Commit dbd4ea78 authored by KAMEZAWA Hiroyuki's avatar KAMEZAWA Hiroyuki Committed by Linus Torvalds
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memcg: add lock to synchronize page accounting and migration 



Introduce a new bit spin lock, PCG_MOVE_LOCK, to synchronize the page
accounting and migration code.  This reworks the locking scheme of
_update_stat() and _move_account() by adding new lock bit PCG_MOVE_LOCK,
which is always taken under IRQ disable.

1. If pages are being migrated from a memcg, then updates to that
   memcg page statistics are protected by grabbing PCG_MOVE_LOCK using
   move_lock_page_cgroup().  In an upcoming commit, memcg dirty page
   accounting will be updating memcg page accounting (specifically: num
   writeback pages) from IRQ context (softirq).  Avoid a deadlocking
   nested spin lock attempt by disabling irq on the local processor when
   grabbing the PCG_MOVE_LOCK.

2. lock for update_page_stat is used only for avoiding race with
   move_account().  So, IRQ awareness of lock_page_cgroup() itself is not
   a problem.  The problem is between mem_cgroup_update_page_stat() and
   mem_cgroup_move_account_page().

Trade-off:
  * Changing lock_page_cgroup() to always disable IRQ (or
    local_bh) has some impacts on performance and I think
    it's bad to disable IRQ when it's not necessary.
  * adding a new lock makes move_account() slower.  Score is
    here.

Performance Impact: moving a 8G anon process.

Before:
	real    0m0.792s
	user    0m0.000s
	sys     0m0.780s

After:
	real    0m0.854s
	user    0m0.000s
	sys     0m0.842s

This score is bad but planned patches for optimization can reduce
this impact.

Signed-off-by: default avatarKAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
Signed-off-by: default avatarGreg Thelen <gthelen@google.com>
Reviewed-by: default avatarMinchan Kim <minchan.kim@gmail.com>
Acked-by: default avatarDaisuke Nishimura <nishimura@mxp.nes.nec.co.jp>
Cc: Andrea Righi <arighi@develer.com>
Cc: Balbir Singh <balbir@linux.vnet.ibm.com>
Cc: Wu Fengguang <fengguang.wu@intel.com>
Signed-off-by: default avatarAndrew Morton <akpm@linux-foundation.org>
Signed-off-by: default avatarLinus Torvalds <torvalds@linux-foundation.org>
parent 2a7106f2

include/linux/page_cgroup.h

+28 −3
Original line number Diff line number Diff line
@@ -35,15 +35,18 @@ struct page_cgroup *lookup_page_cgroup(struct page *page); 


enum {

	/* flags for mem_cgroup */

	PCG_LOCK,  /* page cgroup is locked */

	PCG_LOCK,  /* Lock for pc->mem_cgroup and following bits. */

	PCG_CACHE, /* charged as cache */

	PCG_USED, /* this object is in use. */

	PCG_ACCT_LRU, /* page has been accounted for */

	PCG_MIGRATION, /* under page migration */

	/* 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" */

	PCG_FILE_DIRTY, /* page is dirty */

	PCG_FILE_WRITEBACK, /* page is under writeback */

	PCG_FILE_UNSTABLE_NFS, /* page is NFS unstable */

	PCG_MIGRATION, /* under page migration */

	/* No lock in page_cgroup */

	PCG_ACCT_LRU, /* page has been accounted for (under lru_lock) */

};



#define TESTPCGFLAG(uname, lname)			\
@@ -117,6 +120,10 @@ static inline enum zone_type page_cgroup_zid(struct page_cgroup *pc) 


static inline void lock_page_cgroup(struct page_cgroup *pc)

{

	/*

	 * Don't take this lock in IRQ context.

	 * This lock is for pc->mem_cgroup, USED, CACHE, MIGRATION

	 */

	bit_spin_lock(PCG_LOCK, &pc->flags);

}
@@ -130,6 +137,24 @@ static inline int page_is_cgroup_locked(struct page_cgroup *pc) 
	return bit_spin_is_locked(PCG_LOCK, &pc->flags);

}



static inline void move_lock_page_cgroup(struct page_cgroup *pc,

	unsigned long *flags)

{

	/*

	 * We know updates to pc->flags of page cache's stats are from both of

	 * usual context or IRQ context. Disable IRQ to avoid deadlock.

	 */

	local_irq_save(*flags);

	bit_spin_lock(PCG_MOVE_LOCK, &pc->flags);

}



static inline void move_unlock_page_cgroup(struct page_cgroup *pc,

	unsigned long *flags)

{

	bit_spin_unlock(PCG_MOVE_LOCK, &pc->flags);

	local_irq_restore(*flags);

}



#else /* CONFIG_CGROUP_MEM_RES_CTLR */

struct page_cgroup;

mm/memcontrol.c

+7 −2
Original line number Diff line number Diff line
@@ -1606,6 +1606,7 @@ void mem_cgroup_update_page_stat(struct page *page, 
	struct mem_cgroup *mem;

	struct page_cgroup *pc = lookup_page_cgroup(page);

	bool need_unlock = false;

	unsigned long uninitialized_var(flags);



	if (unlikely(!pc))

		return;
@@ -1617,7 +1618,7 @@ void mem_cgroup_update_page_stat(struct page *page, 
	/* pc->mem_cgroup is unstable ? */

	if (unlikely(mem_cgroup_stealed(mem))) {

		/* take a lock against to access pc->mem_cgroup */

		lock_page_cgroup(pc);

		move_lock_page_cgroup(pc, &flags);

		need_unlock = true;

		mem = pc->mem_cgroup;

		if (!mem || !PageCgroupUsed(pc))
@@ -1640,7 +1641,7 @@ void mem_cgroup_update_page_stat(struct page *page, 


out:

	if (unlikely(need_unlock))

		unlock_page_cgroup(pc);

		move_unlock_page_cgroup(pc, &flags);

	rcu_read_unlock();

	return;

}
@@ -2211,9 +2212,13 @@ static int mem_cgroup_move_account(struct page_cgroup *pc, 
		struct mem_cgroup *from, struct mem_cgroup *to, bool uncharge)

{

	int ret = -EINVAL;

	unsigned long flags;



	lock_page_cgroup(pc);

	if (PageCgroupUsed(pc) && pc->mem_cgroup == from) {

		move_lock_page_cgroup(pc, &flags);

		__mem_cgroup_move_account(pc, from, to, uncharge);

		move_unlock_page_cgroup(pc, &flags);

		ret = 0;

	}

	unlock_page_cgroup(pc);

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