linux-kernel - Re: [PATCH] cgroup/cpuset: Remove redundant cpu/node masks setup in cpuset_init

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Date:   Mon, 25 Apr 2022 15:30:11 +0800
From:   Feng Tang <feng.tang@...el.com>
To:     Waiman Long <longman@...hat.com>
Cc:     Tejun Heo <tj@...nel.org>, Zefan Li <lizefan.x@...edance.com>,
        Johannes Weiner <hannes@...xchg.org>, cgroups@...r.kernel.org,
        linux-mm@...ck.org, linux-kernel@...r.kernel.org,
        Andrew Morton <akpm@...ux-foundation.org>,
        Michal Hocko <mhocko@...nel.org>,
        Dave Hansen <dave.hansen@...el.com>, ying.huang@...el.com,
        stable@...r.kernel.org
Subject: Re: [PATCH] cgroup/cpuset: Remove redundant cpu/node masks setup in
 cpuset_init_smp()

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@...r.kernel.org
> Signed-off-by: Waiman Long <longman@...hat.com>
> ---
>  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.

Thanks,
Feng