New subject: Linaro-open-discussions Digest, Vol 28, Issue 3

19 Jan 2023

      Hi all,
Any topics needs to be discussed on next Tuesday?
Thanks:)
Joyce
...
在 2023年1月17日，上午8:00，linaro-open-discussions-request@op-lists.linaro.org 写道：
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Today's Topics:

Re: [RFC PATCH 2/2] MPAM/resctrl: allocate a domain per component
(Hesham Almatary)

Message: 1
Date: Mon, 16 Jan 2023 11:31:06 +0000
From: Hesham Almatary hesham.almatary@huawei.com
Subject: [Linaro-open-discussions] Re: [RFC PATCH 2/2] MPAM/resctrl:
   allocate a domain per component
To: James Morse james.morse@arm.com
Cc: linaro-open-discussions@op-lists.linaro.org, linuxarm@huawei.com
Message-ID: 20230116113106.00001a2d@huawei.com
Content-Type: text/plain; charset="US-ASCII"
Hello James,
Many thanks for clarifying things up, that definitely helps with my
understanding.
On Thu, 12 Jan 2023 13:38:17 +0000
James Morse james.morse@arm.com wrote:
...
Hi Hesham,
On 12/01/2023 10:34, Hesham Almatary wrote:
...
Thanks for getting back to me on this. I have done some changes to
my ACPI tables and got your code working fine without this patch. In
particular, I matched the proximity domain field with the NUMA node
ID for each memory controller. If they differ, the code won't work
(as it has the assumption that the proximity domain is the same as
NUMA id,
Right, if there is an extra level of indirection in there, its
something I wasn't aware of. I'll need to dig into it. I agree this
explains what you were seeing.
...
from which the affinity/accessibility is set). This leaves me with a
few questions regarding the design and implementation of the driver.
I'd appreciate your input on that.
...

What does a memory MSC correspond to? A class (with a unique ID)

or a component? From the code, it seems like it maps to a component
to me.
An MSC is the device, it has registers and generates interrupts. If
its part of your memory controller, it gets described like that in
the ACPI tables, which lets linux guess that this MSC (or the RIS
within it) control some policy in the memory controller.
Components exist to group devices that should be configured the same.
This happens where designs are sliced up, but this slicing makes no
sense to the software. Classes are a group of components that do the
same thing, but not to the same resource. e.g. they control memory
controllers.
The ACPI tables should describe the MSC, its up to the driver to
build the class and component structures from what it can infer from
the other ACPI tables.
...

Could we have a use case in which we have different class IDs

with the same class type? If yes could you please give an example?
Your L2 and L3 are both caches, but use the level number as the id.
I doubt anyone builds a system with MSC on both, but its possible by
the architecture, and we could expose both via resctrl.
...

What should a component ID for a memory MSC be/represent? The

code assumes it's a (NUMA?) node ID.
The component-ids are some number that makes sense to linux, and
matches something in the ACPI tables. These are exposed via the
schema file to user-space. For the caches, its the cache-id property
from the PPTT table. This is exposed to user-space via
/sys/devices/system/cpu/cpu0/cache/index3/id or equivalent.
Its important that user-space can work out which CPUs share a
component/domain in the schema. Using a sensible id is the
pre-requisite for that.
Intel's memory bandwidth control appears to be implemented on the L3,
so they re-use the id of the L3 cache. These seem to correspond to
NUMA nodes already.
For MPAM - we have no idea if the memory controllers map 1:1 with any
level in the cache. Instead, the driver expects to use the numa node
number directly.
(I'll put this on the list of KNOWN_ISSUES, the Intel side of this
ought to be cleaned up so it doesn't break if they build a SoC where
L3 doesn't map 1:1 with Numa nodes. It looks like they are getting
away with it because Atom doesn't support L3 or memory bandwidth)
That's very useful and informative. Some form of documentation (in
KNOWN_ISSUES, comments, or a README) would be quite useful for such
assumptions as the ACPI/MPAM spec doesn't mention that (i.e., the
logical ID assignments from OS point of view).
...
...

What should a class ID represent for a memory MSC? Which is

different from the class type itself.
The class id is private to the driver, for the caches it needs to be
the cache level. Because of that memory is shoved at the end, on the
assumption no-one has an L255 cache, and 'unknown' devices are shoved
at the beginning... L0 caches probably do exist, but I doubt anyone
would add an MSC to them.
Classes can't be arbitrarily created, as the resctrl picking code
needs to know how they map to resctrl schemas, as we can't invent new
schemas without messing up user-space.
...

How would 4 memory MSCs (with different proximity domains) map to

classes and components?
Each MSC would be a device. There would be one device per component,
because each proximity domain is different. They would all be the
same class, as you'd described them all with a memory type in the
ACPI tables.
If you see a problem with this, let me know! The folk who write the
ACPI specs didn't find any systems where this would lead to
problems... that doesn't mean you haven't build something that looks
quite different.
...

How would 2 Memory MSCs with(in) the same proximity domain and/or

same NUMA node work, if at all?
If you build this, I bet your hardware people say those two MSC must
be programmed the same for the regulation to work. (if not - how is
software expected to understand the hashing scheme used to map
physical-addresses to memory controllers?!)
Each MSC would be a device. They would both be part of the same
component as they have the same proximity domain.
Configuration is applied to the component, so each device/MSC within
the component is always configured the same.
...

Should the ACPI/MPAM MSC's "identifier" field be mapped to class

IDs or component IDs at all?
Classes, no - these are just for the driver to keep track of the
groups. Components, probably ... but another number may make more
sense. This should line up with something that is already exposed to
user-space via sysfs.
Thanks,
James

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