From 283e29043423a15a67696758c95aff5c792c587f Mon Sep 17 00:00:00 2001 From: Viktor Mihajlovski Date: Fri, 25 Nov 2016 14:57:05 +0100 Subject: [PATCH] qemu: Allow use of hot plugged host CPUs if no affinity set 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 Tested-by: Matthew Rosato --- src/qemu/qemu_process.c | 34 +++++++++++++++++++++++++--------- 1 file changed, 25 insertions(+), 9 deletions(-) diff --git a/src/qemu/qemu_process.c b/src/qemu/qemu_process.c index e5b77d6376..3871034818 100644 --- a/src/qemu/qemu_process.c +++ b/src/qemu/qemu_process.c @@ -2195,6 +2195,7 @@ qemuProcessInitCpuAffinity(virDomainObjPtr vm) int ret = -1; virBitmapPtr cpumap = NULL; virBitmapPtr cpumapToSet = NULL; + virBitmapPtr hostcpumap = NULL; qemuDomainObjPrivatePtr priv = vm->privateData; if (!vm->pid) { @@ -2217,20 +2218,34 @@ qemuProcessInitCpuAffinity(virDomainObjPtr vm) * 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) + if (virHostCPUHasBitmap()) { + 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; + } } } @@ -2241,6 +2256,7 @@ qemuProcessInitCpuAffinity(virDomainObjPtr vm) cleanup: virBitmapFree(cpumap); + virBitmapFree(hostcpumap); return ret; } -- GitLab