Re: [PATCH] sched/rt: Add a new sysctl to control uclamp_util_min

From: Peter Zijlstra <peterz@infradead.org>
To: Qais Yousef <qais.yousef@arm.com>
Cc: Ingo Molnar <mingo@redhat.com>,
	Dietmar Eggemann <dietmar.eggemann@arm.com>,
	Steven Rostedt <rostedt@goodmis.org>,
	Luis Chamberlain <mcgrof@kernel.org>,
	Kees Cook <keescook@chromium.org>,
	Iurii Zaikin <yzaikin@google.com>,
	Juri Lelli <juri.lelli@redhat.com>,
	Vincent Guittot <vincent.guittot@linaro.org>,
	Ben Segall <bsegall@google.com>, Mel Gorman <mgorman@suse.de>,
	valentin.schneider@arm.com, qperret@google.com,
	Patrick Bellasi <patrick.bellasi@matbug.net>,
	linux-kernel@vger.kernel.org, linux-fsdevel@vger.kernel.org
Subject: Re: [PATCH] sched/rt: Add a new sysctl to control uclamp_util_min
Date: Fri, 10 Jan 2020 14:39:56 +0100	[thread overview]
Message-ID: <20200110133956.GL2844@hirez.programming.kicks-ass.net> (raw)
In-Reply-To: <20200109130052.feebuwuuvwvm324w@e107158-lin.cambridge.arm.com>

On Thu, Jan 09, 2020 at 01:00:58PM +0000, Qais Yousef wrote:
> On 01/08/20 14:44, Peter Zijlstra wrote:
> > On Fri, Dec 20, 2019 at 04:48:38PM +0000, Qais Yousef wrote:
> > > RT tasks by default try to run at the highest capacity/performance
> > > level. When uclamp is selected this default behavior is retained by
> > > enforcing the uclamp_util_min of the RT tasks to be
> > > uclamp_none(UCLAMP_MAX), which is SCHED_CAPACITY_SCALE; the maximum
> > > value.
> > > 
> > > See commit 1a00d999971c ("sched/uclamp: Set default clamps for RT tasks").
> > > 
> > > On battery powered devices, this default behavior could consume more
> > > power, and it is desired to be able to tune it down. While uclamp allows
> > > tuning this by changing the uclamp_util_min of the individual tasks, but
> > > this is cumbersome and error prone.
> > > 
> > > To control the default behavior globally by system admins and device
> > > integrators, introduce the new sysctl_sched_rt_uclamp_util_min to
> > > change the default uclamp_util_min value of the RT tasks.
> > > 
> > > Whenever the new default changes, it'd be applied on the next wakeup of
> > > the RT task, assuming that it still uses the system default value and
> > > not a user applied one.
> > 
> > This is because these RT tasks are not in a cgroup or not affected by
> > cgroup settings? I feel the justification is a little thin here.
> 
> The uclamp_min for RT tasks is always hardcoded to 1024 at the moment. So even
> if they belong to a cgroup->uclamp_min = 0, they'll still run at max frequency,
> no?

Argh, this is that counter intuitive max aggregate nonsense biting me.