Re: [PATCH] arch/arm64: Fix topology initialization for core scheduling

From: Dietmar Eggemann <dietmar.eggemann@arm.com>
To: Phil Auld <pauld@redhat.com>
Cc: linux-kernel@vger.kernel.org,
	Catalin Marinas <catalin.marinas@arm.com>,
	Will Deacon <will@kernel.org>,
	Mark Rutland <mark.rutland@arm.com>,
	Peter Zijlstra <peterz@infradead.org>,
	linux-arm-kernel@lists.infradead.org
Subject: Re: [PATCH] arch/arm64: Fix topology initialization for core scheduling
Date: Tue, 29 Mar 2022 20:55:08 +0200	[thread overview]
Message-ID: <5a5381cd-813d-7cef-9948-01c3e5e910ef@arm.com> (raw)
In-Reply-To: <YkMjqfBSyT3NOrWB@lorien.usersys.redhat.com>

On 29/03/2022 17:20, Phil Auld wrote:
> On Tue, Mar 29, 2022 at 04:02:22PM +0200 Dietmar Eggemann wrote:
>> On 22/03/2022 17:03, Phil Auld wrote:

[...]

>> I assume this is for a machine which relies on MPIDR-based setup
>> (package_id == -1)? I.e. it doesn't have proper ACPI/(DT) data for
>> topology setup.
> 
> Yes, that's my understanding. No PPTT.
> 
>>
>> Tried on a ThunderX2 by disabling parse_acpi_topology() but then I end
>> up with a machine w/o SMT, so `stress-ng --prctl N` doesn't show this issue.
>>
>> Which machine were you using?
> 
> This instance is an HPE Apollo 70 set to smt-4.  I believe it's ThunderX2
> chips.
> 
> ARM (CN9980-2200LG4077-Y21-G) 
I'm using the same processor just with ACPI/PPTT.

# sudo dmidecode -t 4 | grep "Part Number"
	Part Number: CN9980-2200LG4077-21-Y-G
	Part Number: CN9980-2200LG4077-21-Y-G

# cat /sys/devices/system/cpu/cpu0/topology/thread_siblings
0,32,64,96

# cat /sys/kernel/debug/sched/domains/cpu0/domain*/name
SMT
MC
NUMA

But no matter whether I disable parse_acpi_topology() or just force
`cpu_topology[cpu].package_id = -1` in this function, I always end up with:

# cat /sys/kernel/debug/sched/domains/cpu0/domain*/name
MC
NUMA

# cat /sys/devices/system/cpu/cpu0/topology/thread_siblings_list
0

so no SMT sched domain. The MPIDR-based topology fallback code in
store_cpu_topology() forces `cpuid_topo->thread_id  = -1`.

IMHO this is why on my machine I don't see this issue while running:

root@oss-apollo7007:~# stress-ng --prctl 256 -t 60
stress-ng: info:  [2388042] dispatching hogs: 256 prctl

Is there something I miss in my setup to provoke this issue?

[...]

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