If the cpuset cgroup controller is disabled in /etc/libvirt/qemu.conf QEMU virtual machines can in principle use all host CPUs, even if they are hot plugged, if they have no explicit CPU affinity defined. However, there's libvirt code supposed to handle the situation where the libvirt daemon itself is not using all host CPUs. The code in qemuProcessInitCpuAffinity attempts to set an affinity mask including all defined host CPUs. Unfortunately, the resulting affinity mask for the process will not contain the offline CPUs. See also the sched_setaffinity(2) man page. That means that even if the host CPUs come online again, they won't be used by the QEMU process anymore. The same is true for newly hot plugged CPUs. So we are effectively preventing that QEMU uses all processors instead of enabling it to use them. It only makes sense to set the QEMU process affinity if we're able to actually grow the set of usable CPUs, i.e. if the process affinity is a subset of the online host CPUs. There's still the chance that for some reason the deliberately chosen libvirtd affinity matches the online host CPU mask by accident. In this case the behavior remains as it was before (CPUs offline while setting the affinity will not be used if they show up later on). Signed-off-by: Viktor Mihajlovski <mihajlov(a)linux.vnet.ibm.com&gt; Tested-by: Matthew Rosato <mjrosato(a)linux.vnet.ibm.com&gt; --- V2: - Avoid memory leak by freeing cpumap if the affinity has to be set. - Made an attempt to better describe the issue at hand src/qemu/qemu_process.c | 33 +++++++++++++++++++++++---------- 1 file changed, 23 insertions(+), 10 deletions(-) diff --git a/src/qemu/qemu_process.c b/src/qemu/qemu_process.c index 4758c49..7e55086 100644 --- a/src/qemu/qemu_process.c +++ b/src/qemu/qemu_process.c @@ -2202,6 +2202,7 @@ qemuProcessInitCpuAffinity(virDomainObjPtr vm) int ret = -1; virBitmapPtr cpumap = NULL; virBitmapPtr cpumapToSet = NULL; + virBitmapPtr hostcpumap = NULL; qemuDomainObjPrivatePtr priv = vm->privateData; if (!vm->pid) { @@ -2223,21 +2224,32 @@ qemuProcessInitCpuAffinity(virDomainObjPtr vm) * the spawned QEMU instance to all pCPUs if no map is given in * its config file */ int hostcpus; - - /* setaffinity fails if you set bits for CPUs which - * aren't present, so we have to limit ourselves */ - if ((hostcpus = virHostCPUGetCount()) < 0) + hostcpumap = virHostCPUGetOnlineBitmap(); + cpumap = virProcessGetAffinity(vm->pid); + + if (hostcpumap && cpumap && virBitmapEqual(hostcpumap, cpumap)) { + /* we're using all available CPUs, no reason to set + * mask. If libvirtd is running without explicit + * affinity, we can use hotplugged CPUs for this VM */ + ret = 0; goto cleanup; + } else { + /* setaffinity fails if you set bits for CPUs which + * aren't present, so we have to limit ourselves */ + if ((hostcpus = virHostCPUGetCount()) < 0) + goto cleanup; - if (hostcpus > QEMUD_CPUMASK_LEN) - hostcpus = QEMUD_CPUMASK_LEN; + if (hostcpus > QEMUD_CPUMASK_LEN) + hostcpus = QEMUD_CPUMASK_LEN; - if (!(cpumap = virBitmapNew(hostcpus))) - goto cleanup; + virBitmapFree(cpumap); + if (!(cpumap = virBitmapNew(hostcpus))) + goto cleanup; - virBitmapSetAll(cpumap); + virBitmapSetAll(cpumap); - cpumapToSet = cpumap; + cpumapToSet = cpumap; + } } } @@ -2248,6 +2260,7 @@ qemuProcessInitCpuAffinity(virDomainObjPtr vm) cleanup: virBitmapFree(cpumap); + virBitmapFree(hostcpumap); return ret; } -- 1.9.1