Re: [PATCH v3] topology: make core_mask include at least cluster_siblings

[Dietmar Eggemann <dietmar.eggemann@arm.com>]

On 08/03/2022 18:49, Darren Hart wrote:
> On Tue, Mar 08, 2022 at 05:03:07PM +0100, Dietmar Eggemann wrote:
>> On 08/03/2022 12:04, Vincent Guittot wrote:
>>> On Tue, 8 Mar 2022 at 11:30, Will Deacon <will@kernel.org> wrote:

[...]

>> IMHO, if core_mask weight is 1, MC will be removed/degenerated anyway.
>>
>> This is what I get on my Ampere Altra (I guess I don't have the ACPI
>> changes which would let to a CLS sched domain):
>>
>> # cat /sys/kernel/debug/sched/domains/cpu0/domain*/name
>> DIE
>> NUMA
>> root@oss-altra01:~# zcat /proc/config.gz | grep SCHED_CLUSTER
>> CONFIG_SCHED_CLUSTER=y
> 
> I'd like to follow up on this. Would you share your dmidecode BIOS
> Information section?

# dmidecode -t 0
# dmidecode 3.2
Getting SMBIOS data from sysfs.
SMBIOS 3.2.0 present.

Handle 0x0000, DMI type 0, 26 bytes
BIOS Information
	Vendor: Ampere(TM)
	Version: 0.9.20200724
	Release Date: 2020/07/24
	ROM Size: 7680 kB
	Characteristics:
		PCI is supported
		BIOS is upgradeable
		Boot from CD is supported
		Selectable boot is supported
		ACPI is supported
		UEFI is supported
	BIOS Revision: 5.15
	Firmware Revision: 0.6

> Which kernel version?

v5.17-rc5

[...]

>>> I would not say that I'm happy because this solution skews the core
>>> cpu mask in order to abuse the scheduler so that it will remove a
>>> wrong but useless level when it will build its domains.
>>> But this works so as long as the maintainer are happy, I'm fine
> 
> I did explore the other options and they added considerably more
> complexity without much benefit in my view. I prefer this option which
> maintains the cpu_topology as described by the platform, and maps it
> into something that suits the current scheduler abstraction. I agree
> there is more work to be done here and intend to continue with it.
> 
>> I do not have any better idea than this tweak here either in case the
>> platform can't provide a cleaner setup.
> 
> I'd argue The platform is describing itself accurately in ACPI PPTT
> terms. The topology doesn't fit nicely within the kernel abstractions
> today. This is an area where I hope to continue to improve things going
> forward.

I see. And I assume lying about SCU/LLC boundaries in ACPI is not an
option since it messes up /sys/devices/system/cpu/cpu0/cache/index*/.

[...]