From cfea24086363d2f7b76fb393ae1232640e5226c7 Mon Sep 17 00:00:00 2001
From: Colin Walters <walters@verbum.org>
Date: Jun 19 2018 18:30:23 +0000
Subject: inventory-qcow2: Use physical cores, match libvirt CPU invocation


I was investigating some rpm-ostree CI performance issues in
https://github.com/projectatomic/rpm-ostree/pull/1362
and I discovered that my previous change to use `-smp $cpus` was
suboptimal.  See the linked libvirt discussion; basically we
really want to use physical cores, otherwise the guest kernel
is going to make bad scheduling decisions.

We also take care to also minimize that set by the number of
CPUs available to us; we may be running in a container with
a cgroup for example.

---

diff --git a/inventory/standard-inventory-qcow2 b/inventory/standard-inventory-qcow2
index d2561f8..d73acfb 100755
--- a/inventory/standard-inventory-qcow2
+++ b/inventory/standard-inventory-qcow2
@@ -7,7 +7,7 @@ import os
 import shutil
 import shlex
 import signal
-import multiprocessing
+import StringIO
 import random
 import socket
 import subprocess
@@ -113,6 +113,23 @@ def inv_list(subjects):
             "_meta": {"hostvars": variables}}
 
 
+def get_qemu_smp_arg():
+    """Determine the number of CPUs that should be visible in the guest.
+    See e.g. https://www.redhat.com/archives/libvirt-users/2017-June/msg00025.html
+    We want to match the number of host physical cores.
+    """
+    # We may be run in a cgroup with fewer cores available than physical.
+    available_cpu = int(subprocess.check_output(['nproc']).strip())
+    # https://stackoverflow.com/questions/6481005/how-to-obtain-the-number-of-cpus-cores-in-linux-from-the-command-line
+    core_sockets = set()
+    for line in StringIO.StringIO(subprocess.check_output(['lscpu', '-b', '-p=Core,Socket'])):
+        if line.startswith('#'):
+            continue
+        core_sockets.add(line.strip())
+    sockets = min(available_cpu, len(core_sockets))
+    return '{},sockets={},cores=1,threads=1'.format(sockets, sockets)
+
+
 def write_debug_inventory(file_, host_vars):
     raw_inventory = {"all": {"children": {"localhost": {"hosts": host_vars},
                                           "subjects": {"hosts": host_vars},
@@ -149,7 +166,7 @@ def start_qemu(image, cloudinit, log, portrange=(2222, 5555)):
     # Use -cpu host and -smp by default.
     # virtio-rng-pci: https://wiki.qemu.org/Features/VirtIORNG
     qemu_cmd = ["/usr/bin/qemu-system-x86_64",
-                "-cpu", "host", "-smp", "{}".format(multiprocessing.cpu_count()),
+                "-cpu", "host", "-smp", get_qemu_smp_arg(),
                 "-m", "1024", image, "-enable-kvm", "-snapshot", "-cdrom", cloudinit,
                 "-net", "nic,model=virtio", "-net", "user,hostfwd=tcp:127.0.0.3:{0}-:22".format(port),
                 "-device", "virtio-rng-pci", "-rtc", "base=utc",