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

[Quentin Perret <quentin.perret@arm.com>]

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@arm.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@chromium.org> wrote:
> > > > >
> > > > > Quoting Amit Kucheria (2019-05-13 04:54:12)
> > > > > > On Mon, May 13, 2019 at 4:31 PM Amit Kucheria <amit.kucheria@linaro.org> wrote:
> > > > > > >
> > > > > > > On Tue, Jan 15, 2019 at 12:13 AM Matthias Kaehlcke <mka@chromium.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@chromium.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 :)

Thanks,
Quentin