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

From: Peter Zijlstra <peterz@infradead.org>
To: Qais Yousef <qais.yousef@arm.com>
Cc: Ingo Molnar <mingo@redhat.com>,
	Steven Rostedt <rostedt@goodmis.org>,
	Juri Lelli <juri.lelli@redhat.com>,
	Vincent Guittot <vincent.guittot@linaro.org>,
	Dietmar Eggemann <dietmar.eggemann@arm.com>,
	Ben Segall <bsegall@google.com>, Mel Gorman <mgorman@suse.de>,
	linux-kernel@vger.kernel.org
Subject: Re: [PATCH v2] sched: rt: Make RT capacity aware
Date: Mon, 28 Oct 2019 15:37:49 +0100	[thread overview]
Message-ID: <20191028143749.GE4114@hirez.programming.kicks-ass.net> (raw)
In-Reply-To: <20191009104611.15363-1-qais.yousef@arm.com>

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@arm.com>

Works for me; Steve you OK with this?