hostmem: Allow for specifying a ThreadContext for preallocation

Let's allow for specifying a thread context via the "prealloc-context"
property. When set, preallcoation threads will be crated via the
thread context -- inheriting the same CPU affinity as the thread
context.

Pinning preallcoation threads to CPUs can heavily increase performance
in NUMA setups, because, preallocation from a CPU close to the target
NUMA node(s) is faster then preallocation from a CPU further remote,
simply because of memory bandwidth for initializing memory with zeroes.
This is especially relevant for very large VMs backed by huge/gigantic
pages, whereby preallocation is mandatory.

Reviewed-by: Michal Privoznik <mprivozn@redhat.com>
Message-Id: <20221014134720.168738-7-david@redhat.com>
Signed-off-by: David Hildenbrand <david@redhat.com>

backends/hostmem.c

@@ -232,8 +232,8 @@ static void host_memory_backend_set_prealloc(Object *obj, bool value,
         void *ptr = memory_region_get_ram_ptr(&backend->mr);
         uint64_t sz = memory_region_size(&backend->mr);
 
-        qemu_prealloc_mem(fd, ptr, sz, backend->prealloc_threads, NULL,
-                          &local_err);
+        qemu_prealloc_mem(fd, ptr, sz, backend->prealloc_threads,
+                          backend->prealloc_context, &local_err);
         if (local_err) {
             error_propagate(errp, local_err);
             return;
@@ -385,7 +385,8 @@ host_memory_backend_memory_complete(UserCreatable *uc, Error **errp)
          */
         if (backend->prealloc) {
             qemu_prealloc_mem(memory_region_get_fd(&backend->mr), ptr, sz,
-                              backend->prealloc_threads, NULL, &local_err);
+                              backend->prealloc_threads,
+                              backend->prealloc_context, &local_err);
             if (local_err) {
                 goto out;
             }
@@ -493,6 +494,11 @@ host_memory_backend_class_init(ObjectClass *oc, void *data)
         NULL, NULL);
     object_class_property_set_description(oc, "prealloc-threads",
         "Number of CPU threads to use for prealloc");
+    object_class_property_add_link(oc, "prealloc-context",
+        TYPE_THREAD_CONTEXT, offsetof(HostMemoryBackend, prealloc_context),
+        object_property_allow_set_link, OBJ_PROP_LINK_STRONG);
+    object_class_property_set_description(oc, "prealloc-context",
+        "Context to use for creating CPU threads for preallocation");
     object_class_property_add(oc, "size", "int",
         host_memory_backend_get_size,
         host_memory_backend_set_size,

include/sysemu/hostmem.h

@@ -18,6 +18,7 @@
 #include "qom/object.h"
 #include "exec/memory.h"
 #include "qemu/bitmap.h"
+#include "qemu/thread-context.h"
 
 #define TYPE_MEMORY_BACKEND "memory-backend"
 OBJECT_DECLARE_TYPE(HostMemoryBackend, HostMemoryBackendClass,
@@ -66,6 +67,7 @@ struct HostMemoryBackend {
     bool merge, dump, use_canonical_path;
     bool prealloc, is_mapped, share, reserve;
     uint32_t prealloc_threads;
+    ThreadContext *prealloc_context;
     DECLARE_BITMAP(host_nodes, MAX_NODES + 1);
     HostMemPolicy policy;
 

qapi/qom.json

@@ -578,6 +578,9 @@
 #
 # @prealloc-threads: number of CPU threads to use for prealloc (default: 1)
 #
+# @prealloc-context: thread context to use for creation of preallocation threads
+#                    (default: none) (since 7.2)
+#
 # @share: if false, the memory is private to QEMU; if true, it is shared
 #         (default: false)
 #
@@ -608,6 +611,7 @@
             '*policy': 'HostMemPolicy',
             '*prealloc': 'bool',
             '*prealloc-threads': 'uint32',
+            '*prealloc-context': 'str',
             '*share': 'bool',
             '*reserve': 'bool',
             'size': 'size',