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

Michal Hocko
2011-12-20T16:16:24Z

On Wed 14-12-11 16:52:26, 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.

How the patch guarantees that? I do not see any enforcement. In fact we
depend on the previous patches, don't we.

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 After this patch, PCG_ACCT_LRU bit is obsolete and removed.

It makes things much more simpler. I just think it needs a better
description.

>
> 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(-)
>
> diff --git a/include/linux/page_cgroup.h b/include/linux/page_cgroup.h
> index aaa60da..2cddacf 100644
> --- a/include/linux/page_cgroup.h
> +++ b/include/linux/page_cgroup.h
> @@ -10,8 +10,6 @@ enum {
> /* flags for mem_cgroup and file and I/O status */
> PCG_MOVE_LOCK, /* For race between move_account v.s. following bits */
> PCG_FILE_MAPPED, /* page is accounted as "mapped" */
> - /* No lock in page_cgroup */
> - PCG_ACCT_LRU, /* page has been accounted for (under lru_lock) */
> __NR_PCG_FLAGS,
> };
>
> @@ -75,12 +73,6 @@ TESTPCGFLAG(Used, USED)
> CLEARPCGFLAG(Used, USED)
> SETPCGFLAG(Used, USED)
>
> -SETPCGFLAG(AcctLRU, ACCT_LRU)
> -CLEARPCGFLAG(AcctLRU, ACCT_LRU)
> -TESTPCGFLAG(AcctLRU, ACCT_LRU)
> -TESTCLEARPCGFLAG(AcctLRU, ACCT_LRU)
> -
> -
> SETPCGFLAG(FileMapped, FILE_MAPPED)
> CLEARPCGFLAG(FileMapped, FILE_MAPPED)
> TESTPCGFLAG(FileMapped, FILE_MAPPED)
> diff --git a/mm/memcontrol.c b/mm/memcontrol.c
> index 2ae973d..d5e21e7 100644
> --- a/mm/memcontrol.c
> +++ b/mm/memcontrol.c
> @@ -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);
> /* compound_order() is stabilized through lru_lock */
> MEM_CGROUP_ZSTAT(mz, lru) += 1 << compound_order(page);
> @@ -1024,18 +1002,8 @@ void mem_cgroup_lru_del_list(struct page *page, enum lru_list lru)
> return;
>
> pc = lookup_page_cgroup(page);
> - /*
> - * root_mem_cgroup babysits uncharged LRU pages, but
> - * PageCgroupUsed is cleared when the page is about to get
> - * freed. PageCgroupAcctLRU remembers whether the
> - * LRU-accounting happened against pc->mem_cgroup or
> - * root_mem_cgroup.
> - */
> - if (TestClearPageCgroupAcctLRU(pc)) {
> - VM_BUG_ON(!pc->mem_cgroup);
> - memcg = pc->mem_cgroup;
> - } else
> - memcg = root_mem_cgroup;
> + memcg = pc->mem_cgroup;
> + VM_BUG_ON(!memcg);
> mz = page_cgroup_zoneinfo(memcg, page);
> /* huge page split is done under lru_lock. so, we have no races. */
> MEM_CGROUP_ZSTAT(mz, lru) -= 1 << compound_order(page);
> @@ -2377,6 +2345,7 @@ static void __mem_cgroup_commit_charge(struct mem_cgroup *memcg,
>
> mem_cgroup_charge_statistics(memcg, PageCgroupCache(pc), nr_pages);
> unlock_page_cgroup(pc);
> + WARN_ON_ONCE(PageLRU(page));
> /*
> * "charge_statistics" updated event counter. Then, check it.
> * Insert ancestor (and ancestor's ancestors), to softlimit RB-tree.
> @@ -2388,7 +2357,7 @@ static void __mem_cgroup_commit_charge(struct mem_cgroup *memcg,
> #ifdef CONFIG_TRANSPARENT_HUGEPAGE
>
> #define PCGF_NOCOPY_AT_SPLIT ((1 << PCG_LOCK) | (1 << PCG_MOVE_LOCK) |\
> - (1 << PCG_ACCT_LRU) | (1 << PCG_MIGRATION))
> + (1 << PCG_MIGRATION))
> /*
> * Because tail pages are not marked as "used", set it. We're under
> * zone->lru_lock, 'splitting on pmd' and compound_lock.
> @@ -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;
>
> if (mem_cgroup_disabled())
> @@ -2407,23 +2378,15 @@ void mem_cgroup_split_huge_fixup(struct page *head)
> pc = head_pc + i;
> pc->mem_cgroup = head_pc->mem_cgroup;
> smp_wmb();/* see __commit_charge() */
> - /*
> - * LRU flags cannot be copied because we need to add tail
> - * page to LRU by generic call and our hooks will be called.
> - */
> pc->flags = head_pc->flags & ~PCGF_NOCOPY_AT_SPLIT;
> }
> -
> - if (PageCgroupAcctLRU(head_pc)) {
> - enum lru_list lru;
> - struct mem_cgroup_per_zone *mz;
> - /*
> - * We hold lru_lock, then, reduce counter directly.
> - */
> - lru = page_lru(head);
> - mz = page_cgroup_zoneinfo(head_pc->mem_cgroup, head);
> - MEM_CGROUP_ZSTAT(mz, lru) -= HPAGE_PMD_NR - 1;
> - }
> + /*
> + * Tail pages will be added to LRU.
> + * We hold lru_lock,then,reduce counter directly.
> + */
> + lru = page_lru(head);
> + mz = page_cgroup_zoneinfo(head_pc->mem_cgroup, head);
> + MEM_CGROUP_ZSTAT(mz, lru) -= HPAGE_PMD_NR - 1;
> }
> #endif
>
> @@ -2656,10 +2619,9 @@ int mem_cgroup_cache_charge(struct page *page, struct mm_struct *mm,
> if (!page_is_file_cache(page))
> type = MEM_CGROUP_CHARGE_TYPE_SHMEM;
>
> - if (!PageSwapCache(page)) {
> + if (!PageSwapCache(page))
> ret = mem_cgroup_charge_common(page, mm, gfp_mask, type);
> - WARN_ON_ONCE(PageLRU(page));
> - } else { /* page is swapcache/shmem */
> + else { /* page is swapcache/shmem */
> ret = mem_cgroup_try_charge_swapin(mm, page, gfp_mask, &memcg);
> if (!ret)
> __mem_cgroup_commit_charge_swapin(page, memcg, type);
> --
> 1.7.4.1
>
>
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--
Michal Hocko
SUSE Labs
SUSE LINUX s.r.o.
Lihovarska 1060/12
190 00 Praha 9
Czech Republic