Re: [RFC][PATCH] memcg: Replace mm->owner with mm->memcg

From: Balbir Singh
Date: Wed May 02 2018 - 20:00:15 EST

On Tue, 01 May 2018 12:35:16 -0500
ebiederm@xxxxxxxxxxxx (Eric W. Biederman) wrote:

> Recently it was reported that mm_update_next_owner could get into
> cases where it was executing it's fallback for_each_process part of
> the loop and thus taking up a lot of time.
>
> To deal with this replace mm->owner with mm->memcg. This just reduces
> the complexity of everything. As much as possible I have maintained
> the current semantics. There are two siginificant exceptions. During
> fork the memcg of the process calling fork is charged rather than
> init_css_set. During memory cgroup migration the charges are migrated
> not if the process is the owner of the mm, but if the process being
> migrated has the same memory cgroup as the mm.
>
> I believe it was a bug if init_css_set is charged for memory activity
> during fork, and the old behavior was simply a consequence of the new
> task not having tsk->cgroup not initialized to it's proper cgroup.

That does sound like a bug, I guess we've not seen it because we did
not track any slab allocations initially.

>
> Durhing cgroup migration only thread group leaders are allowed to
> migrate. Which means in practice there should only be one. Linux
> tasks created with CLONE_VM are the only exception, but the common
> cases are already ruled out. Processes created with vfork have a
> suspended parent and can do nothing but call exec so they should never
> show up. Threads of the same cgroup are not the thread group leader
> so also should not show up. That leaves the old LinuxThreads library
> which is probably out of use by now, and someone doing something very
> creative with cgroups, and rolling their own threads with CLONE_VM.
> So in practice I don't think the difference charge migration will
> affect anyone.
>
> To ensure that mm->memcg is updated appropriately I have implemented
> cgroup "attach" and "fork" methods. This ensures that at those
> points the mm pointed to the task has the appropriate memory cgroup.
>
> For simplicity instead of introducing a new mm lock I simply use
> exchange on the pointer where the mm->memcg is updated to get
> atomic updates.
>
> Looking at the history effectively this change is a revert. The
> reason given for adding mm->owner is so that multiple cgroups can be
> attached to the same mm. In the last 8 years a second user of
> mm->owner has not appeared. A feature that has never used, makes the
> code more complicated and has horrible worst case performance should
> go.

The idea was to track the mm to the right cgroup, we did find that
the mm could be confused as belonging to two cgroups. tsk->cgroup is
not sufficient and if when the tgid left, we needed an owner to track
where the current allocations were. But this is from 8 year old history,
I don't have my notes anymore :)

>
> Fixes: cf475ad28ac3 ("cgroups: add an owner to the mm_struct")
> Reported-by: Kirill Tkhai <ktkhai@xxxxxxxxxxxxx>
> Signed-off-by: "Eric W. Biederman" <ebiederm@xxxxxxxxxxxx>
> ---
> fs/exec.c | 1 -
> include/linux/memcontrol.h | 11 ++++--
> include/linux/mm_types.h | 12 +------
> include/linux/sched/mm.h | 8 -----
> kernel/exit.c | 89 ----------------------------------------------
> kernel/fork.c | 17 +++++++--
> mm/memcontrol.c | 86 ++++++++++++++++++++++++++++++++++----------
> 7 files changed, 90 insertions(+), 134 deletions(-)
>
> diff --git a/fs/exec.c b/fs/exec.c
> index 183059c427b9..a8be9318d1a8 100644
> --- a/fs/exec.c
> +++ b/fs/exec.c
> @@ -1040,7 +1040,6 @@ static int exec_mmap(struct mm_struct *mm)
> up_read(&old_mm->mmap_sem);
> BUG_ON(active_mm != old_mm);
> setmax_mm_hiwater_rss(&tsk->signal->maxrss, old_mm);
> - mm_update_next_owner(old_mm);
> mmput(old_mm);
> return 0;
> }
> diff --git a/include/linux/memcontrol.h b/include/linux/memcontrol.h
> index d99b71bc2c66..147e04bfcaee 100644
> --- a/include/linux/memcontrol.h
> +++ b/include/linux/memcontrol.h
> @@ -341,7 +341,6 @@ static inline struct lruvec *mem_cgroup_lruvec(struct pglist_data *pgdat,
> struct lruvec *mem_cgroup_page_lruvec(struct page *, struct pglist_data *);
>
> bool task_in_mem_cgroup(struct task_struct *task, struct mem_cgroup *memcg);
> -struct mem_cgroup *mem_cgroup_from_task(struct task_struct *p);
>
> static inline
> struct mem_cgroup *mem_cgroup_from_css(struct cgroup_subsys_state *css){
> @@ -402,6 +401,8 @@ static inline bool mem_cgroup_is_descendant(struct mem_cgroup *memcg,
> return cgroup_is_descendant(memcg->css.cgroup, root->css.cgroup);
> }
>
> +void mm_update_memcg(struct mm_struct *mm, struct mem_cgroup *new);
> +
> static inline bool mm_match_cgroup(struct mm_struct *mm,
> struct mem_cgroup *memcg)
> {
> @@ -409,7 +410,7 @@ static inline bool mm_match_cgroup(struct mm_struct *mm,
> bool match = false;
>
> rcu_read_lock();
> - task_memcg = mem_cgroup_from_task(rcu_dereference(mm->owner));
> + task_memcg = rcu_dereference(mm->memcg);
> if (task_memcg)
> match = mem_cgroup_is_descendant(task_memcg, memcg);
> rcu_read_unlock();
> @@ -693,7 +694,7 @@ static inline void count_memcg_event_mm(struct mm_struct *mm,
> return;
>
> rcu_read_lock();
> - memcg = mem_cgroup_from_task(rcu_dereference(mm->owner));
> + memcg = rcu_dereference(mm->memcg);
> if (likely(memcg)) {
> count_memcg_events(memcg, idx, 1);
> if (idx == OOM_KILL)
> @@ -781,6 +782,10 @@ static inline struct lruvec *mem_cgroup_page_lruvec(struct page *page,
> return &pgdat->lruvec;
> }
>
> +static inline void mm_update_memcg(struct mm_struct *mm, struct mem_cgroup *new)
> +{
> +}
> +
> static inline bool mm_match_cgroup(struct mm_struct *mm,
> struct mem_cgroup *memcg)
> {
> diff --git a/include/linux/mm_types.h b/include/linux/mm_types.h
> index 21612347d311..ea5efd40a5d1 100644
> --- a/include/linux/mm_types.h
> +++ b/include/linux/mm_types.h
> @@ -443,17 +443,7 @@ struct mm_struct {
> struct kioctx_table __rcu *ioctx_table;
> #endif
> #ifdef CONFIG_MEMCG
> - /*
> - * "owner" points to a task that is regarded as the canonical
> - * user/owner of this mm. All of the following must be true in
> - * order for it to be changed:
> - *
> - * current == mm->owner
> - * current->mm != mm
> - * new_owner->mm == mm
> - * new_owner->alloc_lock is held
> - */
> - struct task_struct __rcu *owner;
> + struct mem_cgroup __rcu *memcg;
> #endif
> struct user_namespace *user_ns;
>
> diff --git a/include/linux/sched/mm.h b/include/linux/sched/mm.h
> index 2c570cd934af..cc8e68d36fc2 100644
> --- a/include/linux/sched/mm.h
> +++ b/include/linux/sched/mm.h
> @@ -95,14 +95,6 @@ extern struct mm_struct *mm_access(struct task_struct *task, unsigned int mode);
> /* Remove the current tasks stale references to the old mm_struct */
> extern void mm_release(struct task_struct *, struct mm_struct *);
>
> -#ifdef CONFIG_MEMCG
> -extern void mm_update_next_owner(struct mm_struct *mm);
> -#else
> -static inline void mm_update_next_owner(struct mm_struct *mm)
> -{
> -}
> -#endif /* CONFIG_MEMCG */
> -
> #ifdef CONFIG_MMU
> extern void arch_pick_mmap_layout(struct mm_struct *mm,
> struct rlimit *rlim_stack);
> diff --git a/kernel/exit.c b/kernel/exit.c
> index c3c7ac560114..be967d2da0ce 100644
> --- a/kernel/exit.c
> +++ b/kernel/exit.c
> @@ -399,94 +399,6 @@ kill_orphaned_pgrp(struct task_struct *tsk, struct task_struct *parent)
> }
> }
>
> -#ifdef CONFIG_MEMCG
> -/*
> - * A task is exiting. If it owned this mm, find a new owner for the mm.
> - */
> -void mm_update_next_owner(struct mm_struct *mm)
> -{
> - struct task_struct *c, *g, *p = current;
> -
> -retry:
> - /*
> - * If the exiting or execing task is not the owner, it's
> - * someone else's problem.
> - */
> - if (mm->owner != p)
> - return;
> - /*
> - * The current owner is exiting/execing and there are no other
> - * candidates. Do not leave the mm pointing to a possibly
> - * freed task structure.
> - */
> - if (atomic_read(&mm->mm_users) <= 1) {
> - mm->owner = NULL;
> - return;
> - }
> -
> - read_lock(&tasklist_lock);
> - /*
> - * Search in the children
> - */
> - list_for_each_entry(c, &p->children, sibling) {
> - if (c->mm == mm)
> - goto assign_new_owner;
> - }
> -
> - /*
> - * Search in the siblings
> - */
> - list_for_each_entry(c, &p->real_parent->children, sibling) {
> - if (c->mm == mm)
> - goto assign_new_owner;
> - }
> -
> - /*
> - * Search through everything else, we should not get here often.
> - */
> - for_each_process(g) {
> - if (g->flags & PF_KTHREAD)
> - continue;
> - for_each_thread(g, c) {
> - if (c->mm == mm)
> - goto assign_new_owner;
> - if (c->mm)
> - break;
> - }
> - }
> - read_unlock(&tasklist_lock);
> - /*
> - * We found no owner yet mm_users > 1: this implies that we are
> - * most likely racing with swapoff (try_to_unuse()) or /proc or
> - * ptrace or page migration (get_task_mm()). Mark owner as NULL.
> - */
> - mm->owner = NULL;
> - return;
> -
> -assign_new_owner:
> - BUG_ON(c == p);
> - get_task_struct(c);
> - /*
> - * The task_lock protects c->mm from changing.
> - * We always want mm->owner->mm == mm
> - */
> - task_lock(c);
> - /*
> - * Delay read_unlock() till we have the task_lock()
> - * to ensure that c does not slip away underneath us
> - */
> - read_unlock(&tasklist_lock);
> - if (c->mm != mm) {
> - task_unlock(c);
> - put_task_struct(c);
> - goto retry;
> - }
> - mm->owner = c;
> - task_unlock(c);
> - put_task_struct(c);
> -}
> -#endif /* CONFIG_MEMCG */
> -
> /*
> * Turn us into a lazy TLB process if we
> * aren't already..
> @@ -540,7 +452,6 @@ static void exit_mm(void)
> up_read(&mm->mmap_sem);
> enter_lazy_tlb(mm, current);
> task_unlock(current);
> - mm_update_next_owner(mm);
> mmput(mm);
> if (test_thread_flag(TIF_MEMDIE))
> exit_oom_victim();
> diff --git a/kernel/fork.c b/kernel/fork.c
> index a5d21c42acfc..f284acf22aad 100644
> --- a/kernel/fork.c
> +++ b/kernel/fork.c
> @@ -868,10 +868,19 @@ static void mm_init_aio(struct mm_struct *mm)
> #endif
> }
>
> -static void mm_init_owner(struct mm_struct *mm, struct task_struct *p)
> +static void mm_init_memcg(struct mm_struct *mm)
> {
> #ifdef CONFIG_MEMCG
> - mm->owner = p;
> + struct cgroup_subsys_state *css;
> +
> + /* Ensure mm->memcg is initialized */
> + mm->memcg = NULL;
> +
> + rcu_read_lock();
> + css = task_css(current, memory_cgrp_id);
> + if (css && css_tryget(css))
> + mm_update_memcg(mm, mem_cgroup_from_css(css));
> + rcu_read_unlock();
> #endif
> }
>
> @@ -901,7 +910,7 @@ static struct mm_struct *mm_init(struct mm_struct *mm, struct task_struct *p,
> spin_lock_init(&mm->page_table_lock);
> mm_init_cpumask(mm);
> mm_init_aio(mm);
> - mm_init_owner(mm, p);
> + mm_init_memcg(mm);
> RCU_INIT_POINTER(mm->exe_file, NULL);
> mmu_notifier_mm_init(mm);
> hmm_mm_init(mm);
> @@ -931,6 +940,7 @@ static struct mm_struct *mm_init(struct mm_struct *mm, struct task_struct *p,
> fail_nocontext:
> mm_free_pgd(mm);
> fail_nopgd:
> + mm_update_memcg(mm, NULL);
> free_mm(mm);
> return NULL;
> }
> @@ -968,6 +978,7 @@ static inline void __mmput(struct mm_struct *mm)
> }
> if (mm->binfmt)
> module_put(mm->binfmt->module);
> + mm_update_memcg(mm, NULL);
> mmdrop(mm);
> }
>
> diff --git a/mm/memcontrol.c b/mm/memcontrol.c
> index 2bd3df3d101a..5dce8a7fa65b 100644
> --- a/mm/memcontrol.c
> +++ b/mm/memcontrol.c
> @@ -664,20 +664,6 @@ static void memcg_check_events(struct mem_cgroup *memcg, struct page *page)
> }
> }
>
> -struct mem_cgroup *mem_cgroup_from_task(struct task_struct *p)
> -{
> - /*
> - * mm_update_next_owner() may clear mm->owner to NULL
> - * if it races with swapoff, page migration, etc.
> - * So this can be called with p == NULL.
> - */
> - if (unlikely(!p))
> - return NULL;
> -
> - return mem_cgroup_from_css(task_css(p, memory_cgrp_id));
> -}
> -EXPORT_SYMBOL(mem_cgroup_from_task);
> -
> static struct mem_cgroup *get_mem_cgroup_from_mm(struct mm_struct *mm)
> {
> struct mem_cgroup *memcg = NULL;
> @@ -692,7 +678,7 @@ static struct mem_cgroup *get_mem_cgroup_from_mm(struct mm_struct *mm)
> if (unlikely(!mm))
> memcg = root_mem_cgroup;
> else {
> - memcg = mem_cgroup_from_task(rcu_dereference(mm->owner));
> + memcg = rcu_dereference(mm->memcg);
> if (unlikely(!memcg))
> memcg = root_mem_cgroup;
> }
> @@ -1011,7 +997,7 @@ bool task_in_mem_cgroup(struct task_struct *task, struct mem_cgroup *memcg)
> * killed to prevent needlessly killing additional tasks.
> */
> rcu_read_lock();
> - task_memcg = mem_cgroup_from_task(task);
> + task_memcg = mem_cgroup_from_css(task_css(task, memory_cgrp_id));
> css_get(&task_memcg->css);
> rcu_read_unlock();
> }
> @@ -4827,15 +4813,16 @@ static int mem_cgroup_can_attach(struct cgroup_taskset *tset)
> if (!move_flags)
> return 0;
>
> - from = mem_cgroup_from_task(p);
> + from = mem_cgroup_from_css(task_css(p, memory_cgrp_id));
>
> VM_BUG_ON(from == memcg);
>
> mm = get_task_mm(p);
> if (!mm)
> return 0;
> +
> /* We move charges only when we move a owner of the mm */
> - if (mm->owner == p) {
> + if (mm->memcg == from) {
> VM_BUG_ON(mc.from);
> VM_BUG_ON(mc.to);
> VM_BUG_ON(mc.precharge);
> @@ -4859,6 +4846,59 @@ static int mem_cgroup_can_attach(struct cgroup_taskset *tset)
> return ret;
> }
>
> +/**
> + * mm_update_memcg - Update the memory cgroup of a mm_struct
> + * @mm: mm struct
> + * @new: new memory cgroup value
> + *
> + * Called whenever mm->memcg needs to change. Consumes a reference
> + * to new (unless new is NULL). The reference to the old memory
> + * cgroup is decreased.
> + */
> +void mm_update_memcg(struct mm_struct *mm, struct mem_cgroup *new)
> +{
> + /* This is the only place where mm->memcg is changed */
> + struct mem_cgroup *old;
> +
> + old = xchg(&mm->memcg, new);
> + if (old)
> + css_put(&old->css);
> +}
> +
> +static void task_update_memcg(struct task_struct *tsk, struct mem_cgroup *new)
> +{
> + struct mm_struct *mm;
> + task_lock(tsk);
> + mm = tsk->mm;
> + if (mm && !(tsk->flags & PF_KTHREAD))
> + mm_update_memcg(mm, new);
> + task_unlock(tsk);
> +}
> +
> +static void mem_cgroup_attach(struct cgroup_taskset *tset)
> +{
> + struct cgroup_subsys_state *css;
> + struct task_struct *tsk;
> +
> + cgroup_taskset_for_each(tsk, css, tset) {
> + struct mem_cgroup *new = mem_cgroup_from_css(css);
> + css_get(css);
> + task_update_memcg(tsk, new);

I'd have to go back and check and I think your comment refers to this,
but we don't expect non tgid tasks to show up here? My concern is I can't
find the guaratee that task_update_memcg(tsk, new) is not

1. Duplicated for each thread in the process or attached to the mm
2. Do not update mm->memcg to point to different places, so the one
that sticks is the one that updated things last.


Balbir Singh