On 11/25/2016 08:57 AM, Viktor Mihajlovski wrote:
> 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@xxxxxxxxxxxxxxxxxx>
> Tested-by: Matthew Rosato <mjrosato@xxxxxxxxxxxxxxxxxx>
> ---
> src/qemu/qemu_process.c | 33 ++++++++++++++++++++++++---------
> 1 file changed, 24 insertions(+), 9 deletions(-)
>
ACK w/ one slight adjustment noted below and both are now pushed.
John
> diff --git a/src/qemu/qemu_process.c b/src/qemu/qemu_process.c
> index 4758c49..9d1bfa4 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 +2224,34 @@ qemuProcessInitCpuAffinity(virDomainObjPtr vm)
> * the spawned QEMU instance to all pCPUs if no map is given in
> * its config file */
> int hostcpus;
> + cpumap = virProcessGetAffinity(vm->pid);
Moved this slightly... It's now:
if (virHostCPUHasBitmap()) {
hostcpumap = virHostCPUGetOnlineBitmap();
cpumap = virProcessGetAffinity(vm->pid);
}
Since cpumap would only be useful in that virBitmapEqual comparison if
the HasBitmap returns true.
It's still Free'd in the } else { prior to refetching and setting all.
>
> - /* 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();
> +
> + 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 +2262,7 @@ qemuProcessInitCpuAffinity(virDomainObjPtr vm)
>
> cleanup:
> virBitmapFree(cpumap);
> + virBitmapFree(hostcpumap);
> return ret;
> }
>
>
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