Re: [PATCH] arch_topology: Make cluster topology span at least SMT CPUs

[Yicong Yang]

On 2022/8/23 16:06, Ionela Voinescu wrote:
> Hi Yicong,
> 
> On Tuesday 23 Aug 2022 at 15:30:44 (+0800), Yicong Yang wrote:
>> From: Yicong Yang <yangyicong@xxxxxxxxxxxxx>
>>
>> Currently cpu_clustergroup_mask() will return CPU mask if cluster span
>> more or the same CPUs as cpu_coregroup_mask(). This will result topology
>> borken on non-Cluster SMT machines.
> 
> Might be worth adding here:.. "when building with CONFIG_SCHED_CLUSTER=y"
> 

will add this qualifier. thanks.

>>
> 
> I thought I had covered this case, but I think I had considered LLC
> spanning more than one core (tested on TX2 as well).
> 
> So you'd only hit this if LLC and cluster level span the same cores (a
> single core in this case, for non-cluster), in the presence of SMT.
> 

Not sure I understand it correctly but in the below case I think the LLC may span
the same core with socket?

> Reviewed-by: Ionela Voinescu <ionela.voinescu@xxxxxxx>
> 
> Many thanks,
> Ionela.
> 
>> Test with:
>> qemu-system-aarch64 -enable-kvm -machine virt \
>>  -net none \
>>  -cpu host \
>>  -bios ./QEMU_EFI.fd \
>>  -m 2G \
>>  -smp 48,sockets=2,cores=12,threads=2 \
>>  -kernel $Image \
>>  -initrd $Rootfs \
>>  -nographic
>>  -append "rdinit=init console=ttyAMA0 sched_verbose loglevel=8"
>>
>> We'll get below error:
>> [    3.084568] BUG: arch topology borken
>> [    3.084570]      the SMT domain not a subset of the CLS domain
>>
>> Since cluster is a level higher than SMT, fix this by making cluster
>> spans at least SMT CPUs.
>>
>> Cc: Sudeep Holla <sudeep.holla@xxxxxxx>
>> Cc: Vincent Guittot <vincent.guittot@xxxxxxxxxx>
>> Cc: Ionela Voinescu <ionela.voinescu@xxxxxxx>
>> Fixes: bfcc4397435d ("arch_topology: Limit span of cpu_clustergroup_mask()")
>> Signed-off-by: Yicong Yang <yangyicong@xxxxxxxxxxxxx>
>> ---
>>  drivers/base/arch_topology.c | 2 +-
>>  1 file changed, 1 insertion(+), 1 deletion(-)
>>
>> diff --git a/drivers/base/arch_topology.c b/drivers/base/arch_topology.c
>> index 0424b59b695e..0056a1273275 100644
>> --- a/drivers/base/arch_topology.c
>> +++ b/drivers/base/arch_topology.c
>> @@ -724,7 +724,7 @@ const struct cpumask *cpu_clustergroup_mask(int cpu)
>>  	 */
>>  	if (cpumask_subset(cpu_coregroup_mask(cpu),
>>  			   &cpu_topology[cpu].cluster_sibling))
>> -		return get_cpu_mask(cpu);
>> +		return cpu_smt_mask(cpu);
>>  
>>  	return &cpu_topology[cpu].cluster_sibling;
>>  }
>> -- 
>> 2.24.0
>>
> 
> .
>