Re: [libvirt] [PATCH 4/9] util: cgroups do not implicitly add task to new machine cgroup

On Tue, Feb 23, 2016 at 04:58:39PM +0100, Henning Schild wrote: > virCgroupNewMachine used to add the pidleader to the newly created > machine cgroup. Do not do this implicit anymore. > > Signed-off-by: Henning Schild <henning.schild(a)siemens.com&gt; > --- > src/lxc/lxc_cgroup.c | 11 +++++++++++ > src/qemu/qemu_cgroup.c | 11 +++++++++++ > src/util/vircgroup.c | 22 ---------------------- > 3 files changed, 22 insertions(+), 22 deletions(-) NACK to this patch once again. This does not actually work as you think it does. > diff --git a/src/util/vircgroup.c b/src/util/vircgroup.c > index 11f33ab..aef8e8c 100644 > --- a/src/util/vircgroup.c > +++ b/src/util/vircgroup.c > @@ -1682,16 +1682,6 @@ virCgroupNewMachineSystemd(const char *name, > } > } > > - if (virCgroupAddTask(*group, pidleader) < 0) { > - virErrorPtr saved = virSaveLastError(); > - virCgroupRemove(*group); > - virCgroupFree(group); > - if (saved) { > - virSetError(saved); > - virFreeError(saved); > - } > - } Just above this we called virSystemdCreateMachine. Systemd will create the cgroup and add the pidleader to those cgroups. Systemd may add the pidleader to just the 'systemd' controller, or it may add the pidleader to *ALL* controllers. We have no way of knowing. This virCgroupAddTask call deals with whatever systemd chose not todo, so we can guarantee consistent behaviour with the pidleader in all cgroups. By removing this you make this method non-deterministic - the pid may or may not be in the cpu controller now. THis is bad because it can lead to QEMU/LXC driver code working in some cases but failing in other cases. Furthermore, this existing does not cause any problems for the scenario you care about. THis cgroup placement is being set in between the time libvirtd calls fork() and exec(). With your later patch 5, we ensure that the PID is moved across into the emulator cgroup, before we call exec(). When we call exec all memory mappings will be replaced, so QEMU will stil start with the correct vCPU placement and memory allocation placement.

Just just drop this patch please. Regards, Daniel