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Date:   Wed, 15 Aug 2018 10:16:05 -0700
From:   Roman Gushchin <guro@fb.com>
To:     Shakeel Butt <shakeelb@google.com>
CC:     Linux MM <linux-mm@kvack.org>, LKML <linux-kernel@vger.kernel.org>,
        <kernel-team@fb.com>, Johannes Weiner <hannes@cmpxchg.org>,
        Michal Hocko <mhocko@kernel.org>, <luto@kernel.org>,
        <koct9i@gmail.com>, Tejun Heo <tj@kernel.org>
Subject: Re: [RFC PATCH 1/2] mm: rework memcg kernel stack accounting
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On Tue, Aug 14, 2018 at 06:18:01PM -0700, Shakeel Butt wrote:
> 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.

I also thought about having per-memcg stack cache, but it seems
that caching 2 * n(cpus) * n(cgroups) stacks is an overkill,
and there is nothing memcg-specific in these stacks except
that they are pre-charged.

> 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").

Sure, will do in v2.

> 
> Reviewed-by: Shakeel Butt <shakeelb@google.com>

Thanks!

> 
> > 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
> >