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

[Quentin Perret]

+CC Morten who had an interest in this

On Thursday 06 Jun 2019 at 10:20:12 (+0200), 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
> > > > > 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

Yeah but a big task can end up being placed on a little CPU even if a
big CPU is idle. So, it's definitely sub-optimal.

> This is maybe not always optimal and should probably be fixed but
> doesn't justifies a wrong topology description IMHO

I totally agree, the proper fix is to get select_idle_sibling() capacity
aware. And actually, it is true the DT should represent the HW, so
perhaps Amit's fix is the right thing to do so we don't let those hacky
topologies creep into mainline ... I just want to make sure we are fully
aware and OK with the fact that the performance on this platform might
be a little inconsistent until select_idle_sibling() is fixed.

Thanks,
Quentin