From: Sai Gurrappadi <sgurrappadi@nvidia.com>
To: Juri Lelli <juri.lelli@arm.com>
Cc: mark.rutland@arm.com, peterz@infradead.org,
catalin.marinas@arm.com, Linus Walleij <linus.walleij@linaro.org>,
will.deacon@arm.com, morten.rasmussen@arm.com,
lorenzo.pieralisi@arm.com, vincent.guittot@linaro.org,
Ian Campbell <ijc+devicetree@hellion.org.uk>,
Chen-Yu Tsai <wens@csie.org>,
devicetree@vger.kernel.org, Pawel Moll <pawel.moll@arm.com>,
linux-pm@vger.kernel.org, Kumar Gala <galak@codeaurora.org>,
broonie@kernel.org,
Gregory CLEMENT <gregory.clement@free-electrons.com>,
linux@arm.linux.org.uk, dietmar.eggemann@arm.com,
linux-arm-kernel@lists.infradead.org,
Thomas Petazzoni <thomas.petazzoni@free-electrons.com>,
Paul Walmsley <paul@pwsan.com>,
pboonstoppel@nvidia.com, linux-kernel@vger.kernel.org,
robh+dt@kernel.org, sudeep.holla@arm.com,
Olof Johansson <olof@lixom.net>,
Maxime Ripard <maxime.ripard@free-electrons.com>
Subject: Re: [PATCH v4 2/8] Documentation: arm: define DT cpu capacity bindings
Date: Mon, 21 Mar 2016 12:20:57 -0700 [thread overview]
Message-ID: <56F04999.6090902@nvidia.com> (raw)
In-Reply-To: <20160321105330.GB12319@e106622-lin>
On 03/21/2016 03:53 AM, Juri Lelli wrote:
> Hi Sai,
>
> On 18/03/16 10:49, Sai Gurrappadi wrote:
>> Hi Juri,
>>
>> On 03/18/2016 07:24 AM, Juri Lelli wrote:
>>
>> <snip>
>>
>>> +
>>> +==========================================
>>> +2 - CPU capacity definition
>>> +==========================================
>>> +
>>> +CPU capacity is a number that provides the scheduler information about CPUs
>>> +heterogeneity. Such heterogeneity can come from micro-architectural differences
>>> +(e.g., ARM big.LITTLE systems) or maximum frequency at which CPUs can run
>>> +(e.g., SMP systems with multiple frequency domains). Heterogeneity in this
>>> +context is about differing performance characteristics; this binding tries to
>>> +capture a first-order approximation of the relative performance of CPUs.
>>
>> Any reason why this capacity number is not dynamically generated based on the
>> max frequency for each CPU? The DT property would then instead specify just
>> the micro-architectural differences between the CPU types.
>>
>
> I'm not sure I clearly understand your question, so I'll try to
> reiterate it.
>
> Are you asking why we don't dynamically profile the system, at boot for
> example, to get this number? Or do you ask why this number couldn't be
> only describing micro-arch differences (so, if I get it right, we should
> then multiply it by max freq to get the capacity of a CPU)?
>
> We already played with the first option (please refer to v2 and v3), but
> we ended up agreeing that dynamic profiling adds overhead to the boot
> process (while a DT approach can provide information to speed up boot)
> and it is in general not repeatable/reliable (as numbers can vary from
> boot to boot for different reasons).
>
> The second option I think can be feasible, but I'm not sure what we gain
> in practice. We will still need to specify a per-platform number, right?
I meant the second bit. We only need some per-platform fudge factor for
micro-architectural differences. Tying in the Fmax like this statically means
that we need to manually synchronize DVFS tables and this number which seems
unnecessary given that the kernel already has this info on boot.
>
> Best,
>
> - Juri
>
next prev parent reply other threads:[~2016-03-21 19:20 UTC|newest]
Thread overview: 20+ messages / expand[flat|nested] mbox.gz Atom feed top
2016-03-18 14:24 [PATCH v4 0/8] CPUs capacity information for heterogeneous systems Juri Lelli
2016-03-18 14:24 ` [PATCH v4 1/8] ARM: initialize cpu_scale to its default Juri Lelli
2016-03-18 14:24 ` [PATCH v4 2/8] Documentation: arm: define DT cpu capacity bindings Juri Lelli
2016-03-18 17:49 ` Sai Gurrappadi
[not found] ` <56EC3F9E.4050209-DDmLM1+adcrQT0dZR+AlfA@public.gmane.org>
2016-03-21 10:53 ` Juri Lelli
2016-03-21 11:09 ` Vincent Guittot
2016-03-21 11:49 ` Juri Lelli
2016-03-21 12:12 ` Mark Brown
[not found] ` <20160321121210.GQ2566-GFdadSzt00ze9xe1eoZjHA@public.gmane.org>
2016-03-21 17:24 ` Juri Lelli
2016-03-21 17:51 ` Mark Brown
[not found] ` <20160321175112.GY2566-GFdadSzt00ze9xe1eoZjHA@public.gmane.org>
2016-03-22 9:50 ` Juri Lelli
2016-03-21 19:20 ` Sai Gurrappadi [this message]
2016-03-20 1:15 ` Rob Herring
2016-03-21 11:39 ` Juri Lelli
2016-03-18 14:24 ` [PATCH v4 3/8] arm: parse cpu capacity from DT Juri Lelli
2016-03-18 14:24 ` [PATCH v4 4/8] arm, dts: add TC2 cpu capacity information Juri Lelli
2016-03-18 14:24 ` [PATCH v4 5/8] arm64: parse cpu capacity from DT Juri Lelli
2016-03-18 14:24 ` [PATCH v4 6/8] arm64, dts: add Juno cpu capacity information Juri Lelli
2016-03-18 14:24 ` [PATCH v4 7/8] arm: add sysfs cpu_capacity attribute Juri Lelli
2016-03-18 14:24 ` [PATCH v4 8/8] arm64: " Juri Lelli
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