[Devel] Re: [PATCH 06/11] memcg: kmem controller infrastructure

[Andrew Morton]

On Mon, 25 Jun 2012 18:15:23 +0400
Glauber Costa <glommer at parallels.com> 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 jump labels, so we don't
> incur any overhead when no mem cgroups are being used.
> 
>
> ...
>
> @@ -416,6 +423,43 @@ static inline void sock_update_memcg(struct sock *sk)
>  static inline void sock_release_memcg(struct sock *sk)
>  {
>  }
> +
> +#define mem_cgroup_kmem_on 0
> +#define __mem_cgroup_new_kmem_page(a, b, c) false
> +#define __mem_cgroup_free_kmem_page(a,b )
> +#define __mem_cgroup_commit_kmem_page(a, b, c)

I suggest that the naming consistently follow the model
"mem_cgroup_kmem_foo".  So "mem_cgroup_kmem_" becomes the well-known
identifier for this subsystem.

Then, s/mem_cgroup/memcg/g/ - show us some mercy here!

> +#define is_kmem_tracked_alloc (false)

memcg_kmem_tracked_alloc, perhaps.  But what does this actually do?

<looks>

eww, ick.

> +#define is_kmem_tracked_alloc (gfp & __GFP_KMEMCG)

What Tejun said.  This:

/*
 * Nice comment goes here
 */
static inline bool memcg_kmem_tracked_alloc(gfp_t gfp)
{
	return gfp & __GFP_KMEMCG;
}

>  #endif /* CONFIG_CGROUP_MEM_RES_CTLR_KMEM */
> +
> +static __always_inline
> +bool mem_cgroup_new_kmem_page(gfp_t gfp, void *handle, int order)

memcg_kmem_new_page().

> +{
> +	if (!mem_cgroup_kmem_on)
> +		return true;
> +	if (!is_kmem_tracked_alloc)
> +		return true;
> +	if (!current->mm)
> +		return true;
> +	if (in_interrupt())
> +		return true;
> +	if (gfp & __GFP_NOFAIL)
> +		return true;
> +	return __mem_cgroup_new_kmem_page(gfp, handle, order);
> +}

Add documentation, please.  The semantics of the return value are
inscrutable.

> +static __always_inline
> +void mem_cgroup_free_kmem_page(struct page *page, int order)
> +{
> +	if (mem_cgroup_kmem_on)
> +		__mem_cgroup_free_kmem_page(page, order);
> +}

memcg_kmem_free_page().

> +static __always_inline
> +void mem_cgroup_commit_kmem_page(struct page *page, struct mem_cgroup *handle,
> +				 int order)
> +{
> +	if (mem_cgroup_kmem_on)
> +		__mem_cgroup_commit_kmem_page(page, handle, order);
> +}

memcg_kmem_commit_page().

>  #endif /* _LINUX_MEMCONTROL_H */
>  
>
> ...
>
> +static inline bool mem_cgroup_kmem_enabled(struct mem_cgroup *memcg)
> +{
> +	return !mem_cgroup_disabled() && memcg &&
> +	       !mem_cgroup_is_root(memcg) && memcg->kmem_accounted;
> +}

Does this really need to handle a memcg==NULL?

> +bool __mem_cgroup_new_kmem_page(gfp_t gfp, void *_handle, int order)
> +{
> +	struct mem_cgroup *memcg;
> +	struct mem_cgroup **handle = (struct mem_cgroup **)_handle;
> +	bool ret = true;
> +	size_t size;
> +	struct task_struct *p;
> +
> +	*handle = NULL;
> +	rcu_read_lock();
> +	p = rcu_dereference(current->mm->owner);
> +	memcg = mem_cgroup_from_task(p);
> +	if (!mem_cgroup_kmem_enabled(memcg))
> +		goto out;
> +
> +	mem_cgroup_get(memcg);
> +
> +	size = (1 << order) << PAGE_SHIFT;

	size = PAGE_SIZE << order;

is simpler and more typical.

> +	ret = memcg_charge_kmem(memcg, gfp, size) == 0;

Odd.  memcg_charge_kmem() returns a nice errno, buit this conversion
just drops that information on the floor.  If the
mem_cgroup_new_kmem_page() return value had been documented, I might
have been able to understand the thinking here.  But it wasn't, so I
couldn't.

> +	if (!ret) {
> +		mem_cgroup_put(memcg);
> +		goto out;
> +	}
> +
> +	*handle = memcg;
> +out:
> +	rcu_read_unlock();
> +	return ret;
> +}
> +EXPORT_SYMBOL(__mem_cgroup_new_kmem_page);
> +
> +void __mem_cgroup_commit_kmem_page(struct page *page, void *handle, int order)
> +{
> +	struct page_cgroup *pc;
> +	struct mem_cgroup *memcg = handle;
> +	size_t size;
> +
> +	if (!memcg)
> +		return;
> +
> +	WARN_ON(mem_cgroup_is_root(memcg));
> +	/* The page allocation must have failed. Revert */
> +	if (!page) {
> +		size = (1 << order) << PAGE_SHIFT;

	PAGE_SIZE << order

> +		memcg_uncharge_kmem(memcg, size);
> +		mem_cgroup_put(memcg);
> +		return;
> +	}

This all looks a bit odd.  After a failure you run a commit which
undoes the speculative charging.  I guess it makes sense.  It's the
sort of thing which can be expounded upon in the documentation whcih
isn't there, sigh.

> +	pc = lookup_page_cgroup(page);
> +	lock_page_cgroup(pc);
> +	pc->mem_cgroup = memcg;
> +	SetPageCgroupUsed(pc);
> +	unlock_page_cgroup(pc);
> +}

missed a newline there.

> +void __mem_cgroup_free_kmem_page(struct page *page, int order)
> +{
> +	struct mem_cgroup *memcg;
> +	size_t size;
> +	struct page_cgroup *pc;
> +
> +	if (mem_cgroup_disabled())
> +		return;
> +
> +	pc = lookup_page_cgroup(page);
> +	lock_page_cgroup(pc);
> +	memcg = pc->mem_cgroup;
> +	pc->mem_cgroup = NULL;
> +	if (!PageCgroupUsed(pc)) {
> +		unlock_page_cgroup(pc);
> +		return;
> +	}
> +	ClearPageCgroupUsed(pc);
> +	unlock_page_cgroup(pc);
> +
> +	/*
> +	 * The classical disabled check won't work

What is "The classical disabled check"?  Be specific.  Testing
mem_cgroup_kmem_on?

> +	 * for uncharge, since it is possible that the user enabled
> +	 * kmem tracking, allocated, and then disabled.
> +	 *
> +	 * 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));
> +	size = (1 << order) << PAGE_SHIFT;

PAGE_SIZE << order

> +	memcg_uncharge_kmem(memcg, size);
> +	mem_cgroup_put(memcg);
> +}
> +EXPORT_SYMBOL(__mem_cgroup_free_kmem_page);
>  #endif /* CONFIG_CGROUP_MEM_RES_CTLR_KMEM */
>  
>  #if defined(CONFIG_INET) && defined(CONFIG_CGROUP_MEM_RES_CTLR_KMEM)
> @@ -5645,3 +5751,69 @@ static int __init enable_swap_account(char *s)
>  __setup("swapaccount=", enable_swap_account);
>  
>  #endif
> +
> +#ifdef CONFIG_CGROUP_MEM_RES_CTLR_KMEM

gargh.  CONFIG_MEMCG_KMEM, please!

> +int memcg_charge_kmem(struct mem_cgroup *memcg, gfp_t gfp, s64 delta)
> +{
> +	struct res_counter *fail_res;
> +	struct mem_cgroup *_memcg;
> +	int may_oom, ret;
> +	bool nofail = false;
> +
> +	may_oom = (gfp & __GFP_WAIT) && (gfp & __GFP_FS) &&
> +	    !(gfp & __GFP_NORETRY);

may_oom should have bool type.

> +	ret = 0;
> +
> +	if (!memcg)
> +		return ret;
> +
> +	_memcg = memcg;
> +	ret = __mem_cgroup_try_charge(NULL, gfp, delta / PAGE_SIZE,
> +	    &_memcg, may_oom);
> +
> +	if (ret == -EINTR)  {
> +		nofail = true;
> +		/*
> +		 * __mem_cgroup_try_charge() chose to bypass to root due
> +		 * to OOM kill or fatal signal.

Is "bypass" correct?  Maybe "fall back"?

> +		 * 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, delta, &fail_res);
> +		if (do_swap_account)
> +			res_counter_charge_nofail(&memcg->memsw, delta,
> +						  &fail_res);
> +		ret = 0;
> +	} else if (ret == -ENOMEM)
> +		return ret;
> +
> +	if (nofail)
> +		res_counter_charge_nofail(&memcg->kmem, delta, &fail_res);
> +	else
> +		ret = res_counter_charge(&memcg->kmem, delta, &fail_res);
> +
> +	if (ret) {
> +		res_counter_uncharge(&memcg->res, delta);
> +		if (do_swap_account)
> +			res_counter_uncharge(&memcg->memsw, delta);
> +	}
> +
> +	return ret;
> +}
>
> ...
>