Re: [PATCH v3 1/1] sched/uclamp: add SCHED_FLAG_UTIL_CLAMP_RESET flag to reset uclamp

[Yun Hsiang <hsiang023167@gmail.com>]

Hi Qais,

On Thu, Oct 29, 2020 at 11:08:18AM +0000, Qais Yousef wrote:
> Hi Yun
> 
> Sorry for chipping in late.
> 
> On 10/25/20 15:36, Yun Hsiang wrote:
> > If the user wants to stop controlling uclamp and let the task inherit
> > the value from the group, we need a method to reset.
> > 
> > Add SCHED_FLAG_UTIL_CLAMP_RESET flag to allow the user to reset uclamp via
> > sched_setattr syscall.
> > 
> > The policy is
> > _CLAMP_RESET                           => reset both min and max
> > _CLAMP_RESET | _CLAMP_MIN              => reset min value
> > _CLAMP_RESET | _CLAMP_MAX              => reset max value
> > _CLAMP_RESET | _CLAMP_MIN | _CLAMP_MAX => reset both min and max
> > 
> > Signed-off-by: Yun Hsiang <hsiang023167@gmail.com>
> > ---
> >  include/uapi/linux/sched.h |  7 +++++--
> >  kernel/sched/core.c        | 41 +++++++++++++++++++++++++++++++-------
> >  2 files changed, 39 insertions(+), 9 deletions(-)
> > 
> > diff --git a/include/uapi/linux/sched.h b/include/uapi/linux/sched.h
> > index 3bac0a8ceab2..6c823ddb1a1e 100644
> > --- a/include/uapi/linux/sched.h
> > +++ b/include/uapi/linux/sched.h
> > @@ -132,17 +132,20 @@ struct clone_args {
> >  #define SCHED_FLAG_KEEP_PARAMS		0x10
> >  #define SCHED_FLAG_UTIL_CLAMP_MIN	0x20
> >  #define SCHED_FLAG_UTIL_CLAMP_MAX	0x40
> > +#define SCHED_FLAG_UTIL_CLAMP_RESET	0x80
> >  
> >  #define SCHED_FLAG_KEEP_ALL	(SCHED_FLAG_KEEP_POLICY | \
> >  				 SCHED_FLAG_KEEP_PARAMS)
> >  
> >  #define SCHED_FLAG_UTIL_CLAMP	(SCHED_FLAG_UTIL_CLAMP_MIN | \
> > -				 SCHED_FLAG_UTIL_CLAMP_MAX)
> > +				 SCHED_FLAG_UTIL_CLAMP_MAX | \
> > +				 SCHED_FLAG_UTIL_CLAMP_RESET)
> 
> Is it safe to change this define in a uapi header without a potential
> consequence?
> 
> FWIW I still have concerns about this approach. We're doing a reset to control
> cgroup behavior, I don't see any correlation between the two. Besides the
> difference between RESET and setting uclamp_min=0 without RESET is not obvious
> nor intuitive for someone who didn't look at the code.
> 
> I propose something like the below which is more explicit about what is being
> requested and delivered here. And if we decide to deprecate this behavior,
> it'd be much easier to just ignore this flag.
> 
> You must set this flag with your uclamp request to retain the cgroup
> inheritance behavior. If the flag is not set, we automatically clear it.

I think this behavior may not meet android requirement. Becasue in
android there is group like top-app. And we want to boost the
group by setting group uclamp_min. If group inheritance is explicit, we
need to set this flag for all the tasks in top-app. This might be
costly.

> 
> Only compile tested.
> 
> Thanks
> 
> --
> Qais Yousef
> 
> 
> --------->8-----------
> 
> From fe48fde7dea582b3b075e80e6936e1199f24363d Mon Sep 17 00:00:00 2001
> From: Qais Yousef <qais.yousef@arm.com>
> Date: Wed, 28 Oct 2020 22:36:26 +0000
> Subject: [PATCH] sched/uclamp: Add new flag to control cgroup behavior
> 
> Signed-off-by: Qais Yousef <qais.yousef@arm.com>
> ---
>  include/linux/sched.h      |  1 +
>  include/uapi/linux/sched.h |  6 ++++++
>  kernel/sched/core.c        | 25 ++++++++++++++++---------
>  3 files changed, 23 insertions(+), 9 deletions(-)
> 
> diff --git a/include/linux/sched.h b/include/linux/sched.h
> index afe01e232935..6eb35dfaa893 100644
> --- a/include/linux/sched.h
> +++ b/include/linux/sched.h
> @@ -604,6 +604,7 @@ struct uclamp_se {
>  	unsigned int bucket_id		: bits_per(UCLAMP_BUCKETS);
>  	unsigned int active		: 1;
>  	unsigned int user_defined	: 1;
> +	unsigned int follow_cgroup	: 1;
>  };
>  #endif /* CONFIG_UCLAMP_TASK */
>  
> diff --git a/include/uapi/linux/sched.h b/include/uapi/linux/sched.h
> index 3bac0a8ceab2..33ff716a7574 100644
> --- a/include/uapi/linux/sched.h
> +++ b/include/uapi/linux/sched.h
> @@ -132,6 +132,12 @@ struct clone_args {
>  #define SCHED_FLAG_KEEP_PARAMS		0x10
>  #define SCHED_FLAG_UTIL_CLAMP_MIN	0x20
>  #define SCHED_FLAG_UTIL_CLAMP_MAX	0x40
> +/*
> + * Control whether a task follows/inherits the cgroup uclamp.min/max or not.
> + * By default this flag is set for all tasks. Any explicit modification to a
> + * task's UCLAMP_MIN/MAX must set this flag to retain the bahavior.
> + */
> +#define SCHED_FLAG_UTIL_FOLLOW_CGROUP	0x80
>  
>  #define SCHED_FLAG_KEEP_ALL	(SCHED_FLAG_KEEP_POLICY | \
>  				 SCHED_FLAG_KEEP_PARAMS)
> diff --git a/kernel/sched/core.c b/kernel/sched/core.c
> index 2d95dc3f4644..1c77d6b8bd96 100644
> --- a/kernel/sched/core.c
> +++ b/kernel/sched/core.c
> @@ -1083,7 +1083,7 @@ uclamp_tg_restrict(struct task_struct *p, enum uclamp_id clamp_id)
>  		return uc_req;
>  
>  	uc_max = task_group(p)->uclamp[clamp_id];
> -	if (uc_req.value > uc_max.value || !uc_req.user_defined)
> +	if (uc_req.value > uc_max.value || uc_req.follow_cgroup)
>  		return uc_max;
>  #endif
>  
> @@ -1446,6 +1446,7 @@ static void __setscheduler_uclamp(struct task_struct *p,
>  				  const struct sched_attr *attr)
>  {
>  	enum uclamp_id clamp_id;
> +	bool follow_cgroup;
>  
>  	/*
>  	 * On scheduling class change, reset to default clamps for tasks
> @@ -1472,14 +1473,18 @@ static void __setscheduler_uclamp(struct task_struct *p,
>  	if (likely(!(attr->sched_flags & SCHED_FLAG_UTIL_CLAMP)))
>  		return;
>  
> +	follow_cgroup = attr->sched_flags & SCHED_FLAG_UTIL_FOLLOW_CGROUP;
> +
>  	if (attr->sched_flags & SCHED_FLAG_UTIL_CLAMP_MIN) {
> -		uclamp_se_set(&p->uclamp_req[UCLAMP_MIN],
> -			      attr->sched_util_min, true);
> +		struct uclamp_se *uc_se = &p->uclamp_req[UCLAMP_MIN];
> +		uc_se->follow_cgroup = follow_cgroup;
> +		uclamp_se_set(uc_se, attr->sched_util_min, true);
>  	}
>  
>  	if (attr->sched_flags & SCHED_FLAG_UTIL_CLAMP_MAX) {
> -		uclamp_se_set(&p->uclamp_req[UCLAMP_MAX],
> -			      attr->sched_util_max, true);
> +		struct uclamp_se *uc_se = &p->uclamp_req[UCLAMP_MAX];
> +		uc_se->follow_cgroup = follow_cgroup;
> +		uclamp_se_set(uc_se, attr->sched_util_max, true);
>  	}
>  }
>  
> @@ -1498,8 +1503,9 @@ static void uclamp_fork(struct task_struct *p)
>  		return;
>  
>  	for_each_clamp_id(clamp_id) {
> -		uclamp_se_set(&p->uclamp_req[clamp_id],
> -			      uclamp_none(clamp_id), false);
> +		struct uclamp_se *uc_se = &p->uclamp_req[clamp_id];
> +		uc_se->follow_cgroup = true;
> +		uclamp_se_set(uc_se, uclamp_none(clamp_id), false);
>  	}
>  }
>  
> @@ -1532,8 +1538,9 @@ static void __init init_uclamp(void)
>  		init_uclamp_rq(cpu_rq(cpu));
>  
>  	for_each_clamp_id(clamp_id) {
> -		uclamp_se_set(&init_task.uclamp_req[clamp_id],
> -			      uclamp_none(clamp_id), false);
> +		struct uclamp_se *uc_se = &init_task.uclamp_req[clamp_id];
> +		uc_se->follow_cgroup = true;
> +		uclamp_se_set(uc_se, uclamp_none(clamp_id), false);
>  	}
>  
>  	/* System defaults allow max clamp values for both indexes */
> -- 
> 2.25.1