Re: [PATCH] cgroup/cpuset: Remove redundant cpu/node masks setup in cpuset_init_smp()

[Waiman Long]

On 4/25/22 03:30, Feng Tang wrote:
> Hi Waiman,
>
> Thanks for the patch!
>
> On Sun, Apr 24, 2022 at 10:09:26PM -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 assuming that effective_mems and effective_cpus have
> > been set up properly which is not strictly the case here. 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.
> >
> > To fix this problem, the redundant cpus_allowed and mems_allowed
> > mask setup in cpuset_init_smp() are removed. The effective_cpus and
> > effective_mems setup there are moved to cpuset_bind().
> >
> > cc: stable@xxxxxxxxxxxxxxx
> > Signed-off-by: Waiman Long <longman@xxxxxxxxxx>
> > ---
> >   kernel/cgroup/cpuset.c | 10 +++-------
> >   1 file changed, 3 insertions(+), 7 deletions(-)
> >
> > diff --git a/kernel/cgroup/cpuset.c b/kernel/cgroup/cpuset.c
> > index 9390bfd9f1cd..a2e15a43397e 100644
> > --- a/kernel/cgroup/cpuset.c
> > +++ b/kernel/cgroup/cpuset.c
> > @@ -2961,6 +2961,9 @@ static void cpuset_bind(struct cgroup_subsys_state *root_css)
> >   	percpu_down_write(&cpuset_rwsem);
> >   	spin_lock_irq(&callback_lock);
> >   
> > +	cpumask_copy(top_cpuset.effective_cpus, cpu_active_mask);
> > +	top_cpuset.effective_mems = node_states[N_MEMORY];
> > +
> >   	if (is_in_v2_mode()) {
> >   		cpumask_copy(top_cpuset.cpus_allowed, cpu_possible_mask);
> >   		top_cpuset.mems_allowed = node_possible_map;
> > @@ -3390,13 +3393,6 @@ 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];
> > -	top_cpuset.old_mems_allowed = top_cpuset.mems_allowed;
> > -
> > -	cpumask_copy(top_cpuset.effective_cpus, cpu_active_mask);
> > -	top_cpuset.effective_mems = node_states[N_MEMORY];
> IIUC, the init order is:
> 	cpuset_bind()
> 	smp_init()
> 	cpuset_init_smp()
>
> while all cpus except boot cpu is brought up in smp_init(), so I'm
> thinking moving the cpus_allowed init from cpuset_init_smp() to
> cpuset_bind() may cause some problem.

Good point. So cpuset_init_smp() is still useful for setting the right 
effective_cpus and probably effective_mems. I will update the patch 
accordingly.

Thanks,
Longman

> Thanks,
> Feng