[PATCH] arch_topology: Limit threads to one specific cluster

[Gavin Shan]

Hi Sudeep,

On 5/24/22 4:51 PM, Sudeep Holla wrote:
> On Tue, May 24, 2022 at 04:12:12PM +0800, Gavin Shan wrote:
>> The sibling information for one particular CPU is updated after ACPI
>> PPTT table is parsed. struct cpu_topology::thread_sibling tracks the
>> the CPUs in same core. However, cluster isn't considered when it's
>> populated. In this case, multiple threads belonging to different
>> clusters can be put together through the sibling information. It
>> eventually leads to unexpected warning from sched subsystem.
>>
>> For example, the following warning is observed in a VM where we have
>> 2 sockets, 4 clusters, 8 cores and 16 threads and the CPU topology
>> is populated as below.
>>
>>     CPU  Socket-ID  Cluster-ID  Core-ID  Thread-ID
>>     ----------------------------------------------
>>      0      0          0          0        0
>>      1      0          0          0        1
>>      2      0          0          1        0
>>      3      0          0          1        1
>>      4      0          1          0        0
>>      5      0          1          0        1
>>      6      0          1          1        0
>>      7      0          1          1        1
>>      8      1          0          0        0
>>      9      1          0          0        1
>>     10      1          0          1        0
>>     11      1          0          1        1
>>     12      1          1          0        0
>>     13      1          1          0        1
>>     14      1          1          1        0
>>     15      1          1          1        1
>>
>>    [    0.592181] CPU: All CPU(s) started at EL1
>>    [    0.593766] alternatives: patching kernel code
>>    [    0.595890] BUG: arch topology borken
>>    [    0.597210]      the SMT domain not a subset of the CLS domain
>>    [    0.599286]      child=0-1,4-5    sd=0-3
>>
>>    # cat /sys/devices/system/cpu/cpu0/topology/cluster_cpus_list
>>    0-3
>>    # cat /sys/devices/system/cpu/cpu0/topology/thread_siblings_list
>>    0-1,4-5
>>
>> This fixes the issue by limiting threads to one specific cluster.
>> With this applied, the unexpected warning disappears in the VM.
>>
> 
> I have similar fix but as part of bigger series[1] to get DT support in
> line with ACPI.
> 

Your patch resolves the issue I have. So please ignore mine. Sorry
for the noise.

https://lore.kernel.org/lkml/20220518093325.2070336-3-sudeep.holla@arm.com

# cat /sys/devices/system/cpu/cpu0/topology/cluster_cpus_list
0-3
# cat /sys/devices/system/cpu/cpu0/topology/thread_siblings_list
0-1

Thanks,
Gavin