Date:	Wed, 9 May 2007 16:05:05 +0100 (IST)
From:	Mel Gorman <mel@....ul.ie>
To:	clameter@....com
cc:	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Linux Memory Management List <linux-mm@...ck.org>,
	dgc@....com, Eric Dumazet <dada1@...mosbay.com>
Subject: Re: [RFC 2/3] SLUB: Implement targeted reclaim and partial list
 defragmentation

On Fri, 4 May 2007, clameter@....com wrote:

> Targeted reclaim allows to target a single slab for reclaim. This is done by
> calling
>
> kmem_cache_vacate(slab, page);
>
> It will return 1 on success, 0 if the operation failed.
>
> The vacate functionality is also used for slab shrinking. During the shrink
> operation SLUB will generate a list sorted by the number of objects in use.
>
> We extract pages off that list that are only filled less than a quarter. These
> objects are then processed using kmem_cache_vacate.
>
> In order for a slabcache to support this functionality two functions must
> be defined via slab_operations.
>
> get_reference(void *)
>
> Must obtain a reference to the object if it has not been freed yet. It is up
> to the slabcache to resolve the race.

Is there also a race here between the object being searched for and the 
reference being taken? Would it be appropriate to call this 
find_get_slub_object() to mirror what find_get_page() does except instead 
of offset, it receives some key meaningful to the cache? It might help set 
a developers expectation of what the function is required to do when 
creating such a cache and simplify locking.

(written much later) Looking through, it's not super-clear if 
get_reference() is only for use by SLUB to get an exclusive reference to 
an object or something that is generally called to reference count it. It 
looks like only one reference is ever taken.

> SLUB guarantees that the objects is
> still allocated. However, another thread may be blocked in slab_free
> attempting to free the same object. It may succeed as soon as
> get_reference() returns to the slab allocator.
>

Fun. If it was find_get_slub_object() instead of get_reference(), the 
caller could block if the object is currently being freed until slab_free 
completed. It would take a hit because the object has to be recreated but 
maybe it would be an easier race to deal with?

> get_reference() processing must recognize this situation (i.e. check refcount
> for zero) and fail in such a sitation (no problem since the object will
> be freed as soon we drop the slab lock before doing kick calls).
>

Block or fail?

> No slab operations may be performed in get_reference(). The slab lock
> for the page with the object is taken. Any slab operations may lead to
> a deadlock.
>
> 2. kick_object(void *)
>
> After SLUB has established references to the remaining objects in a slab it
> will drop all locks and then use kick_object on each of the objects for which
> we obtained a reference. The existence of the objects is guaranteed by
> virtue of the earlier obtained reference. The callback may perform any
> slab operation since no locks are held at the time of call.
>
> The callback should remove the object from the slab in some way. This may
> be accomplished by reclaiming the object and then running kmem_cache_free()
> or reallocating it and then running kmem_cache_free(). Reallocation
> is advantageous at this point because it will then allocate from the partial
> slabs with the most objects because we have just finished slab shrinking.
>

>From the comments alone, it sounds like the page cache API should be 
copied in name at least like slub_object_get(), slub_object_release() and 
maybe even something like slub_object_get_unless_vacating() if the caller 
must not sleep. It seems the page cache shares similar problems to what 
SLUB is doing here.


> NOTE: This patch is for conceptual review. I'd appreciate any feedback
> especially on the locking approach taken here. It will be critical to
> resolve the locking issue for this approach to become feasable.
>

Take everything I say here with a grain of salt. My understanding of SLUB 
is non-existent at the moment and it makes reviewing this a bit tricky.

> Signed-off-by: Christoph Lameter <clameter@....com>
>
> ---
> include/linux/slab.h |    3
> mm/slub.c            |  159 ++++++++++++++++++++++++++++++++++++++++++++++++---
> 2 files changed, 154 insertions(+), 8 deletions(-)
>
> Index: slub/include/linux/slab.h
> ===================================================================
> --- slub.orig/include/linux/slab.h	2007-05-04 13:32:34.000000000 -0700
> +++ slub/include/linux/slab.h	2007-05-04 13:32:50.000000000 -0700
> @@ -42,6 +42,8 @@ struct slab_ops {
> 	void (*ctor)(void *, struct kmem_cache *, unsigned long);
> 	/* FIXME: Remove all destructors ? */
> 	void (*dtor)(void *, struct kmem_cache *, unsigned long);
> +	int (*get_reference)(void *);
> +	void (*kick_object)(void *);
> };
>
> struct kmem_cache *__kmem_cache_create(const char *, size_t, size_t,
> @@ -54,6 +56,7 @@ void kmem_cache_free(struct kmem_cache *
> unsigned int kmem_cache_size(struct kmem_cache *);
> const char *kmem_cache_name(struct kmem_cache *);
> int kmem_ptr_validate(struct kmem_cache *cachep, const void *ptr);
> +int kmem_cache_vacate(struct page *);
>
> /*
>  * Please use this macro to create slab caches. Simply specify the
> Index: slub/mm/slub.c
> ===================================================================
> --- slub.orig/mm/slub.c	2007-05-04 13:32:34.000000000 -0700
> +++ slub/mm/slub.c	2007-05-04 13:56:25.000000000 -0700
> @@ -173,7 +173,7 @@ static struct notifier_block slab_notifi
> static enum {
> 	DOWN,		/* No slab functionality available */
> 	PARTIAL,	/* kmem_cache_open() works but kmalloc does not */
> -	UP,		/* Everything works */
> +	UP,		/* Everything works but does not show up in sysfs */

Should this be here?

> 	SYSFS		/* Sysfs up */
> } slab_state = DOWN;
>
> @@ -211,6 +211,8 @@ static inline struct kmem_cache_node *ge
>
> struct slab_ops default_slab_ops = {
> 	NULL,
> +	NULL,
> +	NULL,
> 	NULL
> };
>
> @@ -839,13 +841,10 @@ static struct page *new_slab(struct kmem
> 	n = get_node(s, page_to_nid(page));
> 	if (n)
> 		atomic_long_inc(&n->nr_slabs);
> +
> 	page->offset = s->offset / sizeof(void *);
> 	page->slab = s;
> -	page->flags |= 1 << PG_slab;
> -	if (s->flags & (SLAB_DEBUG_FREE | SLAB_RED_ZONE | SLAB_POISON |
> -			SLAB_STORE_USER | SLAB_TRACE))
> -		page->flags |= 1 << PG_error;
> -
> +	page->inuse = 0;

You probably have explained this a million times already and in the main 
SLUB patches, but why is _count not used?

> 	start = page_address(page);
> 	end = start + s->objects * s->size;
>
> @@ -862,7 +861,17 @@ static struct page *new_slab(struct kmem
> 	set_freepointer(s, last, NULL);
>
> 	page->freelist = start;
> -	page->inuse = 0;
> +
> +	/*
> +	 * pages->inuse must be visible when PageSlab(page) becomes
> +	 * true for targeted reclaim
> +	 */
> +	smp_wmb();
> +	page->flags |= 1 << PG_slab;
> +	if (s->flags & (SLAB_DEBUG_FREE | SLAB_RED_ZONE | SLAB_POISON |
> +			SLAB_STORE_USER | SLAB_TRACE))
> +		page->flags |= 1 << PG_error;
> +
> out:
> 	if (flags & __GFP_WAIT)
> 		local_irq_disable();
> @@ -2124,6 +2133,111 @@ void kfree(const void *x)
> EXPORT_SYMBOL(kfree);
>
> /*
> + * Vacate all objects in the given slab. Slab must be locked.
> + *
> + * Will drop and regain and drop the slab lock.
> + * Slab must be marked PageActive() to avoid concurrent slab_free from

I just noticed this PageActive() overloading of the page->flags. Is it 
worth defining SlabPerCPU() as an alias or something?

> + * remove the slab from the lists. At the end the slab will either
> + * be freed or have been returned to the partial lists.
> + *
> + * Return error code or number of remaining objects
> + */
> +static int __kmem_cache_vacate(struct kmem_cache *s, struct page *page)
> +{
> +	void *p;
> +	void *addr = page_address(page);
> +	unsigned long map[BITS_TO_LONGS(s->objects)];
> +	int leftover;
> +
> +	if (!page->inuse)
> +		return 0;
> +
> +	/* Determine free objects */
> +	bitmap_zero(map, s->objects);
> +	for(p = page->freelist; p; p = get_freepointer(s, p))
> +		set_bit((p - addr) / s->size, map);
> +
> +	/*
> +	 * Get a refcount for all used objects. If that fails then
> +	 * no KICK callback can be performed.
> +	 */
> +	for(p = addr; p < addr + s->objects * s->size; p += s->size)
> +		if (!test_bit((p - addr) / s->size, map))
> +			if (!s->slab_ops->get_reference(p))
> +				set_bit((p - addr) / s->size, map);
> +

The comment and code implies that only one caller can hold a reference at 
a time and the count is either 1 or 0.

(later)

It looks like get_reference() is only intended for use by SLUB. I don't 
currently see how an implementer of a cache could determine if an object 
is being actively referenced or not when deciding whether to return 1 for 
get_reference() unless the cache interally implemented a referencing count 
API. That might lead to many, similar but subtly different implementations 
of reference counting.

> +	/* Got all the references we need. Now we can drop the slab lock */
> +	slab_unlock(page);
> +

Where did we check we got all the references? It looks more like we got 
all the references we could.

> +	/* Perform the KICK callbacks to remove the objects */
> +	for(p = addr; p < addr + s->objects * s->size; p += s->size)
> +		if (!test_bit((p - addr) / s->size, map))
> +			s->slab_ops->kick_object(p);
> +
> +	slab_lock(page);
> +	leftover = page->inuse;
> +	ClearPageActive(page);
> +	putback_slab(s, page);
> +	return leftover;
> +}

Ok, how this works is clear from the comment in kmem_cache_vacate(). A 
note saying to look at the comment there may be useful.

> +
> +/*
> + * Remove a page from the lists. Must be holding slab lock.
> + */
> +static void remove_from_lists(struct kmem_cache *s, struct page *page)
> +{
> +	if (page->inuse < s->objects)
> +		remove_partial(s, page);
> +	else if (s->flags & SLAB_STORE_USER)
> +		remove_full(s, page);
> +}
> +
> +/*
> + * Attempt to free objects in a page. Return 1 when succesful.
> + */

when or if? Looks more like if

> +int kmem_cache_vacate(struct page *page)
> +{
> +	struct kmem_cache *s;
> +	int rc = 0;
> +
> +	/* Get a reference to the page. Return if its freed or being freed */
> +	if (!get_page_unless_zero(page))
> +		return 0;
> +
> +	/* Check that this is truly a slab page */
> +	if (!PageSlab(page))
> +		goto out;
> +
> +	slab_lock(page);
> +
> +	/*
> +	 * We may now have locked a page that is in various stages of being
> +	 * freed. If the PageSlab bit is off then we have already reached
> +	 * the page allocator. If page->inuse is zero then we are
> +	 * in SLUB but freeing or allocating the page.
> +	 * page->inuse is never modified without the slab lock held.
> +	 *
> +	 * Also abort if the page happens to be a per cpu slab
> +	 */
> +	if (!PageSlab(page) || PageActive(page) || !page->inuse) {
> +		slab_unlock(page);
> +		goto out;
> +	}
> +

The PageActive() part is going to confuse someone eventually because 
they'll interpret it to mean that slab pages are on the LRU now.

> +	/*
> +	 * We are holding a lock on a slab page that is not in the
> +	 * process of being allocated or freed.
> +	 */
> +	s = page->slab;
> +	remove_from_lists(s, page);
> +	SetPageActive(page);
> +	rc = __kmem_cache_vacate(s, page) == 0;

The name rc is misleading here. I am reading it as reference count, but 
it's not a reference count at all.

> +out:
> +	put_page(page);
> +	return rc;
> +}

Where do partially freed slab pages get put back on the lists? Do they 
just remain off the lists until they are totally freed as a type of lazy 
free? If so, a wait_on_slab_page_free() call may be needed later.

> +
> +/*
>  *  kmem_cache_shrink removes empty slabs from the partial lists
>  *  and then sorts the partially allocated slabs by the number
>  *  of items in use. The slabs with the most items in use
> @@ -2137,11 +2251,12 @@ int kmem_cache_shrink(struct kmem_cache
> 	int node;
> 	int i;
> 	struct kmem_cache_node *n;
> -	struct page *page;
> +	struct page *page, *page2;

page2, is a pretty bad name.

> 	struct page *t;
> 	struct list_head *slabs_by_inuse =
> 		kmalloc(sizeof(struct list_head) * s->objects, GFP_KERNEL);
> 	unsigned long flags;
> +	LIST_HEAD(zaplist);
>
> 	if (!slabs_by_inuse)
> 		return -ENOMEM;
> @@ -2194,8 +2309,36 @@ int kmem_cache_shrink(struct kmem_cache
> 		for (i = s->objects - 1; i >= 0; i--)
> 			list_splice(slabs_by_inuse + i, n->partial.prev);
>
> +		if (!s->slab_ops->get_reference || !s->slab_ops->kick_object)
> +			goto out;
> +
> +		/* Take objects with just a few objects off the tail */
> +		while (n->nr_partial > MAX_PARTIAL) {
> +			page = container_of(n->partial.prev, struct page, lru);
> +
> +			/*
> +			 * We are holding the list_lock so we can only
> +			 * trylock the slab
> +			 */
> +			if (!slab_trylock(page))
> +				break;
> +
> +			if (page->inuse > s->objects / 4)
> +				break;
> +

I get the idea of the check but it may have unexpected consequences if all 
of the slabs in use happen to be half full.

> +			list_move(&page->lru, &zaplist);
> +			n->nr_partial--;
> +			SetPageActive(page);
> +			slab_unlock(page);
> +		}

Ok, not sure if this is a problem or not. I don't get why SetPageActive() 
is being called and it's because I don't understand SLUB yet.

> 	out:
> 		spin_unlock_irqrestore(&n->list_lock, flags);
> +
> +		/* Now we can free objects in the slabs on the zaplist */
> +		list_for_each_entry_safe(page, page2, &zaplist, lru) {
> +			slab_lock(page);
> +			__kmem_cache_vacate(s, page);
> +		}

No checking the return value here which is why I think that slab pages 
once vacated are expected to be off the lists and totally freed. If that 
is the case, it might have consequences if the cache is badly behaving and 
never freeing objects.

> 	}
>
> 	kfree(slabs_by_inuse);
>
> -- 
>

-- 
Mel Gorman
Part-time Phd Student                          Linux Technology Center
University of Limerick                         IBM Dublin Software Lab
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