From 19f7e6dc950baf346738f462ac5c6b815c04edcc Mon Sep 17 00:00:00 2001
Message-Id: <19f7e6dc950baf346738f462ac5c6b815c04edcc@dist-git>
From: Michal Privoznik <mprivozn@redhat.com>
Date: Mon, 21 Mar 2022 16:49:25 +0100
Subject: [PATCH] conf: Introduce memory allocation threads

Since its v5.0.0 release QEMU is capable of specifying number of
threads used to allocate memory. It defaults to 1, which may be
too low for humongous guests with gigantic pages.

In general, on QEMU cmd line level it is possible to use
different number of threads per each memory-backend-* object, in
practical terms it's not useful. Therefore, use <memoryBacking/>
to set guest wide value and let all memory devices 'inherit' it,
silently. IOW, don't introduce per device knob because that would
only complicate things for a little or no benefit.

Signed-off-by: Michal Privoznik <mprivozn@redhat.com>
Reviewed-by: Martin Kletzander <mkletzan@redhat.com>
(cherry picked from commit ba7f98126fa84d354ce72929b77cc111a9a557a9)
Resolves: https://bugzilla.redhat.com/show_bug.cgi?id=2067126
Signed-off-by: Michal Privoznik <mprivozn@redhat.com>
---
 docs/formatdomain.rst                        |  8 +++++---
 docs/schemas/domaincommon.rng                | 19 +++++++++++++------
 src/conf/domain_conf.c                       | 15 ++++++++++++++-
 src/conf/domain_conf.h                       |  1 +
 tests/qemuxml2argvdata/memfd-memory-numa.xml |  2 +-
 5 files changed, 34 insertions(+), 11 deletions(-)

diff --git a/docs/formatdomain.rst b/docs/formatdomain.rst
index 8128e43da4..17e89a0c0d 100644
--- a/docs/formatdomain.rst
+++ b/docs/formatdomain.rst
@@ -977,7 +977,7 @@ Memory Backing
        <locked/>
        <source type="file|anonymous|memfd"/>
        <access mode="shared|private"/>
-       <allocation mode="immediate|ondemand"/>
+       <allocation mode="immediate|ondemand" threads='8'/>
        <discard/>
      </memoryBacking>
      ...
@@ -1026,8 +1026,10 @@ influence how virtual memory pages are backed by host pages.
    Using the ``mode`` attribute, specify if the memory is to be "shared" or
    "private". This can be overridden per numa node by ``memAccess``.
 ``allocation``
-   Using the ``mode`` attribute, specify when to allocate the memory by
-   supplying either "immediate" or "ondemand".
+   Using the optional ``mode`` attribute, specify when to allocate the memory by
+   supplying either "immediate" or "ondemand". :since:`Since 8.2.0` it is
+   possible to set the number of threads that hypervisor uses to allocate
+   memory via ``threads`` attribute.
 ``discard``
    When set and supported by hypervisor the memory content is discarded just
    before guest shuts down (or when DIMM module is unplugged). Please note that
diff --git a/docs/schemas/domaincommon.rng b/docs/schemas/domaincommon.rng
index 7fa5c2b8b5..c9c1529979 100644
--- a/docs/schemas/domaincommon.rng
+++ b/docs/schemas/domaincommon.rng
@@ -745,12 +745,19 @@
             </optional>
             <optional>
               <element name="allocation">
-                <attribute name="mode">
-                  <choice>
-                    <value>immediate</value>
-                    <value>ondemand</value>
-                  </choice>
-                </attribute>
+                <optional>
+                  <attribute name="mode">
+                    <choice>
+                      <value>immediate</value>
+                      <value>ondemand</value>
+                    </choice>
+                  </attribute>
+                </optional>
+                <optional>
+                  <attribute name="threads">
+                    <ref name="unsignedInt"/>
+                  </attribute>
+                </optional>
               </element>
             </optional>
             <optional>
diff --git a/src/conf/domain_conf.c b/src/conf/domain_conf.c
index 5691b8d2d5..805a15848e 100644
--- a/src/conf/domain_conf.c
+++ b/src/conf/domain_conf.c
@@ -19095,6 +19095,13 @@ virDomainDefParseMemory(virDomainDef *def,
         VIR_FREE(tmp);
     }
 
+    if (virXPathUInt("string(./memoryBacking/allocation/@threads)",
+                     ctxt, &def->mem.allocation_threads) == -2) {
+        virReportError(VIR_ERR_XML_ERROR, "%s",
+                       _("Failed to parse memory allocation threads"));
+        return -1;
+    }
+
     if (virXPathNode("./memoryBacking/hugepages", ctxt)) {
         /* hugepages will be used */
         if ((n = virXPathNodeSet("./memoryBacking/hugepages/page", ctxt, &nodes)) < 0) {
@@ -27639,6 +27646,7 @@ virDomainMemorybackingFormat(virBuffer *buf,
                              const virDomainMemtune *mem)
 {
     g_auto(virBuffer) childBuf = VIR_BUFFER_INIT_CHILD(buf);
+    g_auto(virBuffer) allocAttrBuf = VIR_BUFFER_INITIALIZER;
 
     if (mem->nhugepages)
         virDomainHugepagesFormat(&childBuf, mem->hugepages, mem->nhugepages);
@@ -27653,8 +27661,13 @@ virDomainMemorybackingFormat(virBuffer *buf,
         virBufferAsprintf(&childBuf, "<access mode='%s'/>\n",
                           virDomainMemoryAccessTypeToString(mem->access));
     if (mem->allocation)
-        virBufferAsprintf(&childBuf, "<allocation mode='%s'/>\n",
+        virBufferAsprintf(&allocAttrBuf, " mode='%s'",
                           virDomainMemoryAllocationTypeToString(mem->allocation));
+    if (mem->allocation_threads > 0)
+        virBufferAsprintf(&allocAttrBuf, " threads='%u'", mem->allocation_threads);
+
+    virXMLFormatElement(&childBuf, "allocation", &allocAttrBuf, NULL);
+
     if (mem->discard)
         virBufferAddLit(&childBuf, "<discard/>\n");
 
diff --git a/src/conf/domain_conf.h b/src/conf/domain_conf.h
index 144ba4dd12..10af94e2e4 100644
--- a/src/conf/domain_conf.h
+++ b/src/conf/domain_conf.h
@@ -2677,6 +2677,7 @@ struct _virDomainMemtune {
     int source; /* enum virDomainMemorySource */
     int access; /* enum virDomainMemoryAccess */
     int allocation; /* enum virDomainMemoryAllocation */
+    unsigned int allocation_threads;
 
     virTristateBool discard;
 };
diff --git a/tests/qemuxml2argvdata/memfd-memory-numa.xml b/tests/qemuxml2argvdata/memfd-memory-numa.xml
index 1ebcee8939..1ac87e3aef 100644
--- a/tests/qemuxml2argvdata/memfd-memory-numa.xml
+++ b/tests/qemuxml2argvdata/memfd-memory-numa.xml
@@ -10,7 +10,7 @@
     </hugepages>
     <source type='memfd'/>
     <access mode='shared'/>
-    <allocation mode='immediate'/>
+    <allocation mode='immediate' threads='8'/>
   </memoryBacking>
   <vcpu placement='static'>8</vcpu>
   <numatune>
-- 
2.35.1