[PATCH] arch_topology: Limit threads to one specific cluster

[Gavin Shan]

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.

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

Signed-off-by: Gavin Shan <gshan at redhat.com>
---
 drivers/base/arch_topology.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/drivers/base/arch_topology.c b/drivers/base/arch_topology.c
index f73b836047cf..25a63a89ae7d 100644
--- a/drivers/base/arch_topology.c
+++ b/drivers/base/arch_topology.c
@@ -710,7 +710,8 @@ void update_siblings_masks(unsigned int cpuid)
 		cpumask_set_cpu(cpuid, &cpu_topo->core_sibling);
 		cpumask_set_cpu(cpu, &cpuid_topo->core_sibling);
 
-		if (cpuid_topo->core_id != cpu_topo->core_id)
+		if (cpuid_topo->cluster_id != cpu_topo->cluster_id ||
+		    cpuid_topo->core_id != cpu_topo->core_id)
 			continue;
 
 		cpumask_set_cpu(cpuid, &cpu_topo->thread_sibling);
-- 
2.23.0