > +
> >  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@google.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
> >  	{

-- 
#include <best/regards.h>

Patrick Bellasi