Re: [PATCH] arm64: dts: sdm845: Add CPU topology

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On 6/6/19 10:20 AM, Vincent Guittot wrote:
On Thu, 6 Jun 2019 at 09:49, Quentin Perret <quentin.perret@xxxxxxx> 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@xxxxxxx> 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@xxxxxxxxxxxx> wrote:

Quoting Amit Kucheria (2019-05-13 04:54:12)
On Mon, May 13, 2019 at 4:31 PM Amit Kucheria <amit.kucheria@xxxxxxxxxx> wrote:

On Tue, Jan 15, 2019 at 12:13 AM Matthias Kaehlcke <mka@xxxxxxxxxxxx> 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@xxxxxxxxxxxx>

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

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.

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