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Date:   Fri, 5 Mar 2021 11:48:49 -0800
From:   Roman Gushchin <guro@fb.com>
To:     Muchun Song <songmuchun@bytedance.com>
CC:     <hannes@cmpxchg.org>, <mhocko@kernel.org>,
        <akpm@linux-foundation.org>, <shakeelb@google.com>,
        <vdavydov.dev@gmail.com>, <linux-kernel@vger.kernel.org>,
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Subject: Re: [PATCH v2 4/5] mm: memcontrol: introduce remote objcg charging
 API
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On Wed, Mar 03, 2021 at 01:59:16PM +0800, Muchun Song wrote:
> The remote memcg charing APIs is a mechanism to charge pages to a given
> memcg. Since all kernel memory are charged by using obj_cgroup APIs.
> Actually, we want to charge kernel memory to the remote object cgroup
> instead of memory cgroup. So introduce remote objcg charging APIs to
> charge the kmem pages by using objcg_cgroup APIs. And if remote memcg
> and objcg are both set, objcg takes precedence over memcg to charge
> the kmem pages.
> 
> In the later patch, we will use those API to charge kernel memory to
> the remote objcg.

I'd abandon/postpone the rest of the patchset (patches 4 and 5) as now.
They add a lot of new code to solve a theoretical problem (please, fix
me if I'm wrong), which is not a panic or data corruption, but
a sub-optimal garbage collection behavior. I think we need a better
motivation or/and an implementation which makes the code simpler
and smaller.

> 
> Signed-off-by: Muchun Song <songmuchun@bytedance.com>
> ---
>  include/linux/sched.h    |  4 ++++
>  include/linux/sched/mm.h | 38 ++++++++++++++++++++++++++++++++++++++
>  kernel/fork.c            |  3 +++
>  mm/memcontrol.c          | 44 ++++++++++++++++++++++++++++++++++++++++----
>  4 files changed, 85 insertions(+), 4 deletions(-)
> 
> diff --git a/include/linux/sched.h b/include/linux/sched.h
> index ee46f5cab95b..8edcc71a0a1d 100644
> --- a/include/linux/sched.h
> +++ b/include/linux/sched.h
> @@ -1318,6 +1318,10 @@ struct task_struct {
>  	/* Used by memcontrol for targeted memcg charge: */
>  	struct mem_cgroup		*active_memcg;
>  #endif
> +#ifdef CONFIG_MEMCG_KMEM
> +	/* Used by memcontrol for targeted objcg charge: */
> +	struct obj_cgroup		*active_objcg;
> +#endif
>  
>  #ifdef CONFIG_BLK_CGROUP
>  	struct request_queue		*throttle_queue;
> diff --git a/include/linux/sched/mm.h b/include/linux/sched/mm.h
> index 1ae08b8462a4..be1189598b09 100644
> --- a/include/linux/sched/mm.h
> +++ b/include/linux/sched/mm.h
> @@ -330,6 +330,44 @@ set_active_memcg(struct mem_cgroup *memcg)
>  }
>  #endif
>  
> +#ifdef CONFIG_MEMCG_KMEM
> +DECLARE_PER_CPU(struct obj_cgroup *, int_active_objcg);
> +
> +/**
> + * set_active_objcg - Starts the remote objcg kmem pages charging scope.
> + * @objcg: objcg to charge.
> + *
> + * This function marks the beginning of the remote objcg charging scope. All the
> + * __GFP_ACCOUNT kmem page allocations till the end of the scope will be charged
> + * to the given objcg.
> + *
> + * NOTE: This function can nest. Users must save the return value and
> + * reset the previous value after their own charging scope is over.
> + *
> + * If remote memcg and objcg are both set, objcg takes precedence over memcg
> + * to charge the kmem pages.
> + */
> +static inline struct obj_cgroup *set_active_objcg(struct obj_cgroup *objcg)
> +{
> +	struct obj_cgroup *old;
> +
> +	if (in_interrupt()) {
> +		old = this_cpu_read(int_active_objcg);
> +		this_cpu_write(int_active_objcg, objcg);
> +	} else {
> +		old = current->active_objcg;
> +		current->active_objcg = objcg;
> +	}
> +
> +	return old;
> +}
> +#else
> +static inline struct obj_cgroup *set_active_objcg(struct obj_cgroup *objcg)
> +{
> +	return NULL;
> +}
> +#endif
> +
>  #ifdef CONFIG_MEMBARRIER
>  enum {
>  	MEMBARRIER_STATE_PRIVATE_EXPEDITED_READY		= (1U << 0),
> diff --git a/kernel/fork.c b/kernel/fork.c
> index d66cd1014211..b4b9dd5d122f 100644
> --- a/kernel/fork.c
> +++ b/kernel/fork.c
> @@ -945,6 +945,9 @@ static struct task_struct *dup_task_struct(struct task_struct *orig, int node)
>  #ifdef CONFIG_MEMCG
>  	tsk->active_memcg = NULL;
>  #endif
> +#ifdef CONFIG_MEMCG_KMEM
> +	tsk->active_objcg = NULL;
> +#endif
>  	return tsk;
>  
>  free_stack:
> diff --git a/mm/memcontrol.c b/mm/memcontrol.c
> index 0cf342d22547..e48d4ab0af76 100644
> --- a/mm/memcontrol.c
> +++ b/mm/memcontrol.c
> @@ -79,6 +79,11 @@ struct mem_cgroup *root_mem_cgroup __read_mostly;
>  /* Active memory cgroup to use from an interrupt context */
>  DEFINE_PER_CPU(struct mem_cgroup *, int_active_memcg);
>  
> +#ifdef CONFIG_MEMCG_KMEM
> +/* Active object cgroup to use from an interrupt context */
> +DEFINE_PER_CPU(struct obj_cgroup *, int_active_objcg);
> +#endif
> +
>  /* Socket memory accounting disabled? */
>  static bool cgroup_memory_nosocket;
>  
> @@ -1076,7 +1081,7 @@ static __always_inline struct mem_cgroup *get_active_memcg(void)
>  	return memcg;
>  }
>  
> -static __always_inline bool memcg_kmem_bypass(void)
> +static __always_inline bool memcg_charge_bypass(void)
>  {
>  	/* Allow remote memcg charging from any context. */
>  	if (unlikely(active_memcg()))
> @@ -1094,7 +1099,7 @@ static __always_inline bool memcg_kmem_bypass(void)
>   */
>  static __always_inline struct mem_cgroup *get_mem_cgroup_from_current(void)
>  {
> -	if (memcg_kmem_bypass())
> +	if (memcg_charge_bypass())
>  		return NULL;
>  
>  	if (unlikely(active_memcg()))
> @@ -1103,6 +1108,29 @@ static __always_inline struct mem_cgroup *get_mem_cgroup_from_current(void)
>  	return get_mem_cgroup_from_mm(current->mm);
>  }
>  
> +#ifdef CONFIG_MEMCG_KMEM
> +static __always_inline struct obj_cgroup *active_objcg(void)
> +{
> +	if (in_interrupt())
> +		return this_cpu_read(int_active_objcg);
> +	else
> +		return current->active_objcg;
> +}
> +
> +static __always_inline bool kmem_charge_bypass(void)
> +{
> +	/* Allow remote charging from any context. */
> +	if (unlikely(active_objcg() || active_memcg()))
> +		return false;
> +
> +	/* Memcg to charge can't be determined. */
> +	if (in_interrupt() || !current->mm || (current->flags & PF_KTHREAD))
> +		return true;
> +
> +	return false;
> +}
> +#endif
> +
>  /**
>   * mem_cgroup_iter - iterate over memory cgroup hierarchy
>   * @root: hierarchy root
> @@ -2997,13 +3025,20 @@ struct mem_cgroup *mem_cgroup_from_obj(void *p)
>  
>  __always_inline struct obj_cgroup *get_obj_cgroup_from_current(void)
>  {
> -	struct obj_cgroup *objcg = NULL;
> +	struct obj_cgroup *objcg;
>  	struct mem_cgroup *memcg;
>  
> -	if (memcg_kmem_bypass())
> +	if (kmem_charge_bypass())
>  		return NULL;
>  
>  	rcu_read_lock();
> +	objcg = active_objcg();
> +	if (unlikely(objcg)) {
> +		/* remote object cgroup must hold a reference. */
> +		obj_cgroup_get(objcg);
> +		goto out;
> +	}
> +
>  	if (unlikely(active_memcg()))
>  		memcg = active_memcg();
>  	else
> @@ -3015,6 +3050,7 @@ __always_inline struct obj_cgroup *get_obj_cgroup_from_current(void)
>  			break;
>  		objcg = NULL;
>  	}
> +out:
>  	rcu_read_unlock();
>  
>  	return objcg;
> -- 
> 2.11.0
>