[PATCH] qemu: Create or remove cgroup when doing vcpu (un)hotpluging

[Osier Yang <jyang@xxxxxxxxxx>]

Various APIs use cgroup to either set or get the statistics of
host or guest. Hotplug or unhotplug new vcpus without creating
or removing the cgroup for the vcpus could cause problems for
those APIs. E.g.

% virsh vcpucount dom
maximum      config        10
maximum      live          10
current      config         1
current      live           1

% virsh setvcpu dom 2

% virsh schedinfo dom --set vcpu_quota=1000
Scheduler      : posix
error: Unable to find vcpu cgroup for rhel6.2(vcpu: 1): No such file or
directory

This patch fixes the problem by creating cgroups for each of the
onlined vcpus, and destroying cgroups for each of the offlined
vcpus.
---
 src/qemu/qemu_driver.c |   53 ++++++++++++++++++++++++++++++++++++++++++++++++
 1 files changed, 53 insertions(+), 0 deletions(-)

diff --git a/src/qemu/qemu_driver.c b/src/qemu/qemu_driver.c
index 97ad23e..b8ceca1 100644
--- a/src/qemu/qemu_driver.c
+++ b/src/qemu/qemu_driver.c
@@ -3601,6 +3601,8 @@ static int qemudDomainHotplugVcpus(struct qemud_driver *driver,
     int vcpus = oldvcpus;
     pid_t *cpupids = NULL;
     int ncpupids;
+    virCgroupPtr cgroup = NULL;
+    virCgroupPtr cgroup_vcpu = NULL;
 
     qemuDomainObjEnterMonitor(driver, vm);
 
@@ -3654,6 +3656,53 @@ static int qemudDomainHotplugVcpus(struct qemud_driver *driver,
         goto cleanup;
     }
 
+    if (virCgroupForDomain(driver->cgroup, vm->def->name, &cgroup, 0) == 0) {
+        int rv = -1;
+
+        if (nvcpus > oldvcpus) {
+            for (i = oldvcpus; i < nvcpus; i++) {
+                /* Create cgroup for the onlined vcpu */
+                rv = virCgroupForVcpu(cgroup, i, &cgroup_vcpu, 1);
+                if (rv < 0) {
+                    virReportSystemError(-rv,
+                                         _("Unable to create vcpu cgroup for %s(vcpu:"
+                                           " %d)"),
+                                         vm->def->name, i);
+                    goto cleanup;
+                }
+
+                /* Add vcpu thread to the cgroup */
+                rv = virCgroupAddTask(cgroup_vcpu, cpupids[i]);
+                if (rv < 0) {
+                    virReportSystemError(-rv,
+                                         _("unable to add vcpu %d task %d to cgroup"),
+                                         i, cpupids[i]);
+                    virCgroupRemove(cgroup_vcpu);
+                    goto cleanup;
+                }
+
+                virCgroupFree(&cgroup_vcpu);
+            }
+        } else {
+            for (i = oldvcpus - 1; i >= nvcpus; i--) {
+                rv = virCgroupForVcpu(cgroup, i, &cgroup_vcpu, 0);
+                if (rv < 0) {
+                    virReportSystemError(-rv,
+                                         _("Unable to access vcpu cgroup for %s(vcpu:"
+                                           " %d)"),
+                                         vm->def->name, i);
+                    goto cleanup;
+                }
+
+                /* Remove cgroup for the offlined vcpu */
+                virCgroupRemove(cgroup_vcpu);
+                virCgroupFree(&cgroup_vcpu);
+            }
+        }
+
+        virCgroupFree(&cgroup);
+    }
+
     priv->nvcpupids = ncpupids;
     VIR_FREE(priv->vcpupids);
     priv->vcpupids = cpupids;
@@ -3664,6 +3713,10 @@ cleanup:
     vm->def->vcpus = vcpus;
     VIR_FREE(cpupids);
     virDomainAuditVcpu(vm, oldvcpus, nvcpus, "update", rc == 1);
+    if (cgroup)
+        virCgroupFree(&cgroup);
+    if (cgroup_vcpu)
+        virCgroupFree(&cgroup_vcpu);
     return ret;
 
 unsupported:
-- 
1.7.7.6

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