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Date: Wed, 15 Aug 2018 10:16:05 -0700
From: Roman Gushchin <guro@...com>
To: Shakeel Butt <shakeelb@...gle.com>
CC: Linux MM <linux-mm@...ck.org>, LKML <linux-kernel@...r.kernel.org>,
<kernel-team@...com>, Johannes Weiner <hannes@...xchg.org>,
Michal Hocko <mhocko@...nel.org>, <luto@...nel.org>,
<koct9i@...il.com>, Tejun Heo <tj@...nel.org>
Subject: Re: [RFC PATCH 1/2] mm: rework memcg kernel stack accounting
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@...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@...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@...gle.com>
Thanks!
>
> > Cc: Johannes Weiner <hannes@...xchg.org>
> > Cc: Michal Hocko <mhocko@...nel.org>
> > Cc: Andy Lutomirski <luto@...nel.org>
> > Cc: Konstantin Khlebnikov <koct9i@...il.com>
> > Cc: Tejun Heo <tj@...nel.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
> >
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