Re: [PATCH v2] cgroup/cpuset: Remove cpus_allowed/mems_allowed setup in cpuset_init_smp()

[Feng Tang]

On Tue, Apr 26, 2022 at 10:34:21PM -0400, Waiman Long wrote:
> On 4/26/22 21:06, Feng Tang wrote:
> > On Tue, Apr 26, 2022 at 10:58:21PM +0800, Waiman Long wrote:
> > > On 4/25/22 23:23, Feng Tang wrote:
> > > > Hi Waiman,
> > > > 
> > > > On Mon, Apr 25, 2022 at 11:55:05AM -0400, Waiman Long wrote:
> > > > > There are 3 places where the cpu and node masks of the top cpuset can
> > > > > be initialized in the order they are executed:
> > > > >    1) start_kernel -> cpuset_init()
> > > > >    2) start_kernel -> cgroup_init() -> cpuset_bind()
> > > > >    3) kernel_init_freeable() -> do_basic_setup() -> cpuset_init_smp()
> > > > > 
> > > > > The first cpuset_init() function just sets all the bits in the masks.
> > > > > The last one executed is cpuset_init_smp() which sets up cpu and node
> > > > > masks suitable for v1, but not v2.  cpuset_bind() does the right setup
> > > > > for both v1 and v2.
> > > > > 
> > > > > For systems with cgroup v2 setup, cpuset_bind() is called once. For
> > > > > systems with cgroup v1 setup, cpuset_bind() is called twice. It is
> > > > > first called before cpuset_init_smp() in cgroup v2 mode.  Then it is
> > > > > called again when cgroup v1 filesystem is mounted in v1 mode after
> > > > > cpuset_init_smp().
> > > > > 
> > > > >     [    2.609781] cpuset_bind() called - v2 = 1
> > > > >     [    3.079473] cpuset_init_smp() called
> > > > >     [    7.103710] cpuset_bind() called - v2 = 0
> > > > I run some test, on a server with centOS, this did happen that
> > > > cpuset_bind() is called twice, first as v2 during kernel boot,
> > > > and then as v1 post-boot.
> > > > 
> > > > However on a QEMU running with a basic debian rootfs image,
> > > > the second  call of cpuset_bind() didn't happen.
> > > The first time cpuset_bind() is called in cgroup_init(), the kernel
> > > doesn't know if userspace is going to mount v1 or v2 cgroup. By default,
> > > it is assumed to be v2. However, if userspace mounts the cgroup v1
> > > filesystem for cpuset, cpuset_bind() will be run at this point by
> > > rebind_subsystem() to set up cgroup v1 environment and
> > > cpus_allowed/mems_allowed will be correctly set at this point. Mounting
> > > the cgroup v2 filesystem, however, does not cause rebind_subsystem() to
> > > run and hence cpuset_bind() is not called again.
> > > 
> > > Is the QEMU setup not mounting any cgroup filesystem at all? If so, does
> > > it matter whether v1 or v2 setup is used?
> > When I got the cpuset binding error report, I tried first on qemu to
> > reproduce and failed (due to there was no memory hotplug), then I
> > reproduced it on a real server. For both system, I used "cgroup_no_v1=all"
> > cmdline parameter to test cgroup-v2, could this be the reason? (TBH,
> > this is the first time I use cgroup-v2).
> > 
> > Here is the info dump:
> > 
> > # mount | grep cgroup
> > tmpfs on /sys/fs/cgroup type tmpfs (ro,nosuid,nodev,noexec,mode=755)
> > cgroup on /sys/fs/cgroup/systemd type cgroup (rw,nosuid,nodev,noexec,relatime,xattr,release_agent=/lib/systemd/systemd-cgroups-agent,name=systemd)
> > 
> > #cat /proc/filesystems | grep cgroup
> > nodev   cgroup
> > nodev   cgroup2
> > 
> > Thanks,
> > Feng
> 
> For cgroup v2, cpus_allowed should be set to cpu_possible_mask and
> mems_allowed to node_possible_map as is done in the first invocation of
> cpuset_bind(). That is the correct behavior.
 
OK. For the cgroup v2 mem binding problem with hot-added nodes, I
retested today, and it can't be reproduced with this patch. So feel
free to add:
  
  Tested-by: Feng Tang <feng.tang@xxxxxxxxx>

Thanks,
Feng


> Cheers,
> Longman
>