linux-kernel - Re: [PATCH v3 0/6] CPUs capacity information for heterogeneous systems

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Date:	Tue, 9 Feb 2016 09:30:05 -0800
From:	Steve Muckle <steve.muckle@...aro.org>
To:	Juri Lelli <juri.lelli@....com>
Cc:	linux-kernel@...r.kernel.org, linux-pm@...r.kernel.org,
	linux-arm-kernel@...ts.infradead.org, devicetree@...r.kernel.org,
	peterz@...radead.org, vincent.guittot@...aro.org,
	robh+dt@...nel.org, mark.rutland@....com, linux@....linux.org.uk,
	sudeep.holla@....com, lorenzo.pieralisi@....com,
	catalin.marinas@....com, will.deacon@....com,
	morten.rasmussen@....com, dietmar.eggemann@....com,
	broonie@...nel.org
Subject: Re: [PATCH v3 0/6] CPUs capacity information for heterogeneous
 systems

On 02/09/2016 02:37 AM, Juri Lelli wrote:
>> I'm still concerned that there's no way to obtain optimal boot time on a
>> > heterogeneous system. Either the dynamic benchmarking is enabled, adding
>> > 1 sec, or the benchmarking is skipped, and task distribution on the
>> > heterogeneous CPUs is determined by the platform's CPU numbering and
>> > chance, potentially impacting performance nondeterministically until
>> > userspace sets the correct capacity values via sysfs.
>> > 
>> > I believe you tested the impact on boot time of using equal capacity
>> > values and saw little difference. I'm wondering though, what was the CPU
>> > numbering on that target?
>> > 
>
> My targets (Juno and TC2) had big cluster on 1,2 and little on the
> remaining cpus. Why do you think this might matter?

There's a natural bias in the scheduler AFAIK towards lower-numbered
CPUs since they are typically scanned in numerically ascending order. So
when all capacities are initially defaulted to be the same I think
you'll be more likely to use the lower numbered CPUs.

I'd be curious what the performance penalty is on a b.L system where the
lowest numbered CPUs are small. I don't have such a target but maybe
it's possible to compare booting just with bigs vs just with littles, at
least until userspace intializes and a script can bring up the others,
which is the same point at which capacities could be properly set. That
would give something of an upper bound.

> Anyway, IMHO boot time performance is not what we are targeting here, so
> I wouldn't be too worried about this particular point.

It may not be the most important thing but it is a factor worth
considering - as mentioned earlier there are applications where boot
time is critical such as automotive. It seems unfortunate that actual
performance may be left on the table due to (IMO anyway) a tenuous
concern over DT semantics. But it looks like that may just be my
position :/ .

thanks,
Steve