hw/core/machine: Introduce CPU cluster topology support

The new Cluster-Aware Scheduling support has landed in Linux 5.16,
which has been proved to benefit the scheduling performance (e.g.
load balance and wake_affine strategy) on both x86_64 and AArch64.

So now in Linux 5.16 we have four-level arch-neutral CPU topology
definition like below and a new scheduler level for clusters.
struct cpu_topology {
    int thread_id;
    int core_id;
    int cluster_id;
    int package_id;
    int llc_id;
    cpumask_t thread_sibling;
    cpumask_t core_sibling;
    cpumask_t cluster_sibling;
    cpumask_t llc_sibling;
}

A cluster generally means a group of CPU cores which share L2 cache
or other mid-level resources, and it is the shared resources that
is used to improve scheduler's behavior. From the point of view of
the size range, it's between CPU die and CPU core. For example, on
some ARM64 Kunpeng servers, we have 6 clusters in each NUMA node,
and 4 CPU cores in each cluster. The 4 CPU cores share a separate
L2 cache and a L3 cache tag, which brings cache affinity advantage.

In virtualization, on the Hosts which have pClusters (physical
clusters), if we can design a vCPU topology with cluster level for
guest kernel and have a dedicated vCPU pinning. A Cluster-Aware
Guest kernel can also make use of the cache affinity of CPU clusters
to gain similar scheduling performance.

This patch adds infrastructure for CPU cluster level topology
configuration and parsing, so that the user can specify cluster
parameter if their machines support it.

Signed-off-by: Yanan Wang <wangyanan55@huawei.com>
Message-Id: <20211228092221.21068-3-wangyanan55@huawei.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
[PMD: Added '(since 7.0)' to @clusters in qapi/machine.json]
Signed-off-by: Philippe Mathieu-Daudé <philmd@redhat.com>

hw/core/machine-smp.c

@@ -37,6 +37,10 @@ static char *cpu_hierarchy_to_string(MachineState *ms)
         g_string_append_printf(s, " * dies (%u)", ms->smp.dies);
     }
 
+    if (mc->smp_props.clusters_supported) {
+        g_string_append_printf(s, " * clusters (%u)", ms->smp.clusters);
+    }
+
     g_string_append_printf(s, " * cores (%u)", ms->smp.cores);
     g_string_append_printf(s, " * threads (%u)", ms->smp.threads);
 
@@ -71,6 +75,7 @@ void machine_parse_smp_config(MachineState *ms,
     unsigned cpus    = config->has_cpus ? config->cpus : 0;
     unsigned sockets = config->has_sockets ? config->sockets : 0;
     unsigned dies    = config->has_dies ? config->dies : 0;
+    unsigned clusters = config->has_clusters ? config->clusters : 0;
     unsigned cores   = config->has_cores ? config->cores : 0;
     unsigned threads = config->has_threads ? config->threads : 0;
     unsigned maxcpus = config->has_maxcpus ? config->maxcpus : 0;
@@ -82,6 +87,7 @@ void machine_parse_smp_config(MachineState *ms,
     if ((config->has_cpus && config->cpus == 0) ||
         (config->has_sockets && config->sockets == 0) ||
         (config->has_dies && config->dies == 0) ||
+        (config->has_clusters && config->clusters == 0) ||
         (config->has_cores && config->cores == 0) ||
         (config->has_threads && config->threads == 0) ||
         (config->has_maxcpus && config->maxcpus == 0)) {
@@ -97,8 +103,13 @@ void machine_parse_smp_config(MachineState *ms,
         error_setg(errp, "dies not supported by this machine's CPU topology");
         return;
     }
+    if (!mc->smp_props.clusters_supported && clusters > 1) {
+        error_setg(errp, "clusters not supported by this machine's CPU topology");
+        return;
+    }
 
     dies = dies > 0 ? dies : 1;
+    clusters = clusters > 0 ? clusters : 1;
 
     /* compute missing values based on the provided ones */
     if (cpus == 0 && maxcpus == 0) {
@@ -113,41 +124,42 @@ void machine_parse_smp_config(MachineState *ms,
             if (sockets == 0) {
                 cores = cores > 0 ? cores : 1;
                 threads = threads > 0 ? threads : 1;
-                sockets = maxcpus / (dies * cores * threads);
+                sockets = maxcpus / (dies * clusters * cores * threads);
             } else if (cores == 0) {
                 threads = threads > 0 ? threads : 1;
-                cores = maxcpus / (sockets * dies * threads);
+                cores = maxcpus / (sockets * dies * clusters * threads);
             }
         } else {
             /* prefer cores over sockets since 6.2 */
             if (cores == 0) {
                 sockets = sockets > 0 ? sockets : 1;
                 threads = threads > 0 ? threads : 1;
-                cores = maxcpus / (sockets * dies * threads);
+                cores = maxcpus / (sockets * dies * clusters * threads);
             } else if (sockets == 0) {
                 threads = threads > 0 ? threads : 1;
-                sockets = maxcpus / (dies * cores * threads);
+                sockets = maxcpus / (dies * clusters * cores * threads);
             }
         }
 
         /* try to calculate omitted threads at last */
         if (threads == 0) {
-            threads = maxcpus / (sockets * dies * cores);
+            threads = maxcpus / (sockets * dies * clusters * cores);
         }
     }
 
-    maxcpus = maxcpus > 0 ? maxcpus : sockets * dies * cores * threads;
+    maxcpus = maxcpus > 0 ? maxcpus : sockets * dies * clusters * cores * threads;
     cpus = cpus > 0 ? cpus : maxcpus;
 
     ms->smp.cpus = cpus;
     ms->smp.sockets = sockets;
     ms->smp.dies = dies;
+    ms->smp.clusters = clusters;
     ms->smp.cores = cores;
     ms->smp.threads = threads;
     ms->smp.max_cpus = maxcpus;
 
     /* sanity-check of the computed topology */
-    if (sockets * dies * cores * threads != maxcpus) {
+    if (sockets * dies * clusters * cores * threads != maxcpus) {
         g_autofree char *topo_msg = cpu_hierarchy_to_string(ms);
         error_setg(errp, "Invalid CPU topology: "
                    "product of the hierarchy must match maxcpus: "

hw/core/machine.c

@@ -742,10 +742,12 @@ static void machine_get_smp(Object *obj, Visitor *v, const char *name,
         .has_cpus = true, .cpus = ms->smp.cpus,
         .has_sockets = true, .sockets = ms->smp.sockets,
         .has_dies = true, .dies = ms->smp.dies,
+        .has_clusters = true, .clusters = ms->smp.clusters,
         .has_cores = true, .cores = ms->smp.cores,
         .has_threads = true, .threads = ms->smp.threads,
         .has_maxcpus = true, .maxcpus = ms->smp.max_cpus,
     };
+
     if (!visit_type_SMPConfiguration(v, name, &config, &error_abort)) {
         return;
     }
@@ -932,6 +934,7 @@ static void machine_initfn(Object *obj)
     ms->smp.max_cpus = mc->default_cpus;
     ms->smp.sockets = 1;
     ms->smp.dies = 1;
+    ms->smp.clusters = 1;
     ms->smp.cores = 1;
     ms->smp.threads = 1;
 }

include/hw/boards.h

@@ -129,10 +129,12 @@ typedef struct {
  * SMPCompatProps:
  * @prefer_sockets - whether sockets are preferred over cores in smp parsing
  * @dies_supported - whether dies are supported by the machine
+ * @clusters_supported - whether clusters are supported by the machine
  */
 typedef struct {
     bool prefer_sockets;
     bool dies_supported;
+    bool clusters_supported;
 } SMPCompatProps;
 
 /**
@@ -299,7 +301,8 @@ typedef struct DeviceMemoryState {
  * @cpus: the number of present logical processors on the machine
  * @sockets: the number of sockets on the machine
  * @dies: the number of dies in one socket
- * @cores: the number of cores in one die
+ * @clusters: the number of clusters in one die
+ * @cores: the number of cores in one cluster
  * @threads: the number of threads in one core
  * @max_cpus: the maximum number of logical processors on the machine
  */
@@ -307,6 +310,7 @@ typedef struct CpuTopology {
     unsigned int cpus;
     unsigned int sockets;
     unsigned int dies;
+    unsigned int clusters;
     unsigned int cores;
     unsigned int threads;
     unsigned int max_cpus;

qapi/machine.json

@@ -1404,7 +1404,9 @@
 #
 # @dies: number of dies per socket in the CPU topology
 #
-# @cores: number of cores per die in the CPU topology
+# @clusters: number of clusters per die in the CPU topology (since 7.0)
+#
+# @cores: number of cores per cluster in the CPU topology
 #
 # @threads: number of threads per core in the CPU topology
 #
@@ -1416,6 +1418,7 @@
      '*cpus': 'int',
      '*sockets': 'int',
      '*dies': 'int',
+     '*clusters': 'int',
      '*cores': 'int',
      '*threads': 'int',
      '*maxcpus': 'int' } }

qemu-options.hx

@@ -206,13 +206,14 @@ SRST
 ERST
 
 DEF("smp", HAS_ARG, QEMU_OPTION_smp,
-    "-smp [[cpus=]n][,maxcpus=maxcpus][,sockets=sockets][,dies=dies][,cores=cores][,threads=threads]\n"
+    "-smp [[cpus=]n][,maxcpus=maxcpus][,sockets=sockets][,dies=dies][,clusters=clusters][,cores=cores][,threads=threads]\n"
     "                set the number of initial CPUs to 'n' [default=1]\n"
     "                maxcpus= maximum number of total CPUs, including\n"
     "                offline CPUs for hotplug, etc\n"
     "                sockets= number of sockets on the machine board\n"
     "                dies= number of dies in one socket\n"
-    "                cores= number of cores in one die\n"
+    "                clusters= number of clusters in one die\n"
+    "                cores= number of cores in one cluster\n"
     "                threads= number of threads in one core\n"
     "Note: Different machines may have different subsets of the CPU topology\n"
     "      parameters supported, so the actual meaning of the supported parameters\n"
@@ -228,7 +229,7 @@ DEF("smp", HAS_ARG, QEMU_OPTION_smp,
     "      must be set as 1 in the purpose of correct parsing.\n",
     QEMU_ARCH_ALL)
 SRST
-``-smp [[cpus=]n][,maxcpus=maxcpus][,sockets=sockets][,dies=dies][,cores=cores][,threads=threads]``
+``-smp [[cpus=]n][,maxcpus=maxcpus][,sockets=sockets][,dies=dies][,clusters=clusters][,cores=cores][,threads=threads]``
     Simulate a SMP system with '\ ``n``\ ' CPUs initially present on
     the machine type board. On boards supporting CPU hotplug, the optional
     '\ ``maxcpus``\ ' parameter can be set to enable further CPUs to be

softmmu/vl.c

@@ -726,6 +726,9 @@ static QemuOptsList qemu_smp_opts = {
         }, {
             .name = "dies",
             .type = QEMU_OPT_NUMBER,
+        }, {
+            .name = "clusters",
+            .type = QEMU_OPT_NUMBER,
         }, {
             .name = "cores",
             .type = QEMU_OPT_NUMBER,