linux-kernel - Re: [PATCH 1/7] sched/core: uclamp: add CPU clamp groups accounting

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Date:   Fri, 13 Apr 2018 11:46:15 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Patrick Bellasi <patrick.bellasi@....com>
Cc:     linux-kernel@...r.kernel.org, linux-pm@...r.kernel.org,
        Ingo Molnar <mingo@...hat.com>, Tejun Heo <tj@...nel.org>,
        "Rafael J . Wysocki" <rafael.j.wysocki@...el.com>,
        Viresh Kumar <viresh.kumar@...aro.org>,
        Vincent Guittot <vincent.guittot@...aro.org>,
        Paul Turner <pjt@...gle.com>,
        Dietmar Eggemann <dietmar.eggemann@....com>,
        Morten Rasmussen <morten.rasmussen@....com>,
        Juri Lelli <juri.lelli@...hat.com>,
        Joel Fernandes <joelaf@...gle.com>,
        Steve Muckle <smuckle@...gle.com>
Subject: Re: [PATCH 1/7] sched/core: uclamp: add CPU clamp groups accounting

On Mon, Apr 09, 2018 at 05:56:09PM +0100, Patrick Bellasi wrote:
> +static inline void uclamp_cpu_get(struct task_struct *p, int cpu, int clamp_id)
> +{
> +	struct uclamp_cpu *uc_cpu = &cpu_rq(cpu)->uclamp[clamp_id];
> +	int clamp_value;
> +	int group_id;
> +
> +	/* Get task's specific clamp value */
> +	clamp_value = p->uclamp[clamp_id].value;
> +	group_id = p->uclamp[clamp_id].group_id;
> +
> +	/* No task specific clamp values: nothing to do */
> +	if (group_id == UCLAMP_NONE)
> +		return;
> +
> +	/* Increment the current group_id */

That I think qualifies being called a bad comment.

> +	uc_cpu->group[group_id].tasks += 1;
> +
> +	/* Mark task as enqueued for this clamp index */
> +	p->uclamp_group_id[clamp_id] = group_id;

Why exactly do we need this? we got group_id from @p in the first place.

I suspect this is because when we update p->uclamp[], we don't update
this active value (when needed), is that worth it?

> +	/*
> +	 * If this is the new max utilization clamp value, then we can update
> +	 * straight away the CPU clamp value. Otherwise, the current CPU clamp
> +	 * value is still valid and we are done.
> +	 */
> +	if (uc_cpu->value < clamp_value)
> +		uc_cpu->value = clamp_value;
> +}