Re: [PATCH v2 1/3] mm: rework memcg kernel stack accounting

[Michal Hocko <mhocko@kernel.org>]

On Tue 21-08-18 14:35:57, Roman Gushchin wrote:
> If CONFIG_VMAP_STACK is set, kernel stacks are allocated
> using __vmalloc_node_range() with __GFP_ACCOUNT. So kernel
> stack pages are charged against corresponding memory cgroups
> on allocation and uncharged on releasing them.
> 
> The problem is that we do cache kernel stacks in small
> per-cpu caches and do reuse them for new tasks, which can
> belong to different memory cgroups.
> 
> Each stack page still holds a reference to the original cgroup,
> so the cgroup can't be released until the vmap area is released.
> 
> To make this happen we need more than two subsequent exits
> without forks in between on the current cpu, which makes it
> very unlikely to happen. As a result, I saw a significant number
> of dying cgroups (in theory, up to 2 * number_of_cpu +
> number_of_tasks), which can't be released even by significant
> memory pressure.
> 
> As a cgroup structure can take a significant amount of memory
> (first of all, per-cpu data like memcg statistics), it leads
> to a noticeable waste of memory.
> 
> Signed-off-by: Roman Gushchin <guro@fb.com>
> Cc: Johannes Weiner <hannes@cmpxchg.org>
> Cc: Michal Hocko <mhocko@kernel.org>
> Cc: Andy Lutomirski <luto@kernel.org>
> Cc: Konstantin Khlebnikov <koct9i@gmail.com>
> Cc: Tejun Heo <tj@kernel.org>
> Cc: Shakeel Butt <shakeelb@google.com>

Looks good to me. Two nits below.

I am not sure stable tree backport is really needed but it would be nice
to put
Fixes: ac496bf48d97 ("fork: Optimize task creation by caching two thread stacks per CPU if CONFIG_VMAP_STACK=y")

Acked-by: Michal Hocko <mhocko@suse.com>

> @@ -248,9 +253,20 @@ static unsigned long *alloc_thread_stack_node(struct task_struct *tsk, int node)
>  static inline void free_thread_stack(struct task_struct *tsk)
>  {
>  #ifdef CONFIG_VMAP_STACK
> -	if (task_stack_vm_area(tsk)) {
> +	struct vm_struct *vm = task_stack_vm_area(tsk);
> +
> +	if (vm) {
>  		int i;
>  
> +		for (i = 0; i < THREAD_SIZE / PAGE_SIZE; i++) {
> +			mod_memcg_page_state(vm->pages[i],
> +					     MEMCG_KERNEL_STACK_KB,
> +					     -(int)(PAGE_SIZE / 1024));
> +
> +			memcg_kmem_uncharge(vm->pages[i],
> +					    compound_order(vm->pages[i]));

when do we have order > 0 here? Also I was wondering how come this
doesn't blow up on partially charged stacks but both
mod_memcg_page_state and memcg_kmem_uncharge check for page->mem_cgroup
so this is safe. Maybe a comment would save people from scratching their
heads.

-- 
Michal Hocko
SUSE Labs