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

On Fri, 26 Feb 2016 13:00:04 +0000 "Daniel P. Berrange" <berrange(a)redhat.com&gt; wrote: > On Fri, Feb 26, 2016 at 01:43:05PM +0100, Henning Schild wrote: > IIUC, the original problem you wanted to address was that vCPU pids > could run the wrong CPU for a short time. ie The original code did > > 1. Libvirtd forks child pid > ... this pid runs on whatever pCPUs that libvirt is permitted to > use... 2. Libvirtd creates root cgroup for VM > ... this pid runs on whatever pCPUs the root cgroup inherited... > 3. Child pid execs QEMU > ... QEMU pid runs on whatever pCPUs the root cgroup inherited... > 4. QEMU creates vCPU pids > ... vCPU pids run on whatever pCPUs the root cgroup inherited... > 4. Libvirtd moves emulator PIDs and vCPU PIDs > ... emulator PIDs are running on assigned pCPUs for emulator... > ... vCPU PIDs are running on assigned pCPUs for vCPUs.... > > With the important change in patch 5 this now looks like > > 1. Libvirtd forks child pid > ... this pid runs on whatever pCPUs that libvirt is permitted to > use... 2. Libvirtd creates root cgroup for VM > ... this pid runs on whatever pCPUs the root cgroup inherited... I am trying to come up with a solution that eliminates the above line from the whole bringup. I.e never allow a pid belonging to the VM outside the pinnings of libvirtd and the VM configuration.

But until step 4 it should probably be "... this pid *just sits* on whatever pCPUs the root cgroup inherited..." If we are sure that it does not "run" before 4. patch 5 does the trick already

> 3. Libvirtd moves pid into emulator group > ... this pid runs on assigned pCPUs for emulator... > 4. Child pid execs QEMU > ... QEMU pid runs on assigned pCPUs for emulator... > 5. QEMU creates vCPU pids > ... vCPU pids are running on assigned pCPUS for emulator... > 6. Libvirtd moves vCPU PIDs > ... emulator PIDs are running on assigned pCPUs for emulator... > ... vCPU PIDs are running on assigned pCPUs for vCPUs.... > > Which is good, because vCPU pids don't ever run on un-restricted > pCPUs. > > > Your patch 4 here is attempting to change step 2 only so that it > looks like > > > 1. Libvirtd forks child pid > ... this pid runs on whatever pCPUs that libvirt is permitted to > use... 2. Libvirtd creates root cgroup for VM > ... this pid runs on whatever pCPUs that libvirt is permitted to > use... or depending on what controller system added > ... this pid runs on whatever pCPUs the root cgroup inherited... > 3. Libvirtd adds pid into emulator group > ... this pid runs on assigned pCPUs for emulator... > 4. Child pid execs QEMU > ... QEMU pid runs on assigned pCPUs for emulator... > 5. QEMU creates vCPU pids > ... vCPU pids are running on assigned pCPUS for emulator... > 6. Libvirtd moves vCPU PIDs > ... emulator PIDs are running on assigned pCPUs for emulator... > ... vCPU PIDs are running on assigned pCPUs for vCPUs.... > > At the time we exec QEMU in step 4 the situation is exactly the same > as before. The vCPU pids are still created in the right place > straight away. > > So this patch 4 doesn't achieve anything useful > > Regards, > Daniel