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Date: Tue, 17 Apr 2007 16:22:48 +0300
From: "Pekka Enberg" <penberg@cs.helsinki.fi>
To: "Pavel Emelianov" <xemul@sw.ru>
Subject: Re: [PATCH] Show slab memory usage on OOM and SysRq-M
Cc: "Andrew Morton" <akpm@osdl.org>,
       "Linux Kernel Mailing List" <linux-kernel@vger.kernel.org>,
       devel@openvz.org, "Kirill Korotaev" <dev@openvz.org>,
       linux-mm@kvack.org
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Hi,

On 4/17/07, Pavel Emelianov <xemul@sw.ru> wrote:
> The out_of_memory() function and SysRq-M handler call
> show_mem() to show the current memory usage state.
>
> This is also helpful to see which slabs are the largest
> in the system.

Makes sense.

On 4/17/07, Pavel Emelianov <xemul@sw.ru> wrote:
> diff --git a/mm/slab.c b/mm/slab.c
> index 21b3c61..9a5829a 100644
> --- a/mm/slab.c
> +++ b/mm/slab.c
> @@ -749,6 +749,7 @@ static inline void init_lock_keys(void)
>   * 2. Protect sanity of cpu_online_map against cpu hotplug events
>   */
>  static DEFINE_MUTEX(cache_chain_mutex);
> +static DEFINE_SPINLOCK(cache_chain_lock);

So, now we have two locks protecting cache_chain? Please explain why
you can't use the mutex.

> +static unsigned long get_cache_size(struct kmem_cache *cachep)
> +{
> +       unsigned long slabs;
> +       struct kmem_list3 *l3;
> +       struct list_head *lh;
> +       int node;
> +
> +       slabs = 0;
> +
> +       for_each_online_node (node) {
> +               l3 = cachep->nodelists[node];
> +               if (l3 == NULL)
> +                       continue;
> +
> +               spin_lock(&l3->list_lock);
> +               list_for_each (lh, &l3->slabs_full)
> +                       slabs++;
> +               list_for_each (lh, &l3->slabs_partial)
> +                       slabs++;
> +               list_for_each (lh, &l3->slabs_free)
> +                       slabs++;
> +               spin_unlock(&l3->list_lock);
> +       }
> +
> +       return slabs * ((PAGE_SIZE << cachep->gfporder) +
> +               (OFF_SLAB(cachep) ? cachep->slabp_cache->buffer_size : 0));
> +}

Considering you're doing this at out_of_memory() time, wouldn't it
make more sense to add a ->nr_pages to struct kmem_cache and do the
tracking in kmem_getpages/kmem_freepages?

I would also drop the OFF_SLAB bits because it really doesn't matter
that much for your purposes. Besides, you're already per-node and
per-CPU caches here which attribute to much more memory on NUMA setups
for example.