[PATCH v4 7/8] qemu: Enable SCHED_CORE for vCPUs

[Michal Privoznik <mprivozn@xxxxxxxxxx>]

For QEMU_SCHED_CORE_VCPUS case, the vCPU threads should be placed
all into one scheduling group, but not the emulator or any of its
threads. Therefore, as soon as vCPU TIDs are detected, fork off a
child which then creates a separate scheduling group and adds all
vCPU threads into it.

Please note, this commit only handles the cold boot case. Hotplug
is going to be implemented in the next commit.

Signed-off-by: Michal Privoznik <mprivozn@xxxxxxxxxx>
Reviewed-by: Daniel P. Berrangé <berrange@xxxxxxxxxx>
---
 src/qemu/qemu_process.c | 39 +++++++++++++++++++++++++++++++++++++++
 1 file changed, 39 insertions(+)

diff --git a/src/qemu/qemu_process.c b/src/qemu/qemu_process.c
index 22845423ce..32136978a9 100644
--- a/src/qemu/qemu_process.c
+++ b/src/qemu/qemu_process.c
@@ -5860,9 +5860,44 @@ qemuProcessSetupVcpu(virDomainObj *vm,
 }
 
 
+static int
+qemuProcessSetupAllVcpusSchedCoreHelper(pid_t ppid G_GNUC_UNUSED,
+                                        void *opaque)
+{
+    virDomainObj *vm = opaque;
+    size_t i;
+
+    /* Since we are setting all vCPU threads at once and from a forked off
+     * child, we don't need the dummy schedCoreChildPID and can create one on
+     * our own. */
+    if (virProcessSchedCoreCreate() < 0) {
+        virReportSystemError(errno, "%s",
+                             _("Unable to set SCHED_CORE"));
+
+        return -1;
+    }
+
+    for (i = 0; i < virDomainDefGetVcpusMax(vm->def); i++) {
+        pid_t vcpupid = qemuDomainGetVcpuPid(vm, i);
+
+        if (vcpupid > 0 &&
+            virProcessSchedCoreShareTo(vcpupid) < 0) {
+            virReportSystemError(errno,
+                                 _("unable to share scheduling cookie to %lld"),
+                                 (long long) vcpupid);
+            return -1;
+        }
+    }
+
+    return 0;
+}
+
+
 static int
 qemuProcessSetupVcpus(virDomainObj *vm)
 {
+    qemuDomainObjPrivate *priv = vm->privateData;
+    g_autoptr(virQEMUDriverConfig) cfg = virQEMUDriverGetConfig(priv->driver);
     virDomainVcpuDef *vcpu;
     unsigned int maxvcpus = virDomainDefGetVcpusMax(vm->def);
     size_t i;
@@ -5905,6 +5940,10 @@ qemuProcessSetupVcpus(virDomainObj *vm)
             return -1;
     }
 
+    if (cfg->schedCore == QEMU_SCHED_CORE_VCPUS &&
+        virProcessRunInFork(qemuProcessSetupAllVcpusSchedCoreHelper, vm) < 0)
+        return -1;
+
     return 0;
 }
 
-- 
2.35.1