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Date: Sat, 22 Jun 2019 08:03:32 -0700 From: Tejun Heo <tj@...nel.org> To: Patrick Bellasi <patrick.bellasi@....com> Cc: linux-kernel@...r.kernel.org, linux-pm@...r.kernel.org, Ingo Molnar <mingo@...hat.com>, Peter Zijlstra <peterz@...radead.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>, Alessio Balsini <balsini@...roid.com> Subject: Re: [PATCH v10 12/16] sched/core: uclamp: Extend CPU's cgroup controller 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
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