Re: [RFC PATCH 1/2] mm: rework memcg kernel stack accounting

[Shakeel Butt <shakeelb@google.com>]

On Tue, Aug 14, 2018 at 5:37 PM Roman Gushchin <guro@fb.com> 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>

I was also looking into this issue. I was thinking of having a
per-memcg per-cpu stack cache. However this solution seems much
simpler. Can you also add the performance number for a similar simple
benchmark done in ac496bf48d97 ("fork: Optimize task creation by
caching two thread stacks per CPU if CONFIG_VMAP_STACK=y").

Reviewed-by: Shakeel Butt <shakeelb@google.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>
> ---
>  kernel/fork.c | 44 ++++++++++++++++++++++++++++++++++++++------
>  1 file changed, 38 insertions(+), 6 deletions(-)
>
> diff --git a/kernel/fork.c b/kernel/fork.c
> index 69b6fea5a181..91872b2b37bd 100644
> --- a/kernel/fork.c
> +++ b/kernel/fork.c
> @@ -224,9 +224,14 @@ static unsigned long *alloc_thread_stack_node(struct task_struct *tsk, int node)
>                 return s->addr;
>         }
>
> +       /*
> +        * Allocated stacks are cached and later reused by new threads,
> +        * so memcg accounting is performed manually on assigning/releasing
> +        * stacks to tasks. Drop __GFP_ACCOUNT.
> +        */
>         stack = __vmalloc_node_range(THREAD_SIZE, THREAD_ALIGN,
>                                      VMALLOC_START, VMALLOC_END,
> -                                    THREADINFO_GFP,
> +                                    THREADINFO_GFP & ~__GFP_ACCOUNT,
>                                      PAGE_KERNEL,
>                                      0, node, __builtin_return_address(0));
>
> @@ -246,12 +251,41 @@ static unsigned long *alloc_thread_stack_node(struct task_struct *tsk, int node)
>  #endif
>  }
>
> +static void memcg_charge_kernel_stack(struct task_struct *tsk)
> +{
> +#ifdef CONFIG_VMAP_STACK
> +       struct vm_struct *vm = task_stack_vm_area(tsk);
> +
> +       if (vm) {
> +               int i;
> +
> +               for (i = 0; i < THREAD_SIZE / PAGE_SIZE; i++)
> +                       memcg_kmem_charge(vm->pages[i], __GFP_NOFAIL,
> +                                         compound_order(vm->pages[i]));
> +
> +               /* All stack pages belong to the same memcg. */
> +               mod_memcg_page_state(vm->pages[0], MEMCG_KERNEL_STACK_KB,
> +                                    THREAD_SIZE / 1024);
> +       }
> +#endif
> +}
> +
>  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;
>
> +               /* All stack pages belong to the same memcg. */
> +               mod_memcg_page_state(vm->pages[0], MEMCG_KERNEL_STACK_KB,
> +                                    -(int)(THREAD_SIZE / 1024));
> +
> +               for (i = 0; i < THREAD_SIZE / PAGE_SIZE; i++)
> +                       memcg_kmem_uncharge(vm->pages[i],
> +                                         compound_order(vm->pages[i]));
> +
>                 for (i = 0; i < NR_CACHED_STACKS; i++) {
>                         if (this_cpu_cmpxchg(cached_stacks[i],
>                                         NULL, tsk->stack_vm_area) != NULL)
> @@ -352,10 +386,6 @@ static void account_kernel_stack(struct task_struct *tsk, int account)
>                                             NR_KERNEL_STACK_KB,
>                                             PAGE_SIZE / 1024 * account);
>                 }
> -
> -               /* All stack pages belong to the same memcg. */
> -               mod_memcg_page_state(vm->pages[0], MEMCG_KERNEL_STACK_KB,
> -                                    account * (THREAD_SIZE / 1024));
>         } else {
>                 /*
>                  * All stack pages are in the same zone and belong to the
> @@ -809,6 +839,8 @@ static struct task_struct *dup_task_struct(struct task_struct *orig, int node)
>         if (!stack)
>                 goto free_tsk;
>
> +       memcg_charge_kernel_stack(tsk);
> +
>         stack_vm_area = task_stack_vm_area(tsk);
>
>         err = arch_dup_task_struct(tsk, orig);
> --
> 2.14.4
>