Re: [PATCH v3 06/13] memcg: kmem controller infrastructure

[Michal Hocko]

On Tue 18-09-12 18:04:03, Glauber Costa wrote:
> This patch introduces infrastructure for tracking kernel memory pages to
> a given memcg. This will happen whenever the caller includes the flag
> __GFP_KMEMCG flag, and the task belong to a memcg other than the root.
> 
> In memcontrol.h those functions are wrapped in inline acessors.  The
> idea is to later on, patch those with static branches, so we don't incur
> any overhead when no mem cgroups with limited kmem are being used.

Could you describe the API a bit here, please? I guess the
kernel user is supposed to call memcg_kmem_newpage_charge and
memcg_kmem_commit_charge resp. memcg_kmem_uncharge_page.
All other kmem functions here are just helpers, right?
> 
> [ v2: improved comments and standardized function names ]
> [ v3: handle no longer opaque, functions not exported,
>   even more comments ]
> [ v4: reworked Used bit handling and surroundings for more clarity ]
> 
> Signed-off-by: Glauber Costa <glommer@xxxxxxxxxxxxx>
> CC: Christoph Lameter <cl@xxxxxxxxx>
> CC: Pekka Enberg <penberg@xxxxxxxxxxxxxx>
> CC: Michal Hocko <mhocko@xxxxxxx>
> CC: Kamezawa Hiroyuki <kamezawa.hiroyu@xxxxxxxxxxxxxx>
> CC: Johannes Weiner <hannes@xxxxxxxxxxx>
> ---
>  include/linux/memcontrol.h |  97 +++++++++++++++++++++++++
>  mm/memcontrol.c            | 177 +++++++++++++++++++++++++++++++++++++++++++++
>  2 files changed, 274 insertions(+)
> 
> diff --git a/include/linux/memcontrol.h b/include/linux/memcontrol.h
> index 8d9489f..82ede9a 100644
> --- a/include/linux/memcontrol.h
> +++ b/include/linux/memcontrol.h
> @@ -21,6 +21,7 @@
>  #define _LINUX_MEMCONTROL_H
>  #include <linux/cgroup.h>
>  #include <linux/vm_event_item.h>
> +#include <linux/hardirq.h>
>
>  struct mem_cgroup;
>  struct page_cgroup;
> @@ -399,6 +400,17 @@ struct sock;
>  #ifdef CONFIG_MEMCG_KMEM
>  void sock_update_memcg(struct sock *sk);
>  void sock_release_memcg(struct sock *sk);
> +
> +static inline bool memcg_kmem_enabled(void)
> +{
> +	return true;
> +}
> +
> +extern bool __memcg_kmem_newpage_charge(gfp_t gfp, struct mem_cgroup **memcg,
> +					int order);
> +extern void __memcg_kmem_commit_charge(struct page *page,
> +				       struct mem_cgroup *memcg, int order);
> +extern void __memcg_kmem_uncharge_page(struct page *page, int order);
>  #else
>  static inline void sock_update_memcg(struct sock *sk)
>  {
> @@ -406,6 +418,91 @@ static inline void sock_update_memcg(struct sock *sk)
>  static inline void sock_release_memcg(struct sock *sk)
>  {
>  }
> +
> +static inline bool memcg_kmem_enabled(void)
> +{
> +	return false;
> +}
> +
> +static inline bool
> +__memcg_kmem_newpage_charge(gfp_t gfp, struct mem_cgroup **memcg, int order)
> +{
> +	return false;
> +}
> +
> +static inline void  __memcg_kmem_uncharge_page(struct page *page, int order)
> +{
> +}
> +
> +static inline void
> +__memcg_kmem_commit_charge(struct page *page, struct mem_cgroup *memcg, int order)
> +{
> +}

I think we shouldn't care about these for !MEMCG_KMEM. It should be
sufficient to define the main three functions bellow as return true
resp. NOOP. This would reduce the code churn a bit and also make it
better maintainable.

>  #endif /* CONFIG_MEMCG_KMEM */
> +
> +/**
> + * memcg_kmem_newpage_charge: verify if a new kmem allocation is allowed.
> + * @gfp: the gfp allocation flags.
> + * @memcg: a pointer to the memcg this was charged against.
> + * @order: allocation order.
> + *
> + * returns true if the memcg where the current task belongs can hold this
> + * allocation.
> + *
> + * We return true automatically if this allocation is not to be accounted to
> + * any memcg.
> + */
> +static __always_inline bool
> +memcg_kmem_newpage_charge(gfp_t gfp, struct mem_cgroup **memcg, int order)
> +{
> +	if (!memcg_kmem_enabled())
> +		return true;
> +
> +	/*
> +	 * __GFP_NOFAIL allocations will move on even if charging is not
> +	 * possible. Therefore we don't even try, and have this allocation
> +	 * unaccounted. We could in theory charge it with
> +	 * res_counter_charge_nofail, but we hope those allocations are rare,
> +	 * and won't be worth the trouble.
> +	 */
> +	if (!(gfp & __GFP_KMEMCG) || (gfp & __GFP_NOFAIL))
> +		return true;
> +	if (in_interrupt() || (!current->mm) || (current->flags & PF_KTHREAD))
> +		return true;
> +	return __memcg_kmem_newpage_charge(gfp, memcg, order);
> +}
> +
> +/**
> + * memcg_kmem_uncharge_page: uncharge pages from memcg
> + * @page: pointer to struct page being freed
> + * @order: allocation order.
> + *
> + * there is no need to specify memcg here, since it is embedded in page_cgroup
> + */
> +static __always_inline void
> +memcg_kmem_uncharge_page(struct page *page, int order)
> +{
> +	if (memcg_kmem_enabled())
> +		__memcg_kmem_uncharge_page(page, order);
> +}
> +
> +/**
> + * memcg_kmem_commit_charge: embeds correct memcg in a page
> + * @memcg: a pointer to the memcg this was charged against.
      ^^^^^^^
remove this one?

> + * @page: pointer to struct page recently allocated
> + * @memcg: the memcg structure we charged against
> + * @order: allocation order.
> + *
> + * Needs to be called after memcg_kmem_newpage_charge, regardless of success or
> + * failure of the allocation. if @page is NULL, this function will revert the
> + * charges. Otherwise, it will commit the memcg given by @memcg to the
> + * corresponding page_cgroup.
> + */
> +static __always_inline void
> +memcg_kmem_commit_charge(struct page *page, struct mem_cgroup *memcg, int order)
> +{
> +	if (memcg_kmem_enabled() && memcg)
> +		__memcg_kmem_commit_charge(page, memcg, order);
> +}
>  #endif /* _LINUX_MEMCONTROL_H */
>  
> diff --git a/mm/memcontrol.c b/mm/memcontrol.c
> index f3fd354..0f36a01 100644
> --- a/mm/memcontrol.c
> +++ b/mm/memcontrol.c
> @@ -10,6 +10,10 @@
>   * Copyright (C) 2009 Nokia Corporation
>   * Author: Kirill A. Shutemov
>   *
> + * Kernel Memory Controller
> + * Copyright (C) 2012 Parallels Inc. and Google Inc.
> + * Authors: Glauber Costa and Suleiman Souhlal
> + *
>   * This program is free software; you can redistribute it and/or modify
>   * it under the terms of the GNU General Public License as published by
>   * the Free Software Foundation; either version 2 of the License, or
> @@ -426,6 +430,9 @@ struct mem_cgroup *mem_cgroup_from_css(struct cgroup_subsys_state *s)
>  #include <net/ip.h>
>  
>  static bool mem_cgroup_is_root(struct mem_cgroup *memcg);
> +static int memcg_charge_kmem(struct mem_cgroup *memcg, gfp_t gfp, u64 size);
> +static void memcg_uncharge_kmem(struct mem_cgroup *memcg, u64 size);
> +

Why the forward declarations here? We can simply move definitions up
before they are used for the first time, can't we? Besides that they are
never used/defined from outside of KMEM_MEMCG.

>  void sock_update_memcg(struct sock *sk)
>  {
>  	if (mem_cgroup_sockets_enabled) {
> @@ -480,6 +487,110 @@ struct cg_proto *tcp_proto_cgroup(struct mem_cgroup *memcg)
>  }
>  EXPORT_SYMBOL(tcp_proto_cgroup);
>  #endif /* CONFIG_INET */
> +
> +static inline bool memcg_can_account_kmem(struct mem_cgroup *memcg)
> +{
> +	return !mem_cgroup_disabled() && !mem_cgroup_is_root(memcg) &&
> +		memcg->kmem_accounted;
> +}
> +
> +/*
> + * We need to verify if the allocation against current->mm->owner's memcg is
> + * possible for the given order. But the page is not allocated yet, so we'll
> + * need a further commit step to do the final arrangements.
> + *
> + * It is possible for the task to switch cgroups in this mean time, so at
> + * commit time, we can't rely on task conversion any longer.  We'll then use
> + * the handle argument to return to the caller which cgroup we should commit
> + * against. We could also return the memcg directly and avoid the pointer
> + * passing, but a boolean return value gives better semantics considering
> + * the compiled-out case as well.
> + *
> + * Returning true means the allocation is possible.
> + */
> +bool
> +__memcg_kmem_newpage_charge(gfp_t gfp, struct mem_cgroup **_memcg, int order)
> +{
> +	struct mem_cgroup *memcg;
> +	bool ret;
> +	struct task_struct *p;

Johannes likes christmas trees ;) and /me would like to remove `p' and
use mem_cgroup_from_task(rcu_dereference(current->mm->owner)) same as we
do at other places (I guess it will be checkpatch safe).

> +
> +	*_memcg = NULL;
> +	rcu_read_lock();
> +	p = rcu_dereference(current->mm->owner);
> +	memcg = mem_cgroup_from_task(p);

mem_cgroup_from_task says it can return NULL. Do we care here? If not
then please put VM_BUG_ON(!memcg) here.

> +	rcu_read_unlock();
> +
> +	if (!memcg_can_account_kmem(memcg))
> +		return true;
> +
> +	mem_cgroup_get(memcg);

I am confused. Why do we take a reference to memcg rather than css_get
here? Ahh it is because we keep the reference while the page is
allocated, right? Comment please.

I am still not sure whether we need css_get here as well. How do you
know that the current is not moved in parallel and it is a last task in
a group which then can go away?

> +
> +	ret = memcg_charge_kmem(memcg, gfp, PAGE_SIZE << order) == 0;
> +	if (ret)
> +		*_memcg = memcg;
> +	else
> +		mem_cgroup_put(memcg);
> +
> +	return ret;
> +}
> +
> +void __memcg_kmem_commit_charge(struct page *page, struct mem_cgroup *memcg,
> +			      int order)
> +{
> +	struct page_cgroup *pc;
> +
> +	WARN_ON(mem_cgroup_is_root(memcg));

Why the warn? Nobody should use this directly and
memcg_kmem_commit_charge takes care of the root same as
__memcg_kmem_newpage_charge does. If it is for correctness then it
should be VM_BUG_ON.

> +
> +	/* The page allocation failed. Revert */
> +	if (!page) {
> +		memcg_uncharge_kmem(memcg, PAGE_SIZE << order);
> +		return;
> +	}
> +
> +	pc = lookup_page_cgroup(page);
> +	lock_page_cgroup(pc);
> +	pc->mem_cgroup = memcg;
> +	SetPageCgroupUsed(pc);
> +	unlock_page_cgroup(pc);
> +}
> +
> +void __memcg_kmem_uncharge_page(struct page *page, int order)
> +{
> +	struct mem_cgroup *memcg = NULL;
> +	struct page_cgroup *pc;
> +
> +
> +	pc = lookup_page_cgroup(page);
> +	/*
> +	 * Fast unlocked return. Theoretically might have changed, have to
> +	 * check again after locking.
> +	 */
> +	if (!PageCgroupUsed(pc))
> +		return;
> +
> +	lock_page_cgroup(pc);
> +	if (PageCgroupUsed(pc)) {
> +		memcg = pc->mem_cgroup;
> +		ClearPageCgroupUsed(pc);
> +	}
> +	unlock_page_cgroup(pc);
> +
> +	/*
> +	 * Checking if kmem accounted is enabled won't work for uncharge, since
> +	 * it is possible that the user enabled kmem tracking, allocated, and
> +	 * then disabled it again.

disabling cannot happen, right?

> +	 *
> +	 * We trust if there is a memcg associated with the page, it is a valid
> +	 * allocation
> +	 */
> +	if (!memcg)
> +		return;
> +
> +	WARN_ON(mem_cgroup_is_root(memcg));

Same as above I do not see a reason for warn here. It just adds a code
and if you want it for debugging then VM_BUG_ON sounds more appropriate.
/me thinks

> +	memcg_uncharge_kmem(memcg, PAGE_SIZE << order);
> +	mem_cgroup_put(memcg);
> +}
>  #endif /* CONFIG_MEMCG_KMEM */
>  
>  #if defined(CONFIG_INET) && defined(CONFIG_MEMCG_KMEM)
> @@ -5700,3 +5811,69 @@ static int __init enable_swap_account(char *s)
>  __setup("swapaccount=", enable_swap_account);
>  
>  #endif
> +
> +#ifdef CONFIG_MEMCG_KMEM
> +int memcg_charge_kmem(struct mem_cgroup *memcg, gfp_t gfp, u64 size)
> +{
> +	struct res_counter *fail_res;
> +	struct mem_cgroup *_memcg;
> +	int ret;
> +	bool may_oom;
> +	bool nofail = false;
> +
> +	may_oom = (gfp & __GFP_WAIT) && (gfp & __GFP_FS) &&
> +	    !(gfp & __GFP_NORETRY);

A comment please? Why __GFP_IO is not considered for example?

> +
> +	ret = 0;
> +
> +	if (!memcg)
> +		return ret;

How can we get a NULL memcg here without blowing in
__memcg_kmem_newpage_charge?

> +
> +	_memcg = memcg;
> +	ret = __mem_cgroup_try_charge(NULL, gfp, size / PAGE_SIZE,

me likes >> PAGE_SHIFT more.

> +	    &_memcg, may_oom);
> +
> +	if (ret == -EINTR)  {
> +		nofail = true;
> +		/*
> +		 * __mem_cgroup_try_charge() chosed to bypass to root due to
> +		 * OOM kill or fatal signal.  Since our only options are to
> +		 * either fail the allocation or charge it to this cgroup, do
> +		 * it as a temporary condition. But we can't fail. From a
> +		 * kmem/slab perspective, the cache has already been selected,
> +		 * by mem_cgroup_get_kmem_cache(), so it is too late to change
> +		 * our minds
> +		 */
> +		res_counter_charge_nofail(&memcg->res, size, &fail_res);
> +		if (do_swap_account)
> +			res_counter_charge_nofail(&memcg->memsw, size,
> +						  &fail_res);
> +		ret = 0;
> +	} else if (ret == -ENOMEM)
> +		return ret;
> +
> +	if (nofail)
> +		res_counter_charge_nofail(&memcg->kmem, size, &fail_res);
> +	else
> +		ret = res_counter_charge(&memcg->kmem, size, &fail_res);
> +
> +	if (ret) {
> +		res_counter_uncharge(&memcg->res, size);
> +		if (do_swap_account)
> +			res_counter_uncharge(&memcg->memsw, size);
> +	}

You could save few lines and get rid of the strange nofail by:
[...]
+		res_counter_charge_nofail(&memcg->res, size, &fail_res);
+		if (do_swap_account)
+			res_counter_charge_nofail(&memcg->memsw, size,
+						  &fail_res);
+		res_counter_charge_nofail(&memcg->kmem, size, &fail_res);
+		return 0;
+	} else if (ret == -ENOMEM)
+		return ret;
+	else
+		ret = res_counter_charge(&memcg->kmem, size, &fail_res);
+
+	if (ret) {
+		res_counter_uncharge(&memcg->res, size);
+		if (do_swap_account)
+			res_counter_uncharge(&memcg->memsw, size);
+	}

> +
> +	return ret;
> +}
> +
> +void memcg_uncharge_kmem(struct mem_cgroup *memcg, u64 size)
> +{
> +	if (!memcg)
> +		return;
> +
> +	res_counter_uncharge(&memcg->kmem, size);
> +	res_counter_uncharge(&memcg->res, size);
> +	if (do_swap_account)
> +		res_counter_uncharge(&memcg->memsw, size);
> +}
> +#endif /* CONFIG_MEMCG_KMEM */
> -- 
> 1.7.11.4
> 
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-- 
Michal Hocko
SUSE Labs
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