Re: [PATCH] Move memory controller allocations to their own slabs(v2)

[Hugh Dickins]

On Tue, 11 Mar 2008, Balbir Singh wrote:
> 
> Move the memory controller data structure page_cgroup to its own slab cache.
> It saves space on the system, allocations are not necessarily pushed to order
> of 2 and should provide performance benefits. Users who disable the memory
> controller can also double check that the memory controller is not allocating
> page_cgroup's.
> 
> Signed-off-by: Balbir Singh <balbir@xxxxxxxxxxxxxxxxxx>

I certainly approve of giving page_cgroups their own kmem_cache
(and agree with Kame that it was overkill for the zones).

But I don't agree with the SLAB_HWCACHE_ALIGN you've slipped into
this version.  That'll be wasting a lot of (all? more than?) the
space you're trying to save with a kmem_cache, won't it?  Let me
talk about that separately, in reply to the mail where you report
the numbers.

Are you proposing this page_cgroup_cache mod for 2.6.25 or for 2.6.26?
I ask because I want to build upon it to fix up some GFP_ flag issues:
I think we end up claiming the page_cgroups are __GFP_MOVABLE when they
should be called __GFP_RECLAIMABLE; but I don't know how seriously we
take MOVABLE/RECLAIMABLE discrepancies at present.

There's a patch I'd also like to build upon from Christoph in -mm
(remove-set_migrateflags.patch), which sheds light on a similar issue
with radix_tree_nodes).  It's importance is again dependent on how
seriously we're taking MOVABLE/RECLAIMABLE discrepancies.

Hugh

> ---
> 
>  mm/memcontrol.c |   12 ++++++++----
>  1 file changed, 8 insertions(+), 4 deletions(-)
> 
> diff -puN mm/memcontrol.c~memory-controller-move-to-own-slab mm/memcontrol.c
> --- linux-2.6.25-rc4/mm/memcontrol.c~memory-controller-move-to-own-slab	2008-03-10 23:22:34.000000000 +0530
> +++ linux-2.6.25-rc4-balbir/mm/memcontrol.c	2008-03-11 10:29:00.000000000 +0530
> @@ -26,6 +26,7 @@
>  #include <linux/backing-dev.h>
>  #include <linux/bit_spinlock.h>
>  #include <linux/rcupdate.h>
> +#include <linux/slab.h>
>  #include <linux/swap.h>
>  #include <linux/spinlock.h>
>  #include <linux/fs.h>
> @@ -35,6 +36,7 @@
>  
>  struct cgroup_subsys mem_cgroup_subsys;
>  static const int MEM_CGROUP_RECLAIM_RETRIES = 5;
> +static struct kmem_cache *page_cgroup_cache;
>  
>  /*
>   * Statistics for memory cgroup.
> @@ -560,7 +562,7 @@ retry:
>  	}
>  	unlock_page_cgroup(page);
>  
> -	pc = kzalloc(sizeof(struct page_cgroup), gfp_mask);
> +	pc = kmem_cache_zalloc(page_cgroup_cache, gfp_mask);
>  	if (pc == NULL)
>  		goto err;
>  
> @@ -622,7 +624,7 @@ retry:
>  		 */
>  		res_counter_uncharge(&mem->res, PAGE_SIZE);
>  		css_put(&mem->css);
> -		kfree(pc);
> +		kmem_cache_free(page_cgroup_cache, pc);
>  		goto retry;
>  	}
>  	page_assign_page_cgroup(page, pc);
> @@ -637,7 +639,7 @@ done:
>  	return 0;
>  out:
>  	css_put(&mem->css);
> -	kfree(pc);
> +	kmem_cache_free(page_cgroup_cache, pc);
>  err:
>  	return -ENOMEM;
>  }
> @@ -695,7 +697,7 @@ void mem_cgroup_uncharge_page(struct pag
>  		res_counter_uncharge(&mem->res, PAGE_SIZE);
>  		css_put(&mem->css);
>  
> -		kfree(pc);
> +		kmem_cache_free(page_cgroup_cache, pc);
>  		return;
>  	}
>  
> @@ -1020,6 +1022,8 @@ mem_cgroup_create(struct cgroup_subsys *
>  	if (unlikely((cont->parent) == NULL)) {
>  		mem = &init_mem_cgroup;
>  		init_mm.mem_cgroup = mem;
> +		page_cgroup_cache = KMEM_CACHE(page_cgroup,
> +					SLAB_HWCACHE_ALIGN | SLAB_PANIC);
>  	} else
>  		mem = kzalloc(sizeof(struct mem_cgroup), GFP_KERNEL);
>  
--
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