On Mon, Feb 13, 2012 at 7:14 PM, KAMEZAWA Hiroyuki <kamezawa.hiroyu@xxxxxxxxxxxxxx> wrote: > From ad2905362ef58a44d96a325193ab384739418050 Mon Sep 17 00:00:00 2001 > From: KAMEZAWA Hiroyuki <kamezawa.hiroyu@xxxxxxxxxxxxxx> > Date: Thu, 2 Feb 2012 11:49:59 +0900 > Subject: [PATCH 4/6] memcg: use new logic for page stat accounting. > > Now, page-stat-per-memcg is recorded into per page_cgroup flag by > duplicating page's status into the flag. The reason is that memcg > has a feature to move a page from a group to another group and we > have race between "move" and "page stat accounting", > > Under current logic, assume CPU-A and CPU-B. CPU-A does "move" > and CPU-B does "page stat accounting". > > When CPU-A goes 1st, > > CPU-A CPU-B > update "struct page" info. > move_lock_mem_cgroup(memcg) > see flags pc->flags? > copy page stat to new group > overwrite pc->mem_cgroup. > move_unlock_mem_cgroup(memcg) > move_lock_mem_cgroup(mem) > set pc->flags > update page stat accounting > move_unlock_mem_cgroup(mem) > > stat accounting is guarded by move_lock_mem_cgroup() and "move" > logic (CPU-A) doesn't see changes in "struct page" information. > > But it's costly to have the same information both in 'struct page' and > 'struct page_cgroup'. And, there is a potential problem. > > For example, assume we have PG_dirty accounting in memcg. > PG_..is a flag for struct page. > PCG_ is a flag for struct page_cgroup. > (This is just an example. The same problem can be found in any > kind of page stat accounting.) > > CPU-A CPU-B > TestSet PG_dirty > (delay) TestClear PG_dirty_ PG_dirty > if (TestClear(PCG_dirty)) > memcg->nr_dirty-- > if (TestSet(PCG_dirty)) > memcg->nr_dirty++ > > @@ -141,6 +141,31 @@ static inline bool mem_cgroup_disabled(void) > return false; > } > > +void __mem_cgroup_begin_update_page_stat(struct page *page, > + bool *lock, unsigned long *flags); > + > +static inline void mem_cgroup_begin_update_page_stat(struct page *page, > + bool *lock, unsigned long *flags) > +{ > + if (mem_cgroup_disabled()) > + return; > + rcu_read_lock(); > + *lock = false; This seems like a strange place to set *lock=false. I think it's clearer if __mem_cgroup_begin_update_page_stat() is the only routine that sets or clears *lock. But I do see that in patch 6/6 'memcg: fix performance of mem_cgroup_begin_update_page_stat()' this position is required. > + return __mem_cgroup_begin_update_page_stat(page, lock, flags); > +} > diff --git a/mm/memcontrol.c b/mm/memcontrol.c > index ecf8856..30afea5 100644 > --- a/mm/memcontrol.c > +++ b/mm/memcontrol.c > @@ -1877,32 +1877,54 @@ bool mem_cgroup_handle_oom(struct mem_cgroup *memcg, gfp_t mask) > * If there is, we take a lock. > */ > > +void __mem_cgroup_begin_update_page_stat(struct page *page, > + bool *lock, unsigned long *flags) > +{ > + struct mem_cgroup *memcg; > + struct page_cgroup *pc; > + > + pc = lookup_page_cgroup(page); > +again: > + memcg = pc->mem_cgroup; > + if (unlikely(!memcg || !PageCgroupUsed(pc))) > + return; > + if (!mem_cgroup_stealed(memcg)) > + return; > + > + move_lock_mem_cgroup(memcg, flags); > + if (memcg != pc->mem_cgroup || !PageCgroupUsed(pc)) { > + move_unlock_mem_cgroup(memcg, flags); > + goto again; > + } > + *lock = true; > +} > + > +void __mem_cgroup_end_update_page_stat(struct page *page, > + bool *lock, unsigned long *flags) 'lock' looks like an unused parameter. If so, then remove it. > +{ > + struct page_cgroup *pc = lookup_page_cgroup(page); > + > + /* > + * It's guaranteed that pc->mem_cgroup never changes while > + * lock is held Please continue comment describing what provides this guarantee. I assume it is because rcu_read_lock() is held by mem_cgroup_begin_update_page_stat(). Maybe it's best to to just make small reference to the locking protocol description in mem_cgroup_start_move(). > + */ > + move_unlock_mem_cgroup(pc->mem_cgroup, flags); > +} > + > + I think it would be useful to add a small comment here declaring that all callers of this routine must be in a mem_cgroup_begin_update_page_stat(), mem_cgroup_end_update_page_stat() critical section to keep pc->mem_cgroup stable. > void mem_cgroup_update_page_stat(struct page *page, > enum mem_cgroup_page_stat_item idx, int val) > { -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@xxxxxxxxx. For more info on Linux MM, see: http://www.linux-mm.org/ . Fight unfair telecom internet charges in Canada: sign http://stopthemeter.ca/ Don't email: <a href