linux-kernel - Re: SLUB: Support for statistics to help analyze allocator behavior

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Date:	Wed, 6 Feb 2008 00:19:48 -0800
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	Christoph Lameter <clameter@....com>
Cc:	Pekka Enberg <penberg@...helsinki.fi>,
	linux-kernel@...r.kernel.org, linux-mm@...ck.org
Subject: Re: SLUB: Support for statistics to help analyze allocator behavior

On Mon, 4 Feb 2008 22:20:04 -0800 (PST) Christoph Lameter <clameter@....com> wrote:

> The statistics provided here allow the monitoring of allocator behavior
> at the cost of some (minimal) loss of performance. Counters are placed in
> SLUB's per cpu data structure that is already written to by other code.

Seems sane.

> The per cpu structure may be extended by the statistics to be more than
> one cacheline which will increase the cache footprint of SLUB.
>
> That is why there is a compile option to enable/disable the inclusion of
> the statistics module. 

The compile-time optionality is really sad.  But no obvious solution
suggests itself.

> @@ -1357,17 +1366,22 @@ static struct page *get_partial(struct k
>  static void unfreeze_slab(struct kmem_cache *s, struct page *page, int tail)
>  {
>  	struct kmem_cache_node *n = get_node(s, page_to_nid(page));
> +	struct kmem_cache_cpu *c = get_cpu_slab(s, smp_processor_id());

So we're never running preemptibly here.

>  	ClearSlabFrozen(page);
>  	if (page->inuse) {
>  
> -		if (page->freelist != page->end)
> +		if (page->freelist != page->end) {
>  			add_partial(n, page, tail);
> -		else if (SlabDebug(page) && (s->flags & SLAB_STORE_USER))
> +			stat(c, tail ? DEACTIVATE_TO_TAIL : DEACTIVATE_TO_HEAD);
> +		} else {
> +			stat(c, DEACTIVATE_FULL);
> +			if (SlabDebug(page) && (s->flags & SLAB_STORE_USER))
>  			add_full(n, page);

missing a tab

> +#ifdef CONFIG_SLUB_STATS
> +
> +#define STAT_ATTR(si, text) 					\
> +static ssize_t text##_show(struct kmem_cache *s, char *buf)	\
> +{								\
> +	unsigned long sum  = 0;					\
> +	int cpu;						\
> +								\
> +	for_each_online_cpu(cpu)				\
> +		sum += get_cpu_slab(s, cpu)->stat[si];		\

maybe cache the get_cpu_slab() result in a local?

> +	return sprintf(buf, "%lu\n", sum);			\
> +}								\
> +SLAB_ATTR_RO(text);						\

this is pretty broken after cpu hot-unplug, isn't it?


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