linux-kernel - Re: [PATCH] arm64: dts: sdm845: Add CPU topology

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Date:   Thu, 6 Jun 2019 10:34:45 +0200
From:   Dietmar Eggemann <dietmar.eggemann@....com>
To:     Vincent Guittot <vincent.guittot@...aro.org>,
        Quentin Perret <quentin.perret@....com>
Cc:     Stephen Boyd <swboyd@...omium.org>,
        Amit Kucheria <amit.kucheria@...aro.org>,
        Andy Gross <agross@...nel.org>,
        Matthias Kaehlcke <mka@...omium.org>,
        David Brown <david.brown@...aro.org>,
        Rob Herring <robh+dt@...nel.org>,
        Mark Rutland <mark.rutland@....com>,
        linux-arm-msm <linux-arm-msm@...r.kernel.org>,
        "open list:ARM/QUALCOMM SUPPORT" <linux-soc@...r.kernel.org>,
        "open list:OPEN FIRMWARE AND FLATTENED DEVICE TREE BINDINGS" 
        <devicetree@...r.kernel.org>, LKML <linux-kernel@...r.kernel.org>,
        Douglas Anderson <dianders@...omium.org>,
        Rajendra Nayak <rnayak@...eaurora.org>,
        Morten Rasmussen <Morten.Rasmussen@....com>
Subject: Re: [PATCH] arm64: dts: sdm845: Add CPU topology

On 6/6/19 10:20 AM, Vincent Guittot wrote:
> On Thu, 6 Jun 2019 at 09:49, Quentin Perret <quentin.perret@....com> wrote:
>>
>> Hi Vincent,
>>
>> On Thursday 06 Jun 2019 at 09:05:16 (+0200), Vincent Guittot wrote:
>>> Hi Quentin,
>>>
>>> On Wed, 5 Jun 2019 at 19:21, Quentin Perret <quentin.perret@....com> wrote:
>>>>
>>>> On Friday 17 May 2019 at 14:55:19 (-0700), Stephen Boyd wrote:
>>>>> Quoting Amit Kucheria (2019-05-16 04:54:45)
>>>>>> (cc'ing Andy's correct email address)
>>>>>>
>>>>>> On Wed, May 15, 2019 at 2:46 AM Stephen Boyd <swboyd@...omium.org> wrote:
>>>>>>>
>>>>>>> Quoting Amit Kucheria (2019-05-13 04:54:12)
>>>>>>>> On Mon, May 13, 2019 at 4:31 PM Amit Kucheria <amit.kucheria@...aro.org> wrote:
>>>>>>>>>
>>>>>>>>> On Tue, Jan 15, 2019 at 12:13 AM Matthias Kaehlcke <mka@...omium.org> wrote:
>>>>>>>>>>
>>>>>>>>>> The 8 CPU cores of the SDM845 are organized in two clusters of 4 big
>>>>>>>>>> ("gold") and 4 little ("silver") cores. Add a cpu-map node to the DT
>>>>>>>>>> that describes this topology.
>>>>>>>>>
>>>>>>>>> This is partly true. There are two groups of gold and silver cores,
>>>>>>>>> but AFAICT they are in a single cluster, not two separate ones. SDM845
>>>>>>>>> is one of the early examples of ARM's Dynamiq architecture.
>>>>>>>>>
>>>>>>>>>> Signed-off-by: Matthias Kaehlcke <mka@...omium.org>
>>>>>>>>>
>>>>>>>>> I noticed that this patch sneaked through for this merge window but
>>>>>>>>> perhaps we can whip up a quick fix for -rc2?
>>>>>>>>>
>>>>>>>>
>>>>>>>> And please find attached a patch to fix this up. Andy, since this
>>>>>>>> hasn't landed yet (can we still squash this into the original patch?),
>>>>>>>> I couldn't add a Fixes tag.
>>>>>>>>
>>>>>>>
>>>>>>> I had the same concern. Thanks for catching this. I suspect this must
>>>>>>> cause some problem for IPA given that it can't discern between the big
>>>>>>> and little "power clusters"?
>>>>>>
>>>>>> Both EAS and IPA, I believe. It influences the scheduler's view of the
>>>>>> the topology.
>>>>>
>>>>> And EAS and IPA are OK with the real topology? I'm just curious if
>>>>> changing the topology to reflect reality will be a problem for those
>>>>> two.
>>>>
>>>> FWIW, neither EAS nor IPA depends on this. Not the upstream version of
>>>> EAS at least (which is used in recent Android kernels -- 4.19+).
>>>>
>>>> But doing this is still required for other things in the scheduler (the
>>>> so-called 'capacity-awareness' code). So until we have a better
>>>> solution, this patch is doing the right thing.
>>>
>>> I'm not sure to catch what you mean ?
>>> Which so-called 'capacity-awareness' code are you speaking about ? and
>>> what is the problem ?
>>
>> I'm talking about the wake-up path. ATM select_idle_sibling() is totally
>> unaware of capacity differences. In its current form, this function
>> basically assumes that all CPUs in a given sd_llc have the same
>> capacity, which would be wrong if we had a single MC level for SDM845.
>> So, until select_idle_sibling() is 'fixed' to be capacity-aware, we need
>> two levels of sd for asymetric systems (including DynamIQ) so the
>> wake_cap() story actually works.
>>
>> I hope that clarifies it :)
> 
> hmm... does this justifies this wrong topology ?
> select_idle_sibling() is called only when system is overloaded and
> scheduler disables the EAS path
> In this case, the scheduler looks either for an idle cpu or for evenly
> spreading the loads
> This is maybe not always optimal and should probably be fixed but
> doesn't justifies a wrong topology description IMHO

The big/Little cluster detection in wake_cap() doesn't work anymore with 
DynamIQ w/o Phanton (DIE) domain. So the decision of going sis() or slow 
path is IMHO broken.
But I support the idea of not introducing Phantom Domains in device tree 
and fix wake_cap() instead.