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

[Feng Tang]

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. 

> As a result, cpu and memory node hot add may fail to update the cpu and
> node masks of the top cpuset to include the newly added cpu or node in
> a cgroup v2 environment.
> 
> smp_init() is called after the first two init functions.  So we don't
> have a complete list of active cpus and memory nodes until later in
> cpuset_init_smp() which is the right time to set up effective_cpus
> and effective_mems.
> 
> To fix this problem, the potentially incorrect cpus_allowed &
> mems_allowed setup in cpuset_init_smp() are removed.  For cgroup v2
> systems, the initial cpuset_bind() call will set them up correctly.
> For cgroup v1 systems, the second call to cpuset_bind() will do the
> right setup.
> 
> cc: stable@xxxxxxxxxxxxxxx
> Signed-off-by: Waiman Long <longman@xxxxxxxxxx>
> ---
>  kernel/cgroup/cpuset.c | 5 +++--
>  1 file changed, 3 insertions(+), 2 deletions(-)
> 
> diff --git a/kernel/cgroup/cpuset.c b/kernel/cgroup/cpuset.c
> index 9390bfd9f1cd..6bd8f5ef40fe 100644
> --- a/kernel/cgroup/cpuset.c
> +++ b/kernel/cgroup/cpuset.c
> @@ -3390,8 +3390,9 @@ static struct notifier_block cpuset_track_online_nodes_nb = {
>   */
>  void __init cpuset_init_smp(void)
>  {
> -	cpumask_copy(top_cpuset.cpus_allowed, cpu_active_mask);
> -	top_cpuset.mems_allowed = node_states[N_MEMORY];

So can we keep line
  cpumask_copy(top_cpuset.cpus_allowed, cpu_active_mask);

and only remove line 
       top_cpuset.mems_allowed = node_states[N_MEMORY];
?

Thanks,
Feng