Re: [PATCH v3 15/16] arch_topology: Set cluster identifier in each core/thread from /cpu-map

Sudeep Holla
2022-06-13T11:26:59Z

On Mon, Jun 13, 2022 at 11:19:36AM +0200, Dietmar Eggemann wrote:
> On 10/06/2022 12:27, Sudeep Holla wrote:
> > On Fri, Jun 10, 2022 at 12:08:44PM +0200, Vincent Guittot wrote:
> >> On Mon, 6 Jun 2022 at 12:22, Sudeep Holla <sudeep.holla@arm.com> wrote:
> >>>
> >
> > [...]
> >
> >>> Why ? Are you suggesting that we shouldn't present the hardware cluster
> >>> to the topology because of the above reason ? If so, sorry that is not a
> >>> valid reason. We could add login to return NULL or appropriate value
> >>> needed in cpu_clustergroup_mask id it matches MC level mask if we can't
> >>> deal that in generic scheduler code. But the topology code can't be
> >>> compromised for that reason as it is user visible.
> >>
> >> I tend to agree with Dietmar. The legacy use of cluster node in DT
> >> refers to the dynamiQ or legacy b.L cluster which is also aligned to
> >> the LLC and the MC scheduling level. The new cluster level that has
> >> been introduced recently does not target this level but some
> >> intermediate levels either inside like for the kupeng920 or the v9
> >> complex or outside like for the ampere altra. So I would say that
> >> there is one cluster node level in DT that refers to the same MC/LLC
> >> level and only an additional child/parent cluster node should be used
> >> to fill the clustergroup_mask.
> >>
> >
> > Again I completely disagree. Let us look at the problems separately.
> > The hardware topology that some of the tools like lscpu and lstopo expects
> > what the hardware looks like and not the scheduler's view of the hardware.
> > So the topology masks that gets exposed to the user-space needs fixing
> > even today. I have reports from various tooling people about the same.
> > E.g. Juno getting exposed as dual socket system is utter non-sense.
> >
> > Yes scheduler uses most of the topology masks as is but that is not a must.
> > There are these *group_mask functions that can implement what scheduler
> > needs to be fed.
> >
> > I am not sure why the 2 issues are getting mixed up and that is the main
> > reason why I jumped into this to make sure the topology masks are
> > not tampered based on the way it needs to be used for scheduler.
>
> I'm all in favor of not mixing up those 2 issues. But I don't understand
> why you have to glue them together.
>

What are you referring as 'glue them together'. As I said this series just
address the hardware topology and if there is any impact on sched domains
then it is do with alignment with ACPI and DT platform behaviour. I am not
adding anything more to glue topology and info needed for sched domains.

> (1) DT systems broken in userspace (lstopo shows Juno with 2 Packages)
>

Correct.

> (2) Introduce CONFIG_SCHED_CLUSTER for DT systems
>

If that is a problem, you need to disable it for DT platforms. Not
supporting proper hardware topology is not the way to workaround the
issues enabling CONFIG_SCHED_CLUSTER for DT systems IMO.

>
> (1) This can be solved with your patch-set w/o setting `(1. level)
> cpu-map cluster nodes`. The `socket nodes` taking over the
> functionality of the `cluster nodes` sorts out the `Juno is seen as
> having 2 packages`.
> This will make core_sibling not suitable for cpu_coregroup_mask()
> anymore. But this is OK since llc from cacheinfo (i.e. llc_sibling)
> takes over here.
> There is no need to involve `cluster nodes` anymore.
>

Again you are just deferring introducing CONFIG_SCHED_CLUSTER on DT
which is fine but I don't agree with your approach.

> (2) This will only make sense for Armv9 L2 complexes if we connect `2.
> level cpu-map cluster nodes` with cluster_id and cluster_sibling.
> And only then clusters would mean the same thing in ACPI and DT.
> I guess this was mentioned already a couple of times.
>

Indeed. But I don't get what you mean by 2 level here. ACPI puts 1st level
cpu nodes in cluster mask. So we are just aligning to it on DT platforms
here. So if you are saying that is an issue, please fix that for ACPI too.

> > Both ACPI and DT on a platform must present exact same hardware topology
> > to the user-space, there is no space for argument there.
> >
> >> IIUC, we don't describe the dynamiQ level in ACPI which uses cache
> >> topology instead to define cpu_coregroup_mask whereas DT described the
> >> dynamiQ instead of using cache topology. If you use cache topology
> >> now, then you should skip the dynamiQ
> >>
> >
> > Yes, unless someone can work out a binding to represent that and convince
> > DT maintainers ;).
> >
> >> Finally, even if CLS and MC have the same scheduling behavior for now,
> >> they might ends up with different scheduling properties which would
> >> mean that replacing MC level by CLS one for current SoC would become
> >> wrong
> >>
> >
> > Again as I mentioned to Dietmar, that is something we can and must deal with
> > in those *group_mask and not expect topology mask to be altered to meet
> > CLS/MC or whatever sched domains needs. Sorry, that is my strong opinion
> > as the topology is already user-space visible and (tooling) people are
> > complaining that DT systems are broken and doesn't match ACPI systems.
> >
> > So unless someone gives me non-scheduler and topology specific reasons
> > to change that, sorry but my opinion on this matter is not going to change ;).
>
> `lstopo` is fine with a now correct /sys/.../topology/package_cpus (or
> core_siblings (old filename). It's not reading
> /sys/.../topology/cluster_cpus (yet) so why set it (wrongly) to 0x39 for
> CPU0 on Juno when it can stay 0x01?
>

On ACPI ? If so, it could be the package ID in the ACPI table which can be
invalid and kernel use the table offset as ID. It is not ideal but doesn't
affect the masks. The current value 0 or 1 on Juno is cluster ID and they
contribute to wrong package cpu masks.

And yes lstopo doesn't read cluster IDs. But we expose them in ACPI system
and not on DT which was my main point.

As pointed out earlier, have you checked ACPI on Juno and with
CONFIG_SCHED_CLUSTER ? If the behaviour with my series on DT and ACPI
differs, then it is an issue. But AFAIU, it doesn't and that is my main
argument. You are just assuming what we have on Juno with DT is correct
which may be w.r.t to scheduler but definitely not with respect to the
hardware topology exposed to the users. So my aim is to get that fixed.
If you are not happy with that, then how can be be happy with what is the
current behaviour on ACPI + and - CONFIG_SCHED_CLUSTER. I haven't got
your opinion yet on that matter.

--
Regards,
Sudeep