Re: [PATCH 4/4] memcg: simplify LRU handling by new rule

[Johannes Weiner <hannes@cmpxchg.org>]

On Wed, Dec 14, 2011 at 04:52:26PM +0900, KAMEZAWA Hiroyuki wrote:
> From: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
> 
> Now, at LRU handling, memory cgroup needs to do complicated works
> to see valid pc->mem_cgroup, which may be overwritten.
> 
> This patch is for relaxing the protocol. This patch guarantees
>    - when pc->mem_cgroup is overwritten, page must not be on LRU.
> 
> By this, LRU routine can believe pc->mem_cgroup and don't need to
> check bits on pc->flags. This new rule may adds small overheads to
> swapin. But in most case, lru handling gets faster.
> 
> After this patch, PCG_ACCT_LRU bit is obsolete and removed.
> 
> Signed-off-by: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
>
> ---
>  include/linux/page_cgroup.h |    8 -----
>  mm/memcontrol.c             |   72 ++++++++++--------------------------------
>  2 files changed, 17 insertions(+), 63 deletions(-)

This, too, speaks for itself and the logic seems sound to me.

Acked-by: Johannes Weiner <hannes@cmpxchg.org>

Minor style things:

> @@ -974,30 +974,8 @@ struct lruvec *mem_cgroup_lru_add_list(struct zone *zone, struct page *page,
>  		return &zone->lruvec;
>  
>  	pc = lookup_page_cgroup(page);
> -	VM_BUG_ON(PageCgroupAcctLRU(pc));
> -	/*
> -	 * putback:				charge:
> -	 * SetPageLRU				SetPageCgroupUsed
> -	 * smp_mb				smp_mb
> -	 * PageCgroupUsed && add to memcg LRU	PageLRU && add to memcg LRU
> -	 *
> -	 * Ensure that one of the two sides adds the page to the memcg
> -	 * LRU during a race.
> -	 */
> -	smp_mb();
> -	/*
> -	 * If the page is uncharged, it may be freed soon, but it
> -	 * could also be swap cache (readahead, swapoff) that needs to
> -	 * be reclaimable in the future.  root_mem_cgroup will babysit
> -	 * it for the time being.
> -	 */
> -	if (PageCgroupUsed(pc)) {
> -		/* Ensure pc->mem_cgroup is visible after reading PCG_USED. */
> -		smp_rmb();
> -		memcg = pc->mem_cgroup;
> -		SetPageCgroupAcctLRU(pc);
> -	} else
> -		memcg = root_mem_cgroup;
> +	memcg = pc->mem_cgroup;
> +	VM_BUG_ON(!memcg);
>  	mz = page_cgroup_zoneinfo(memcg, page);

I think the memcg local variable is not really needed anymore.

Also, please don't add bug-ons for simple NULL tests, they are
redundant when the dereference would blow up just as well.

> @@ -2399,6 +2368,8 @@ void mem_cgroup_split_huge_fixup(struct page *head)
>  {
>  	struct page_cgroup *head_pc = lookup_page_cgroup(head);
>  	struct page_cgroup *pc;
> +	struct mem_cgroup_per_zone *mz;
> +	enum lru_list lru;
>  	int i;

You broke the reverse christmas tree sorting!

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