vircgroup.c: add virCgroupSetupCpuShares()

The code that calls virCgroupSetCpuShares() and virCgroupGetCpuShares()
is repeated in 4 different places. Let's put it in a new
virCgroupSetupCpuShares() to avoid code repetition.

There's a reason of why we execute a Get in the same value we
just executed Set, explained in detail by commit 97814d8ab3.
Let's add a gist of the reasoning behind it as a comment in
this new function as well.

Signed-off-by: Daniel Henrique Barboza <danielhb413@gmail.com>
Signed-off-by: Ján Tomko <jtomko@redhat.com>
Reviewed-by: Ján Tomko <jtomko@redhat.com>

src/libvirt_private.syms

@@ -1732,6 +1732,7 @@ virCgroupSetupBlkioDeviceWeight;
 virCgroupSetupBlkioDeviceWriteBps;
 virCgroupSetupBlkioDeviceWriteIops;
 virCgroupSetupCpusetCpus;
+virCgroupSetupCpuShares;
 virCgroupSupportsCpuBW;
 virCgroupTerminateMachine;
 

src/lxc/lxc_cgroup.c

@@ -40,10 +40,7 @@ static int virLXCCgroupSetupCpuTune(virDomainDefPtr def,
 {
     if (def->cputune.sharesSpecified) {
         unsigned long long val;
-        if (virCgroupSetCpuShares(cgroup, def->cputune.shares) < 0)
-            return -1;
-
-        if (virCgroupGetCpuShares(cgroup, &val) < 0)
+        if (virCgroupSetupCpuShares(cgroup, def->cputune.shares, &val) < 0)
             return -1;
         def->cputune.shares = val;
     }

src/lxc/lxc_driver.c

@@ -1959,10 +1959,8 @@ lxcDomainSetSchedulerParametersFlags(virDomainPtr dom,
         if (STREQ(param->field, VIR_DOMAIN_SCHEDULER_CPU_SHARES)) {
             if (def) {
                 unsigned long long val;
-                if (virCgroupSetCpuShares(priv->cgroup, params[i].value.ul) < 0)
-                    goto endjob;
-
-                if (virCgroupGetCpuShares(priv->cgroup, &val) < 0)
+                if (virCgroupSetupCpuShares(priv->cgroup, params[i].value.ul,
+                                            &val) < 0)
                     goto endjob;
 
                 def->cputune.shares = val;

src/qemu/qemu_cgroup.c

@@ -894,11 +894,10 @@ qemuSetupCpuCgroup(virDomainObjPtr vm)
 
     if (vm->def->cputune.sharesSpecified) {
         unsigned long long val;
-        if (virCgroupSetCpuShares(priv->cgroup, vm->def->cputune.shares) < 0)
+        if (virCgroupSetupCpuShares(priv->cgroup, vm->def->cputune.shares,
+                                    &val) < 0)
             return -1;
 
-        if (virCgroupGetCpuShares(priv->cgroup, &val) < 0)
-            return -1;
         if (vm->def->cputune.shares != val) {
             vm->def->cputune.shares = val;
             if (virTypedParamsAddULLong(&eventParams, &eventNparams,

src/qemu/qemu_driver.c

@@ -10648,10 +10648,7 @@ qemuDomainSetSchedulerParametersFlags(virDomainPtr dom,
         if (STREQ(param->field, VIR_DOMAIN_SCHEDULER_CPU_SHARES)) {
             if (def) {
                 unsigned long long val;
-                if (virCgroupSetCpuShares(priv->cgroup, value_ul) < 0)
-                    goto endjob;
-
-                if (virCgroupGetCpuShares(priv->cgroup, &val) < 0)
+                if (virCgroupSetupCpuShares(priv->cgroup, value_ul, &val) < 0)
                     goto endjob;
 
                 def->cputune.shares = val;

src/util/vircgroup.c

@@ -3681,3 +3681,23 @@ virCgroupSetupCpusetCpus(virCgroupPtr cgroup, virBitmapPtr cpumask)
 
     return 0;
 }
+
+
+/* Per commit 97814d8ab3, the Linux kernel can consider a 'shares'
+ * value of '0' and '1' as 2, and any value larger than a maximum
+ * is reduced to maximum.
+ *
+ * The 'realValue' pointer holds the actual 'shares' value set by
+ * the kernel if the function returned success. */
+int
+virCgroupSetupCpuShares(virCgroupPtr cgroup, unsigned long long shares,
+                        unsigned long long *realValue)
+{
+    if (virCgroupSetCpuShares(cgroup, shares) < 0)
+        return -1;
+
+    if (virCgroupGetCpuShares(cgroup, realValue) < 0)
+        return -1;
+
+    return 0;
+}

src/util/vircgroup.h

@@ -222,6 +222,8 @@ virCgroupGetDomainTotalCpuStats(virCgroupPtr group,
 
 int virCgroupSetCpuShares(virCgroupPtr group, unsigned long long shares);
 int virCgroupGetCpuShares(virCgroupPtr group, unsigned long long *shares);
+int virCgroupSetupCpuShares(virCgroupPtr cgroup, unsigned long long shares,
+                            unsigned long long *realValue);
 
 int virCgroupSetCpuCfsPeriod(virCgroupPtr group, unsigned long long cfs_period);
 int virCgroupGetCpuCfsPeriod(virCgroupPtr group, unsigned long long *cfs_period);