linux-kernel - Re: [PATCH v2] sched: rt: Make RT capacity aware

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Date:   Mon, 28 Oct 2019 14:01:47 -0400
From:   Steven Rostedt <rostedt@...dmis.org>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Qais Yousef <qais.yousef@....com>, Ingo Molnar <mingo@...hat.com>,
        Juri Lelli <juri.lelli@...hat.com>,
        Vincent Guittot <vincent.guittot@...aro.org>,
        Dietmar Eggemann <dietmar.eggemann@....com>,
        Ben Segall <bsegall@...gle.com>, Mel Gorman <mgorman@...e.de>,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2] sched: rt: Make RT capacity aware

On Mon, 28 Oct 2019 15:37:49 +0100
Peter Zijlstra <peterz@...radead.org> wrote:

> On Wed, Oct 09, 2019 at 11:46:11AM +0100, Qais Yousef wrote:
> > Capacity Awareness refers to the fact that on heterogeneous systems
> > (like Arm big.LITTLE), the capacity of the CPUs is not uniform, hence
> > when placing tasks we need to be aware of this difference of CPU
> > capacities.
> > 
> > In such scenarios we want to ensure that the selected CPU has enough
> > capacity to meet the requirement of the running task. Enough capacity
> > means here that capacity_orig_of(cpu) >= task.requirement.
> > 
> > The definition of task.requirement is dependent on the scheduling class.
> > 
> > For CFS, utilization is used to select a CPU that has >= capacity value
> > than the cfs_task.util.
> > 
> > 	capacity_orig_of(cpu) >= cfs_task.util
> > 
> > DL isn't capacity aware at the moment but can make use of the bandwidth
> > reservation to implement that in a similar manner CFS uses utilization.
> > The following patchset implements that:
> > 
> > https://lore.kernel.org/lkml/20190506044836.2914-1-luca.abeni@santannapisa.it/
> > 
> > 	capacity_orig_of(cpu)/SCHED_CAPACITY >= dl_deadline/dl_runtime
> > 
> > For RT we don't have a per task utilization signal and we lack any
> > information in general about what performance requirement the RT task
> > needs. But with the introduction of uclamp, RT tasks can now control
> > that by setting uclamp_min to guarantee a minimum performance point.
> > 
> > ATM the uclamp value are only used for frequency selection; but on
> > heterogeneous systems this is not enough and we need to ensure that the
> > capacity of the CPU is >= uclamp_min. Which is what implemented here.
> > 
> > 	capacity_orig_of(cpu) >= rt_task.uclamp_min
> > 
> > Note that by default uclamp.min is 1024, which means that RT tasks will
> > always be biased towards the big CPUs, which make for a better more
> > predictable behavior for the default case.
> > 
> > Must stress that the bias acts as a hint rather than a definite
> > placement strategy. For example, if all big cores are busy executing
> > other RT tasks we can't guarantee that a new RT task will be placed
> > there.
> > 
> > On non-heterogeneous systems the original behavior of RT should be
> > retained. Similarly if uclamp is not selected in the config.
> > 
> > Signed-off-by: Qais Yousef <qais.yousef@....com>  
> 
> Works for me; Steve you OK with this?

Nothing against it, but I want to take a deeper look before we accept
it. Are you OK in waiting a week? I'm currently at Open Source Summit
and still have two more talks to write (giving them Thursday). I wont
have time to look till next week.

-- Steve