linux-kernel - Re: [PATCH v7 03/15] sched/core: uclamp: Add system default clamps

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Date:   Wed, 13 Mar 2019 15:32:40 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Patrick Bellasi <patrick.bellasi@....com>
Cc:     linux-kernel@...r.kernel.org, linux-pm@...r.kernel.org,
        linux-api@...r.kernel.org, Ingo Molnar <mingo@...hat.com>,
        Tejun Heo <tj@...nel.org>,
        "Rafael J . Wysocki" <rafael.j.wysocki@...el.com>,
        Vincent Guittot <vincent.guittot@...aro.org>,
        Viresh Kumar <viresh.kumar@...aro.org>,
        Paul Turner <pjt@...gle.com>,
        Quentin Perret <quentin.perret@....com>,
        Dietmar Eggemann <dietmar.eggemann@....com>,
        Morten Rasmussen <morten.rasmussen@....com>,
        Juri Lelli <juri.lelli@...hat.com>,
        Todd Kjos <tkjos@...gle.com>,
        Joel Fernandes <joelaf@...gle.com>,
        Steve Muckle <smuckle@...gle.com>,
        Suren Baghdasaryan <surenb@...gle.com>
Subject: Re: [PATCH v7 03/15] sched/core: uclamp: Add system default clamps

On Fri, Feb 08, 2019 at 10:05:42AM +0000, Patrick Bellasi wrote:

> diff --git a/include/linux/sched.h b/include/linux/sched.h
> index 45460e7a3eee..447261cd23ba 100644
> --- a/include/linux/sched.h
> +++ b/include/linux/sched.h
> @@ -584,14 +584,32 @@ struct sched_dl_entity {
>   * Utilization clamp for a scheduling entity
>   * @value:		clamp value "requested" by a se
>   * @bucket_id:		clamp bucket corresponding to the "requested" value
> + * @effective:		clamp value and bucket actually "assigned" to the se
> + * @active:		the se is currently refcounted in a rq's bucket
>   *
> + * Both bucket_id and effective::bucket_id are the index of the clamp bucket
> + * matching the corresponding clamp value which are pre-computed and stored to
> + * avoid expensive integer divisions from the fast path.
> + *
> + * The active bit is set whenever a task has got an effective::value assigned,
> + * which can be different from the user requested clamp value. This allows to
> + * know a task is actually refcounting the rq's effective::bucket_id bucket.
>   */
>  struct uclamp_se {
> +	/* Clamp value "requested" by a scheduling entity */
>  	unsigned int value		: bits_per(SCHED_CAPACITY_SCALE);
>  	unsigned int bucket_id		: bits_per(UCLAMP_BUCKETS);
> +	unsigned int active		: 1;
> +	/*
> +	 * Clamp value "obtained" by a scheduling entity.
> +	 *
> +	 * This cache the actual clamp value, possibly enforced by system
> +	 * default clamps, a task is subject to while enqueued in a rq.
> +	 */
> +	struct {
> +		unsigned int value	: bits_per(SCHED_CAPACITY_SCALE);
> +		unsigned int bucket_id	: bits_per(UCLAMP_BUCKETS);
> +	} effective;

I still think that this effective thing is backwards.

The existing code already used @value and @bucket_id as 'effective' and
you're now changing all that again. This really doesn't make sense to
me.

Also; if you don't add it inside struct uclamp_se, but add a second
instance,

>  };
>  #endif /* CONFIG_UCLAMP_TASK */
>  


> @@ -803,6 +811,70 @@ static inline void uclamp_rq_update(struct rq *rq, unsigned int clamp_id,
>  	WRITE_ONCE(rq->uclamp[clamp_id].value, max_value);
>  }
>  
> +/*
> + * The effective clamp bucket index of a task depends on, by increasing
> + * priority:
> + * - the task specific clamp value, when explicitly requested from userspace
> + * - the system default clamp value, defined by the sysadmin
> + *
> + * As a side effect, update the task's effective value:
> + *    task_struct::uclamp::effective::value
> + * to represent the clamp value of the task effective bucket index.
> + */
> +static inline void
> +uclamp_effective_get(struct task_struct *p, unsigned int clamp_id,
> +		     unsigned int *clamp_value, unsigned int *bucket_id)
> +{
> +	/* Task specific clamp value */
> +	*bucket_id = p->uclamp[clamp_id].bucket_id;
> +	*clamp_value = p->uclamp[clamp_id].value;
> +
> +	/* Always apply system default restrictions */
> +	if (unlikely(*clamp_value > uclamp_default[clamp_id].value)) {
> +		*clamp_value = uclamp_default[clamp_id].value;
> +		*bucket_id = uclamp_default[clamp_id].bucket_id;
> +	}
> +}

you can avoid horrors like this and simply return a struct uclamp_se by
value.