nodeinfo: Fix output on PPC64 KVM hosts

The nodeinfo is reporting incorrect number of cpus and incorrect host
topology on PPC64 KVM hosts. The KVM hypervisor on PPC64 needs only
the primary thread in a core to be online, and the secondaries offlined.
While scheduling a guest in, the kvm scheduler wakes up the secondaries to
run in guest context.

The host scheduling of the guests happen at the core level(as only primary
thread is online). The kvm scheduler exploits as many threads of the core
as needed by guest. Further, starting POWER8, the processor allows splitting
a physical core into multiple subcores with 2 or 4 threads each. Again, only
the primary thread in a subcore is online in the host. The KVM-PPC
scheduler allows guests to exploit all the offline threads in the subcore,
by bringing them online when needed.
(Kernel patches on split-core http://www.spinics.net/lists/kvm-ppc/msg09121.html)

Recently with dynamic micro-threading changes in ppc-kvm, makes sure
to utilize all the offline cpus across guests, and across guests with
different cpu topologies.
(https://www.mail-archive.com/kvm@vger.kernel.org/msg115978.html)

Since the offline cpus are brought online in the guest context, it is safe
to count them as online. Nodeinfo today discounts these offline cpus from
cpu count/topology calclulation, and the nodeinfo output is not of any help
and the host appears overcommited when it is actually not.

The patch carefully counts those offline threads whose primary threads are
online. The host topology displayed by the nodeinfo is also fixed when the
host is in valid kvm state.
Signed-off-by: NShivaprasad G Bhat <sbhat@linux.vnet.ibm.com>
Signed-off-by: NAndrea Bolognani <abologna@redhat.com>

src/libvirt_private.syms

@@ -1010,6 +1010,7 @@ nodeGetMemoryParameters;
 nodeGetMemoryStats;
 nodeGetOnlineCPUBitmap;
 nodeGetPresentCPUBitmap;
+nodeGetThreadsPerSubcore;
 nodeSetMemoryParameters;
 
 

src/nodeinfo.c

@@ -31,6 +31,12 @@
 #include <dirent.h>
 #include <sys/utsname.h>
 #include "conf/domain_conf.h"
+#include <fcntl.h>
+#include <sys/ioctl.h>
+
+#if HAVE_LINUX_KVM_H
+# include <linux/kvm.h>
+#endif
 
 #if defined(__FreeBSD__) || defined(__APPLE__)
 # include <sys/time.h>
@@ -392,13 +398,14 @@ virNodeParseSocket(const char *dir,
  * filling arguments */
 static int
 ATTRIBUTE_NONNULL(1) ATTRIBUTE_NONNULL(3)
-ATTRIBUTE_NONNULL(4) ATTRIBUTE_NONNULL(5)
-ATTRIBUTE_NONNULL(6) ATTRIBUTE_NONNULL(7)
-ATTRIBUTE_NONNULL(8)
+ATTRIBUTE_NONNULL(4) ATTRIBUTE_NONNULL(6)
+ATTRIBUTE_NONNULL(7) ATTRIBUTE_NONNULL(8)
+ATTRIBUTE_NONNULL(9)
 virNodeParseNode(const char *node,
                  virArch arch,
                  virBitmapPtr present_cpus_map,
                  virBitmapPtr online_cpus_map,
+                 int threads_per_subcore,
                  int *sockets,
                  int *cores,
                  int *threads,
@@ -492,7 +499,18 @@ virNodeParseNode(const char *node,
             continue;
 
         if (!virBitmapIsBitSet(online_cpus_map, cpu)) {
+            if (threads_per_subcore > 0 &&
+                cpu % threads_per_subcore != 0 &&
+                virBitmapIsBitSet(online_cpus_map,
+                                  cpu - (cpu % threads_per_subcore))) {
+                /* Secondary offline threads are counted as online when
+                 * subcores are in use and the corresponding primary
+                 * thread is online */
+                processors++;
+            } else {
+                /* But they are counted as offline otherwise */
                 (*offline)++;
+            }
             continue;
         }
 
@@ -545,6 +563,12 @@ virNodeParseNode(const char *node,
             *cores = core;
     }
 
+    if (threads_per_subcore > 0) {
+        /* The thread count ignores offline threads, which means that only
+         * only primary threads have been considered so far. If subcores
+         * are in use, we need to also account for secondary threads */
+        *threads *= threads_per_subcore;
+    }
     ret = processors;
 
  cleanup:
@@ -563,6 +587,41 @@ virNodeParseNode(const char *node,
     return ret;
 }
 
+/* Check whether the host subcore configuration is valid.
+ *
+ * A valid configuration is one where no secondary thread is online;
+ * the primary thread in a subcore is always the first one */
+static bool
+nodeHasValidSubcoreConfiguration(const char *sysfs_prefix,
+                                 int threads_per_subcore)
+{
+    virBitmapPtr online_cpus = NULL;
+    int cpu = -1;
+    bool ret = false;
+
+    /* No point in checking if subcores are not in use */
+    if (threads_per_subcore <= 0)
+        goto cleanup;
+
+    if (!(online_cpus = nodeGetOnlineCPUBitmap(sysfs_prefix)))
+        goto cleanup;
+
+    while ((cpu = virBitmapNextSetBit(online_cpus, cpu)) >= 0) {
+
+        /* A single online secondary thread is enough to
+         * make the configuration invalid */
+        if (cpu % threads_per_subcore != 0)
+            goto cleanup;
+    }
+
+    ret = true;
+
+ cleanup:
+    virBitmapFree(online_cpus);
+
+    return ret;
+}
+
 int
 linuxNodeInfoCPUPopulate(const char *sysfs_prefix,
                          FILE *cpuinfo,
@@ -576,6 +635,7 @@ linuxNodeInfoCPUPopulate(const char *sysfs_prefix,
     DIR *nodedir = NULL;
     struct dirent *nodedirent = NULL;
     int cpus, cores, socks, threads, offline = 0;
+    int threads_per_subcore = 0;
     unsigned int node;
     int ret = -1;
     char *sysfs_nodedir = NULL;
@@ -683,6 +743,36 @@ linuxNodeInfoCPUPopulate(const char *sysfs_prefix,
         goto fallback;
     }
 
+    /* PPC-KVM needs the secondary threads of a core to be offline on the
+     * host. The kvm scheduler brings the secondary threads online in the
+     * guest context. Moreover, P8 processor has split-core capability
+     * where, there can be 1,2 or 4 subcores per core. The primaries of the
+     * subcores alone will be online on the host for a subcore in the
+     * host. Even though the actual threads per core for P8 processor is 8,
+     * depending on the subcores_per_core = 1, 2 or 4, the threads per
+     * subcore will vary accordingly to 8, 4 and 2 repectively.
+     * So, On host threads_per_core what is arrived at from sysfs in the
+     * current logic is actually the subcores_per_core. Threads per subcore
+     * can only be obtained from the kvm device. For example, on P8 wih 1
+     * core having 8 threads, sub_cores_percore=4, the threads 0,2,4 & 6
+     * will be online. The sysfs reflects this and in the current logic
+     * variable 'threads' will be 4 which is nothing but subcores_per_core.
+     * If the user tampers the cpu online/offline states using chcpu or other
+     * means, then it is an unsupported configuration for kvm.
+     * The code below tries to keep in mind
+     *  - when the libvirtd is run inside a KVM guest or Phyp based guest.
+     *  - Or on the kvm host where user manually tampers the cpu states to
+     *    offline/online randomly.
+     * On hosts other than POWER this will be 0, in which case a simpler
+     * thread-counting logic will be used  */
+    if ((threads_per_subcore = nodeGetThreadsPerSubcore(arch)) < 0)
+        goto cleanup;
+
+    /* If the subcore configuration is not valid, just pretend subcores
+     * are not in use and count threads one by one */
+    if (!nodeHasValidSubcoreConfiguration(sysfs_prefix, threads_per_subcore))
+        threads_per_subcore = 0;
+
     while ((direrr = virDirRead(nodedir, &nodedirent, sysfs_nodedir)) > 0) {
         if (sscanf(nodedirent->d_name, "node%u", &node) != 1)
             continue;
@@ -696,6 +786,7 @@ linuxNodeInfoCPUPopulate(const char *sysfs_prefix,
         if ((cpus = virNodeParseNode(sysfs_cpudir, arch,
                                      present_cpus_map,
                                      online_cpus_map,
+                                     threads_per_subcore,
                                      &socks, &cores,
                                      &threads, &offline)) < 0)
             goto cleanup;
@@ -729,6 +820,7 @@ linuxNodeInfoCPUPopulate(const char *sysfs_prefix,
     if ((cpus = virNodeParseNode(sysfs_cpudir, arch,
                                  present_cpus_map,
                                  online_cpus_map,
+                                 threads_per_subcore,
                                  &socks, &cores,
                                  &threads, &offline)) < 0)
         goto cleanup;
@@ -2248,3 +2340,56 @@ nodeAllocPages(unsigned int npages,
  cleanup:
     return ret;
 }
+
+/* Get the number of threads per subcore.
+ *
+ * This will be 2, 4 or 8 on POWER hosts, depending on the current
+ * micro-threading configuration, and 0 everywhere else.
+ *
+ * Returns the number of threads per subcore if subcores are in use, zero
+ * if subcores are not in use, and a negative value on error */
+int
+nodeGetThreadsPerSubcore(virArch arch)
+{
+    int threads_per_subcore = 0;
+
+#if HAVE_LINUX_KVM_H && defined(KVM_CAP_PPC_SMT)
+    const char *kvmpath = "/dev/kvm";
+    int kvmfd;
+
+    if (ARCH_IS_PPC64(arch)) {
+
+        /* It's okay if /dev/kvm doesn't exist, because
+         *   a. we might be running in a guest
+         *   b. the kvm module might not be installed or enabled
+         * In either case, falling back to the subcore-unaware thread
+         * counting logic is the right thing to do */
+        if (!virFileExists(kvmpath))
+            goto out;
+
+        if ((kvmfd = open(kvmpath, O_RDONLY)) < 0) {
+            /* This can happen when running as a regular user if
+             * permissions are tight enough, in which case erroring out
+             * is better than silently falling back and reporting
+             * different nodeinfo depending on the user */
+            virReportSystemError(errno,
+                                 _("Failed to open '%s'"),
+                                 kvmpath);
+            threads_per_subcore = -1;
+            goto out;
+        }
+
+        /* For Phyp and KVM based guests the ioctl for KVM_CAP_PPC_SMT
+         * returns zero and both primary and secondary threads will be
+         * online */
+        threads_per_subcore = ioctl(kvmfd,
+                                    KVM_CHECK_EXTENSION,
+                                    KVM_CAP_PPC_SMT);
+
+        VIR_FORCE_CLOSE(kvmfd);
+    }
+#endif /* HAVE_LINUX_KVM_H && defined(KVM_CAP_PPC_SMT) */
+
+ out:
+    return threads_per_subcore;
+}

src/nodeinfo.h

@@ -47,6 +47,7 @@ int nodeGetMemory(unsigned long long *mem,
 virBitmapPtr nodeGetPresentCPUBitmap(const char *sysfs_prefix);
 virBitmapPtr nodeGetOnlineCPUBitmap(const char *sysfs_prefix);
 int nodeGetCPUCount(const char *sysfs_prefix);
+int nodeGetThreadsPerSubcore(virArch arch);
 
 int nodeGetMemoryParameters(virTypedParameterPtr params,
                             int *nparams,