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From: Vincent Guittot <vincent.guittot@linaro.org>
Date: Tue, 1 Mar 2016 15:58:23 +0100
Message-ID: <CAKfTPtC7MLeYT6nNKPG777fYgha6wLpeLxHx2H175cg0=_FRDA@mail.gmail.com>
Subject: Re: [PATCH 0/3] cpufreq: Replace timers with utilization update callbacks
To: Peter Zijlstra <peterz@infradead.org>
Cc: Juri Lelli <juri.lelli@arm.com>, Steve Muckle <steve.muckle@linaro.org>,
        "Rafael J. Wysocki" <rjw@rjwysocki.net>,
        "Rafael J. Wysocki" <rafael@kernel.org>,
        Linux PM list <linux-pm@vger.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        Srinivas Pandruvada <srinivas.pandruvada@linux.intel.com>,
        Viresh Kumar <viresh.kumar@linaro.org>,
        Thomas Gleixner <tglx@linutronix.de>
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On 1 March 2016 at 14:58, Peter Zijlstra <peterz@infradead.org> wrote:
> On Fri, Feb 12, 2016 at 03:48:54PM +0100, Vincent Guittot wrote:
>
>> Another point to take into account is that the RT tasks will "steal"
>> the compute capacity that has been requested by the cfs tasks.
>>
>> Let takes the example of a CPU with 3 OPP on which run 2 rt tasks A
>> and B and 1 cfs task C.
>
>> Let assume that the real time constraint of RT task A is too agressive
>> for the lowest OPP0 and that the change of the frequency of the core
>> is too slow compare to this constraint but the real time constraint of
>> RT task B can be handle whatever the OPP. System don't have other
>> choice than setting the cpufreq min freq to OPP1 to be sure that
>> constraint of task A will be covered at anytime.
>
>> Then, we still have 2
>> possible OPPs. The CFS task asks for compute capacity that fits in
>> OPP1 but a part of this capacity will be stolen by RT tasks. If we
>> monitor the load of RT tasks and request capacity for these RT tasks
>> according to their current utilization, we can decide to switch to
>> highest OPP2 to ensure that task C will have enough remaining
>> capacity. A lot of embedded platform faces such kind of use cases
>
> Still doesn't make sense. How would you know the constraint of RT task
> A, and that it cannot be satisfied by OPP0 ? The only information you
> have in the task model is a static priority.

The kernel doesn't have this information so that's why the sysfs
cpufreq/scaling_min_freq has to be used to prevent the kernel (and
cpufreq in particular) to use OPP0.
>From a kernel/sched/cpufreq pov, we assume that all OPPs above
cpufreq/scaling_min can be used with RT tasks of the system. And
performance governor is used if only highest OPP can be used.

>
> The only possible choice the kernel has at this point is max OPP. It
> doesn't have enough (_any_) information about worst case execution of
> that task.
>