Date:	Thu, 8 Oct 2009 16:37:24 -0400
From:	Mathieu Desnoyers <mathieu.desnoyers@...ymtl.ca>
To:	Christoph Lameter <cl@...ux-foundation.org>
Cc:	Peter Zijlstra <peterz@...radead.org>, akpm@...ux-foundation.org,
	linux-kernel@...r.kernel.org,
	Pekka Enberg <penberg@...helsinki.fi>,
	Tejun Heo <tj@...nel.org>, Mel Gorman <mel@....ul.ie>,
	mingo@...e.hu
Subject: Re: [this_cpu_xx V5 19/19] SLUB: Experimental new fastpath w/o
	interrupt disable

* Christoph Lameter (cl@...ux-foundation.org) wrote:
> On Thu, 8 Oct 2009, Mathieu Desnoyers wrote:
> 
> > OK, sounds good,
> 
> Then here the full patch for review (vs. percpu-next):
> 
> 
> From: Christoph Lameter <cl@...ux-foundation.org>
> Subject: SLUB: Experimental new fastpath w/o interrupt disable
> 
> This is a bit of a different tack on things than the last version provided
> by Mathieu.
> 
> Instead of using a cmpxchg we keep a state variable in the per cpu structure
> that is incremented when we enter the hot path. We can then detect that
> a thread is in the fastpath and fall back to alternate allocation / free
> technique that bypasses fastpath caching.
> 
> V1->V2:
> - Use safe preempt_enable / disable.
> - Enable preempt before calling into the page allocator
> - Checkup on hotpath activity changes when returning from page allocator.
> - Add barriers.
> 
> Todo:
> - Verify that this is really safe
> - Is this a benefit?
> 
> Cc: Mathieu Desnoyers <mathieu.desnoyers@...ymtl.ca>
> Cc: Pekka Enberg <penberg@...helsinki.fi>
> Signed-off-by: Christoph Lameter <cl@...ux-foundation.org>
> 
> 
> ---
>  include/linux/slub_def.h |    1
>  mm/slub.c                |  112 +++++++++++++++++++++++++++++++++++++++--------
>  2 files changed, 96 insertions(+), 17 deletions(-)
> 
> Index: linux-2.6/include/linux/slub_def.h
> ===================================================================
> --- linux-2.6.orig/include/linux/slub_def.h	2009-10-08 11:35:59.000000000 -0500
> +++ linux-2.6/include/linux/slub_def.h	2009-10-08 11:35:59.000000000 -0500
> @@ -38,6 +38,7 @@ struct kmem_cache_cpu {
>  	void **freelist;	/* Pointer to first free per cpu object */
>  	struct page *page;	/* The slab from which we are allocating */
>  	int node;		/* The node of the page (or -1 for debug) */
> +	int active;		/* Active fastpaths */
>  #ifdef CONFIG_SLUB_STATS
>  	unsigned stat[NR_SLUB_STAT_ITEMS];
>  #endif
> Index: linux-2.6/mm/slub.c
> ===================================================================
> --- linux-2.6.orig/mm/slub.c	2009-10-08 11:35:59.000000000 -0500
> +++ linux-2.6/mm/slub.c	2009-10-08 14:03:22.000000000 -0500
> @@ -1606,7 +1606,14 @@ static void *__slab_alloc(struct kmem_ca
>  			  unsigned long addr)
>  {
>  	void **object;
> -	struct page *page = __this_cpu_read(s->cpu_slab->page);
> +	struct page *page;
> +	unsigned long flags;
> +	int hotpath;
> +
> +	local_irq_save(flags);

(Recommend adding)

	preempt_enable_no_resched();


The preempt enable right in the middle of a big function is adding an
unnecessary barrier(), which will restrain gcc from doing its
optimizations.  This might hurt performances.

I still recommend the preempt_enable_no_resched() at the beginning of
__slab_alloc(), and simply putting a check_resched() here (which saves
us the odd compiler barrier in the middle of function).


> +	hotpath = __this_cpu_read(s->cpu_slab->active) != 0;
> +	__this_cpu_dec(s->cpu_slab->active);	/* Drop count from hotpath */
> +	page = __this_cpu_read(s->cpu_slab->page);
> 
>  	/* We handle __GFP_ZERO in the caller */
>  	gfpflags &= ~__GFP_ZERO;
> @@ -1627,12 +1634,24 @@ load_freelist:
>  	if (unlikely(SLABDEBUG && PageSlubDebug(page)))
>  		goto debug;
> 
> -	__this_cpu_write(s->cpu_slab->node, page_to_nid(page));
> -	__this_cpu_write(s->cpu_slab->freelist, get_freepointer(s, object));
> -	page->inuse = page->objects;
> -	page->freelist = NULL;
> +	if (unlikely(hotpath)) {
> +		/* Object on page free list available and hotpath busy */
> +		page->inuse++;
> +		page->freelist = get_freepointer(s, object);
> +
> +	} else {
> +
> +		/* Prepare new list of objects for hotpath */
> +		__this_cpu_write(s->cpu_slab->freelist, get_freepointer(s, object));
> +		page->inuse = page->objects;
> +		page->freelist = NULL;
> +		__this_cpu_write(s->cpu_slab->node, page_to_nid(page));
> +
> +	}
> +
>  unlock_out:
>  	slab_unlock(page);
> +	local_irq_restore(flags);
>  	stat(s, ALLOC_SLOWPATH);
>  	return object;
> 
> @@ -1642,21 +1661,38 @@ another_slab:
>  new_slab:
>  	page = get_partial(s, gfpflags, node);
>  	if (page) {
> -		__this_cpu_write(s->cpu_slab->page, page);
>  		stat(s, ALLOC_FROM_PARTIAL);
> +
> +		if (hotpath)
> +			goto hot_lock;
> +
> +		__this_cpu_write(s->cpu_slab->page, page);
>  		goto load_freelist;
>  	}
> 
>  	if (gfpflags & __GFP_WAIT)
>  		local_irq_enable();
> 
> +	preempt_enable();

We could replace the above by:

if (gfpflags & __GFP_WAIT) {
	local_irq_enable();
	preempt_check_resched();
}


>  	page = new_slab(s, gfpflags, node);
> +	preempt_disable();
> +

(remove the above)


> +	/*
> +	 * We have already decremented our count. Someone else
> +	 * could be running right now or we were moved to a
> +	 * processor that is in the hotpath. So check against -1.
> +	 */
> +	hotpath = __this_cpu_read(s->cpu_slab->active) != -1;
> 
>  	if (gfpflags & __GFP_WAIT)
>  		local_irq_disable();
> 
>  	if (page) {
>  		stat(s, ALLOC_SLAB);
> +
> +		if (hotpath)
> +			 goto hot_no_lock;
> +
>  		if (__this_cpu_read(s->cpu_slab->page))
>  			flush_slab(s, __this_cpu_ptr(s->cpu_slab));
>  		slab_lock(page);
> @@ -1664,9 +1700,13 @@ new_slab:
>  		__this_cpu_write(s->cpu_slab->page, page);
>  		goto load_freelist;
>  	}
> +
> +	local_irq_restore(flags);
> +
>  	if (!(gfpflags & __GFP_NOWARN) && printk_ratelimit())
>  		slab_out_of_memory(s, gfpflags, node);
>  	return NULL;
> +
>  debug:
>  	if (!alloc_debug_processing(s, page, object, addr))
>  		goto another_slab;
> @@ -1675,6 +1715,20 @@ debug:
>  	page->freelist = get_freepointer(s, object);
>  	__this_cpu_write(s->cpu_slab->node, -1);
>  	goto unlock_out;
> +
> +	/*
> +	 * Hotpath is busy and we need to avoid touching
> +	 * hotpath variables
> +	 */
> +hot_no_lock:
> +	slab_lock(page);
> +
> +hot_lock:
> +	__ClearPageSlubFrozen(page);
> +	if (get_freepointer(s, page->freelist))
> +		/* Cannot put page into the hotpath. Instead back to partial */
> +		add_partial(get_node(s, page_to_nid(page)), page, 0);
> +	goto load_freelist;
>  }
> 
>  /*
> @@ -1691,7 +1745,6 @@ static __always_inline void *slab_alloc(
>  		gfp_t gfpflags, int node, unsigned long addr)
>  {
>  	void **object;
> -	unsigned long flags;
> 
>  	gfpflags &= gfp_allowed_mask;
> 
> @@ -1701,18 +1754,27 @@ static __always_inline void *slab_alloc(
>  	if (should_failslab(s->objsize, gfpflags))
>  		return NULL;
> 
> -	local_irq_save(flags);
> +	preempt_disable();
> +
> +	irqsafe_cpu_inc(s->cpu_slab->active);
> +	barrier();
>  	object = __this_cpu_read(s->cpu_slab->freelist);
> -	if (unlikely(!object || !node_match(s, node)))
> +	if (unlikely(!object || !node_match(s, node) ||
> +			__this_cpu_read(s->cpu_slab->active)))

Missing a barrier() here ?

The idea is to let gcc know that "active" inc/dec and "freelist" reads
must never be reordered. Even when the decrement is done in the slow
path branch.

> 
>  		object = __slab_alloc(s, gfpflags, node, addr);
> 
>  	else {
> +
>  		__this_cpu_write(s->cpu_slab->freelist,
>  			get_freepointer(s, object));
> +		barrier();
> +		irqsafe_cpu_dec(s->cpu_slab->active);
>  		stat(s, ALLOC_FASTPATH);
> +
>  	}
> -	local_irq_restore(flags);
> +
> +	preempt_enable();

Could move the preempt_enable() above to the else (fast path) branch.

> 
>  	if (unlikely((gfpflags & __GFP_ZERO) && object))
>  		memset(object, 0, s->objsize);
> @@ -1777,7 +1839,9 @@ static void __slab_free(struct kmem_cach
>  {
>  	void *prior;
>  	void **object = (void *)x;
> +	unsigned long flags;
> 
> +	local_irq_save(flags);
>  	stat(s, FREE_SLOWPATH);
>  	slab_lock(page);
> 
> @@ -1809,6 +1873,7 @@ checks_ok:
> 
>  out_unlock:
>  	slab_unlock(page);
> +	local_irq_restore(flags);
>  	return;
> 
>  slab_empty:
> @@ -1820,6 +1885,7 @@ slab_empty:
>  		stat(s, FREE_REMOVE_PARTIAL);
>  	}
>  	slab_unlock(page);
> +	local_irq_restore(flags);
>  	stat(s, FREE_SLAB);
>  	discard_slab(s, page);
>  	return;
> @@ -1845,24 +1911,31 @@ static __always_inline void slab_free(st
>  			struct page *page, void *x, unsigned long addr)
>  {
>  	void **object = (void *)x;
> -	unsigned long flags;
> 
>  	kmemleak_free_recursive(x, s->flags);
> -	local_irq_save(flags);
> +	preempt_disable();
>  	kmemcheck_slab_free(s, object, s->objsize);
>  	debug_check_no_locks_freed(object, s->objsize);
>  	if (!(s->flags & SLAB_DEBUG_OBJECTS))
>  		debug_check_no_obj_freed(object, s->objsize);
> 
> +	irqsafe_cpu_inc(s->cpu_slab->active);
> +	barrier();
>  	if (likely(page == __this_cpu_read(s->cpu_slab->page) &&
> -			__this_cpu_read(s->cpu_slab->node) >= 0)) {
> -		set_freepointer(s, object, __this_cpu_read(s->cpu_slab->freelist));
> +			__this_cpu_read(s->cpu_slab->node) >= 0) &&
> +			!__this_cpu_read(s->cpu_slab->active)) {
> +		set_freepointer(s, object,
> +			__this_cpu_read(s->cpu_slab->freelist));
>  		__this_cpu_write(s->cpu_slab->freelist, object);
> +		barrier();
> +		irqsafe_cpu_dec(s->cpu_slab->active);
> +		preempt_enable();
>  		stat(s, FREE_FASTPATH);
> -	} else
> +	} else {

Perhaps missing a barrier() in the else ?

Thanks,

Mathieu

> +		irqsafe_cpu_dec(s->cpu_slab->active);
> +		preempt_enable();
>  		__slab_free(s, page, x, addr);
> -
> -	local_irq_restore(flags);
> +	}
>  }
> 
>  void kmem_cache_free(struct kmem_cache *s, void *x)
> @@ -2064,6 +2137,8 @@ static DEFINE_PER_CPU(struct kmem_cache_
> 
>  static inline int alloc_kmem_cache_cpus(struct kmem_cache *s, gfp_t flags)
>  {
> +	int cpu;
> +
>  	if (s < kmalloc_caches + KMALLOC_CACHES && s >= kmalloc_caches)
>  		/*
>  		 * Boot time creation of the kmalloc array. Use static per cpu data
> @@ -2073,6 +2148,9 @@ static inline int alloc_kmem_cache_cpus(
>  	else
>  		s->cpu_slab =  alloc_percpu(struct kmem_cache_cpu);
> 
> +	for_each_possible_cpu(cpu)
> +		per_cpu_ptr(s->cpu_slab, cpu)->active = -1;
> +
>  	if (!s->cpu_slab)
>  		return 0;
> 

-- 
Mathieu Desnoyers
OpenPGP key fingerprint: 8CD5 52C3 8E3C 4140 715F  BA06 3F25 A8FE 3BAE 9A68
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