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

Johannes Weiner
2011-12-19T15:48:24Z

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 After this patch, PCG_ACCT_LRU bit is obsolete and removed.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!