Re: [PATCH v10 12/16] sched/core: uclamp: Extend CPU's cgroup controller

From: Tejun Heo <tj@kernel.org>
To: Patrick Bellasi <patrick.bellasi@arm.com>
Cc: linux-kernel@vger.kernel.org, linux-pm@vger.kernel.org,
	Ingo Molnar <mingo@redhat.com>,
	Peter Zijlstra <peterz@infradead.org>,
	"Rafael J . Wysocki" <rafael.j.wysocki@intel.com>,
	Vincent Guittot <vincent.guittot@linaro.org>,
	Viresh Kumar <viresh.kumar@linaro.org>,
	Paul Turner <pjt@google.com>,
	Quentin Perret <quentin.perret@arm.com>,
	Dietmar Eggemann <dietmar.eggemann@arm.com>,
	Morten Rasmussen <morten.rasmussen@arm.com>,
	Juri Lelli <juri.lelli@redhat.com>, Todd Kjos <tkjos@google.com>,
	Joel Fernandes <joelaf@google.com>,
	Steve Muckle <smuckle@google.com>,
	Suren Baghdasaryan <surenb@google.com>,
	Alessio Balsini <balsini@android.com>
Subject: Re: [PATCH v10 12/16] sched/core: uclamp: Extend CPU's cgroup controller
Date: Sat, 22 Jun 2019 08:03:32 -0700	[thread overview]
Message-ID: <20190622150332.GM657710@devbig004.ftw2.facebook.com> (raw)
In-Reply-To: <20190621084217.8167-13-patrick.bellasi@arm.com>

Hello,

Generally looks good to me.  Some nitpicks.

On Fri, Jun 21, 2019 at 09:42:13AM +0100, Patrick Bellasi wrote:
> @@ -951,6 +951,12 @@ controller implements weight and absolute bandwidth limit models for
>  normal scheduling policy and absolute bandwidth allocation model for
>  realtime scheduling policy.
>  
> +Cycles distribution is based, by default, on a temporal base and it
> +does not account for the frequency at which tasks are executed.
> +The (optional) utilization clamping support allows to enforce a minimum
> +bandwidth, which should always be provided by a CPU, and a maximum bandwidth,
> +which should never be exceeded by a CPU.

I kinda wonder whether the term bandwidth is a bit confusing because
it's also used for cpu.max/min.  Would just calling it frequency be
clearer?

> +static ssize_t cpu_uclamp_min_write(struct kernfs_open_file *of,
> +				    char *buf, size_t nbytes,
> +				    loff_t off)
> +{
> +	struct task_group *tg;
> +	u64 min_value;
> +	int ret;
> +
> +	ret = uclamp_scale_from_percent(buf, &min_value);
> +	if (ret)
> +		return ret;
> +	if (min_value > SCHED_CAPACITY_SCALE)
> +		return -ERANGE;
> +
> +	rcu_read_lock();
> +
> +	tg = css_tg(of_css(of));
> +	if (tg == &root_task_group) {
> +		ret = -EINVAL;
> +		goto out;
> +	}

I don't think you need the above check.

> +	if (tg->uclamp_req[UCLAMP_MIN].value == min_value)
> +		goto out;
> +	if (tg->uclamp_req[UCLAMP_MAX].value < min_value) {
> +		ret = -EINVAL;

So, uclamp.max limits the maximum freq% can get and uclamp.min limits
hte maximum freq% protection can get in the subtree.  Let's say
uclamp.max is 50% and uclamp.min is 100%.  It means that protection is
not limited but the actual freq% is limited upto 50%, which isn't
necessarily invalid.  For a simple example, a user might be saying
that they want to get whatever protection they can get from its parent
but wanna limit eventual freq at 50% and it isn't too difficult to
imagine cases where the two knobs are configured separately especially
configuration is being managed hierarchically / automatically.

tl;dr is that we don't need the above restriction and shouldn't
generally be restricting configurations when they don't need to.

Thanks.

-- 
tejun