Re: [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@xxxxxxx

# 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