linux-kernel - Re: [PATCH v3 6/7] arm64: topology: Enable ACPI/PPTT based CPU topology.

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Date:   Fri, 20 Oct 2017 10:14:28 +0100
From:   Lorenzo Pieralisi <lorenzo.pieralisi@....com>
To:     Jeremy Linton <jeremy.linton@....com>
Cc:     linux-acpi@...r.kernel.org, linux-arm-kernel@...ts.infradead.org,
        sudeep.holla@....com, hanjun.guo@...aro.org, rjw@...ysocki.net,
        will.deacon@....com, catalin.marinas@....com,
        gregkh@...uxfoundation.org, viresh.kumar@...aro.org,
        mark.rutland@....com, linux-kernel@...r.kernel.org,
        linux-pm@...r.kernel.org, jhugo@...eaurora.org,
        wangxiongfeng2@...wei.com, Jonathan.Zhang@...ium.com,
        ahs3@...hat.com, Jayachandran.Nair@...ium.com,
        austinwc@...eaurora.org
Subject: Re: [PATCH v3 6/7] arm64: topology: Enable ACPI/PPTT based CPU
 topology.

On Thu, Oct 19, 2017 at 11:13:27AM -0500, Jeremy Linton wrote:
> On 10/19/2017 10:56 AM, Lorenzo Pieralisi wrote:
> >On Thu, Oct 12, 2017 at 02:48:55PM -0500, Jeremy Linton wrote:
> >>Propagate the topology information from the PPTT tree to the
> >>cpu_topology array. We can get the thread id, core_id and
> >>cluster_id by assuming certain levels of the PPTT tree correspond
> >>to those concepts. The package_id is flagged in the tree and can be
> >>found by passing an arbitrary large level to setup_acpi_cpu_topology()
> >>which terminates its search when it finds an ACPI node flagged
> >>as the physical package. If the tree doesn't contain enough
> >>levels to represent all of thread/core/cod/package then the package
> >>id will be used for the missing levels.
> >>
> >>Since server/ACPI machines are more likely to be multisocket and NUMA,
> >
> >I think this stuff is vague enough already so to start with I would drop
> >patch 4 and 5 and stop assuming what machines are more likely to ship
> >with ACPI than DT.
> >
> >I am just saying, for the umpteenth time, that these levels have no
> >architectural meaning _whatsoever_, level is a hierarchy concept
> >with no architectural meaning attached.
> 
> ?
> 
> Did anyone say anything about that? No, I think the only thing being
> guaranteed here is that the kernel's physical_id maps to an ACPI
> defined socket. Which seems to be the mindset of pretty much the
> entire !arm64 community meaning they are optimizing their software
> and the kernel with that concept in mind.
> 
> Are you denying the existence of non-uniformity between threads
> running on different physical sockets?

No, I have not explained my POV clearly, apologies.

AFAIK, the kernel currently deals with 2 (3 - if SMT) topology layers.

1) thread
2) core
3) package

What I wanted to say is, that, to simplify this series, you do not need
to introduce the COD topology level, since it is just another arbitrary
topology level (ie there is no way you can pinpoint which level
corresponds to COD with PPTT - or DT for the sake of this discussion)
that would not be used in the kernel (apart from big.LITTLE cpufreq
driver and PSCI checker whose usage of topology_physical_package_id() is
questionable anyway).

PPTT allows you to define what level corresponds to a package, use
it to initialize the package topology level (that on ARM internal
variables we call cluster) and be done with it.

I do not think that adding another topology level improves anything as
far as ACPI topology detection is concerned, you are not able to use it
in the scheduler or from userspace to group CPUs anyway.

Does this answer your question ?

Thanks,
Lorenzo