qemu_conf: Introduce a knob to set SCHED_CORE

Ideally, we would just pick the best default and users wouldn't
have to intervene at all. But in some cases it may be handy to
not bother with SCHED_CORE at all or place helper processes into
the same group as QEMU. Introduce a knob in qemu.conf to allow
users control this behaviour.

Signed-off-by: Michal Privoznik <mprivozn@redhat.com>
Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>

src/qemu/libvirtd_qemu.aug

@@ -110,6 +110,7 @@ module Libvirtd_qemu =
                  | bool_entry "dump_guest_core"
                  | str_entry "stdio_handler"
                  | int_entry "max_threads_per_process"
+                 | str_entry "sched_core"
 
    let device_entry = bool_entry "mac_filter"
                  | bool_entry "relaxed_acs_check"

src/qemu/qemu.conf.in

@@ -952,3 +952,19 @@
 # DO NOT use in production.
 #
 #deprecation_behavior = "none"
+
+# If this is set then QEMU and its threads will run in a separate scheduling
+# group meaning no other process will share Hyper Threads of a single core with
+# QEMU. Each QEMU has its own group.
+#
+# Possible options are:
+# "none"     - (default) neither QEMU or any of its helper processes are placed
+#              into separate scheduling group
+# "vcpus"    - only QEMU vCPU threads are placed into a separate scheduling group,
+#              emulator threads and helper processes remain outside of the group
+# "emulator" - only QEMU and its threads (emulator + vCPUs) are placed into
+#              separate scheduling group, helper proccesses remain outside of
+#              the group
+# "full"    -  both QEMU and its helper processes are placed into separate
+#              scheduling group
+#sched_core = "none"

src/qemu/qemu_conf.c

@@ -66,6 +66,14 @@ VIR_LOG_INIT("qemu.qemu_conf");
 #define QEMU_MIGRATION_PORT_MIN 49152
 #define QEMU_MIGRATION_PORT_MAX 49215
 
+VIR_ENUM_IMPL(virQEMUSchedCore,
+              QEMU_SCHED_CORE_LAST,
+              "none",
+              "vcpus",
+              "emulator",
+              "full");
+
+
 static virClass *virQEMUDriverConfigClass;
 static void virQEMUDriverConfigDispose(void *obj);
 
@@ -629,6 +637,7 @@ virQEMUDriverConfigLoadProcessEntry(virQEMUDriverConfig *cfg,
     g_auto(GStrv) hugetlbfs = NULL;
     g_autofree char *stdioHandler = NULL;
     g_autofree char *corestr = NULL;
+    g_autofree char *schedCore = NULL;
     size_t i;
 
     if (virConfGetValueStringList(conf, "hugetlbfs_mount", true,
@@ -706,6 +715,35 @@ virQEMUDriverConfigLoadProcessEntry(virQEMUDriverConfig *cfg,
         }
     }
 
+    if (virConfGetValueString(conf, "sched_core", &schedCore) < 0)
+        return -1;
+    if (schedCore) {
+        int val = virQEMUSchedCoreTypeFromString(schedCore);
+
+        if (val < 0) {
+            virReportError(VIR_ERR_CONFIG_UNSUPPORTED,
+                           _("Unknown sched_core value %s"),
+                           schedCore);
+            return -1;
+        }
+
+        if (val != QEMU_SCHED_CORE_NONE) {
+            int rv = virProcessSchedCoreAvailable();
+
+            if (rv < 0) {
+                virReportSystemError(errno, "%s",
+                                     _("Unable to detect SCHED_CORE"));
+                return -1;
+            } else if (rv == 0) {
+                virReportError(VIR_ERR_CONFIG_UNSUPPORTED, "%s",
+                               _("SCHED_CORE not supported by kernel"));
+                return -1;
+            }
+        }
+
+        cfg->schedCore = val;
+    }
+
     return 0;
 }
 

src/qemu/qemu_conf.h

@@ -44,6 +44,17 @@
 
 #define QEMU_DRIVER_NAME "QEMU"
 
+typedef enum {
+    QEMU_SCHED_CORE_NONE = 0,
+    QEMU_SCHED_CORE_VCPUS,
+    QEMU_SCHED_CORE_EMULATOR,
+    QEMU_SCHED_CORE_FULL,
+
+    QEMU_SCHED_CORE_LAST
+} virQEMUSchedCore;
+
+VIR_ENUM_DECL(virQEMUSchedCore);
+
 typedef struct _virQEMUDriver virQEMUDriver;
 
 typedef struct _virQEMUDriverConfig virQEMUDriverConfig;
@@ -216,6 +227,8 @@ struct _virQEMUDriverConfig {
     char **capabilityfilters;
 
     char *deprecationBehavior;
+
+    virQEMUSchedCore schedCore;
 };
 
 G_DEFINE_AUTOPTR_CLEANUP_FUNC(virQEMUDriverConfig, virObjectUnref);

src/qemu/test_libvirtd_qemu.aug.in

@@ -116,3 +116,4 @@ module Test_libvirtd_qemu =
     { "1" = "capname" }
 }
 { "deprecation_behavior" = "none" }
+{ "sched_core" = "none" }