Re: [PATCH 9/10] mm/memcg: move lru_lock into lruvec

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Hugh Dickins wrote:
We're nearly there.  Now move lru_lock and irqflags into struct lruvec,
so they are in every zone (for !MEM_RES_CTLR and mem_cgroup_disabled()
cases) and in every memcg lruvec.

Extend the memcg version of page_relock_lruvec() to drop old and take
new lock whenever changing lruvec.  But the memcg will only be stable
once we already have the lock: so, having got it, check if it's still
the lock we want, and retry if not.  It's for this retry that we route
all page lruvec locking through page_relock_lruvec().

No need for lock_page_cgroup() in here (which would entail reinverting
the lock ordering, and _irq'ing all of its calls): the lrucare protocol
when charging (holding old lock while changing owner then acquiring new)
fits correctly with this retry protocol.  In some places we rely also on
page_count 0 preventing further references, in some places on !PageLRU
protecting a page from outside interference: mem_cgroup_move_account()

What if page_relock_lruvec() were preempted for a while, after reading
a valid mem_cgroup from page_cgroup, but before acquiring the lock?
In that case, a rmdir might free the mem_cgroup and its associated
zoneinfo, and we take a spin_lock in freed memory.  But rcu_read_lock()
before we read mem_cgroup keeps it safe: cgroup.c uses synchronize_rcu()
in between pre_destroy (force_empty) and destroy (freeing structures).
mem_cgroup_force_empty() cannot succeed while there's any charge, or any
page on any of its lrus - and checks list_empty() while holding the lock.

Heh, your code is RCU-protected too. =)

On lumpy/compaction isolate you do:

if (!PageLRU(page))
	continue

__isolate_lru_page()

page_relock_rcu_vec()
	rcu_read_lock()
	rcu_dereference()...
	spin_lock()...
	rcu_read_unlock()

You protect page_relock_rcu_vec with switching pointers back to root.

I do:

catch_page_lru()
	rcu_read_lock()
	if (!PageLRU(page))
		return false
	rcu_dereference()...
	spin_lock()...
	rcu_read_unlock()
	if (PageLRU())
		return true
if true
	__isolate_lru_page()

I protect my catch_page_lruvec() with PageLRU() under single rcu-interval with locking.
Thus my code is better, because it not requires switching pointers back to root memcg.

Meanwhile after seeing your patches, I realized that this rcu-protection is
required only for lock-by-pfn in lumpy/compaction isolation.
Thus my locking should be simplified and optimized.


But although we are now fully prepared, in this patch keep on using
the zone->lru_lock for all of its memcgs: so that the cost or benefit
of split locking can be easily compared with the final patch (but
of course, some costs and benefits come earlier in the series).

Signed-off-by: Hugh Dickins<hughd@xxxxxxxxxx>
---
  include/linux/mmzone.h |    4 +-
  include/linux/swap.h   |   13 +++---
  mm/memcontrol.c        |   74 ++++++++++++++++++++++++++-------------
  mm/page_alloc.c        |    2 -
  4 files changed, 59 insertions(+), 34 deletions(-)

--- mmotm.orig/include/linux/mmzone.h	2012-02-18 11:57:42.675524592 -0800
+++ mmotm/include/linux/mmzone.h	2012-02-18 11:58:09.047525220 -0800
@@ -174,6 +174,8 @@ struct zone_reclaim_stat {

  struct lruvec {
  	struct zone *zone;
+	spinlock_t lru_lock;
+	unsigned long irqflags;
  	struct list_head lists[NR_LRU_LISTS];
  	struct zone_reclaim_stat reclaim_stat;
  };
@@ -373,8 +375,6 @@ struct zone {
  	ZONE_PADDING(_pad1_)

  	/* Fields commonly accessed by the page reclaim scanner */
-	spinlock_t		lru_lock;
-	unsigned long		irqflags;
  	struct lruvec		lruvec;

  	unsigned long		pages_scanned;	   /* since last reclaim */
--- mmotm.orig/include/linux/swap.h	2012-02-18 11:57:42.675524592 -0800
+++ mmotm/include/linux/swap.h	2012-02-18 11:58:09.047525220 -0800
@@ -252,25 +252,24 @@ static inline void lru_cache_add_file(st

  static inline spinlock_t *lru_lockptr(struct lruvec *lruvec)
  {
-	return&lruvec->zone->lru_lock;
+	/* Still use per-zone lru_lock */
+	return&lruvec->zone->lruvec.lru_lock;
  }

  static inline void lock_lruvec(struct lruvec *lruvec)
  {
-	struct zone *zone = lruvec->zone;
  	unsigned long irqflags;

-	spin_lock_irqsave(&zone->lru_lock, irqflags);
-	zone->irqflags = irqflags;
+	spin_lock_irqsave(lru_lockptr(lruvec), irqflags);
+	lruvec->irqflags = irqflags;
  }

  static inline void unlock_lruvec(struct lruvec *lruvec)
  {
-	struct zone *zone = lruvec->zone;
  	unsigned long irqflags;

-	irqflags = zone->irqflags;
-	spin_unlock_irqrestore(&zone->lru_lock, irqflags);
+	irqflags = lruvec->irqflags;
+	spin_unlock_irqrestore(lru_lockptr(lruvec), irqflags);
  }

  #ifdef CONFIG_CGROUP_MEM_RES_CTLR
--- mmotm.orig/mm/memcontrol.c	2012-02-18 11:58:02.451525062 -0800
+++ mmotm/mm/memcontrol.c	2012-02-18 11:58:09.051525220 -0800
@@ -1048,39 +1048,64 @@ void page_relock_lruvec(struct page *pag
  	struct page_cgroup *pc;
  	struct lruvec *lruvec;

-	if (mem_cgroup_disabled())
+	if (unlikely(mem_cgroup_disabled())) {
  		lruvec =&page_zone(page)->lruvec;
-	else {
-		pc = lookup_page_cgroup(page);
-		memcg = pc->mem_cgroup;
-		/*
-		 * At present we start up with all page_cgroups initialized
-		 * to zero: correct that to root_mem_cgroup once we see it.
-		 */
-		if (unlikely(!memcg))
-			memcg = pc->mem_cgroup = root_mem_cgroup;
-		/*
-		 * We must reset pc->mem_cgroup back to root before freeing
-		 * a page: avoid additional callouts from hot paths by doing
-		 * it here when we see the page is frozen (can safely be done
-		 * before taking lru_lock because the page is frozen).
-		 */
-		if (memcg != root_mem_cgroup&&  !page_count(page))
-			pc->mem_cgroup = root_mem_cgroup;
-		mz = page_cgroup_zoneinfo(memcg, page);
-		lruvec =&mz->lruvec;
+		if (*lruvp&&  *lruvp != lruvec) {
+			unlock_lruvec(*lruvp);
+			*lruvp = NULL;
+		}
+		if (!*lruvp) {
+			*lruvp = lruvec;
+			lock_lruvec(lruvec);
+		}
+		return;
  	}

+	pc = lookup_page_cgroup(page);
+	/*
+	 * Imagine being preempted for a long time: we need to make sure that
+	 * the structure at pc->mem_cgroup, and structures it links to, cannot
+	 * be freed while we locate and acquire its zone lru_lock.  cgroup's
+	 * synchronize_rcu() between pre_destroy and destroy makes this safe.
+	 */
+	rcu_read_lock();
+again:
+	memcg = rcu_dereference(pc->mem_cgroup);
  	/*
-	 * For the moment, simply lock by zone just as before.
+	 * At present we start up with all page_cgroups initialized
+	 * to zero: here treat NULL as root_mem_cgroup, then correct
+	 * the page_cgroup below once we really have it locked.
  	 */
-	if (*lruvp&&  (*lruvp)->zone != lruvec->zone) {
+	mz = page_cgroup_zoneinfo(memcg ? : root_mem_cgroup, page);
+	lruvec =&mz->lruvec;
+
+	/*
+	 * Sometimes we are called with non-NULL *lruvp spinlock already held:
+	 * hold on if we want the same lock again, otherwise drop and acquire.
+	 */
+	if (*lruvp&&  *lruvp != lruvec) {
  		unlock_lruvec(*lruvp);
  		*lruvp = NULL;
  	}
-	if (!*lruvp)
+	if (!*lruvp) {
+		*lruvp = lruvec;
  		lock_lruvec(lruvec);
-	*lruvp = lruvec;
+		/*
+		 * But pc->mem_cgroup may have changed since we looked...
+		 */
+		if (unlikely(pc->mem_cgroup != memcg))
+			goto again;
+	}
+
+	/*
+	 * We must reset pc->mem_cgroup back to root before freeing a page:
+	 * avoid additional callouts from hot paths by doing it here when we
+	 * see the page is frozen.  Also initialize pc at first use of page.
+	 */
+	if (memcg != root_mem_cgroup&&  (!memcg || !page_count(page)))
+		pc->mem_cgroup = root_mem_cgroup;
+
+	rcu_read_unlock();
  }

  void mem_cgroup_reset_uncharged_to_root(struct page *page)
@@ -4744,6 +4769,7 @@ static int alloc_mem_cgroup_per_zone_inf
  	for (zone = 0; zone<  MAX_NR_ZONES; zone++) {
  		mz =&pn->zoneinfo[zone];
  		mz->lruvec.zone =&NODE_DATA(node)->node_zones[zone];
+		spin_lock_init(&mz->lruvec.lru_lock);
  		for_each_lru(lru)
  			INIT_LIST_HEAD(&mz->lruvec.lists[lru]);
  		mz->usage_in_excess = 0;
--- mmotm.orig/mm/page_alloc.c	2012-02-18 11:57:28.375524252 -0800
+++ mmotm/mm/page_alloc.c	2012-02-18 11:58:09.051525220 -0800
@@ -4360,12 +4360,12 @@ static void __paginginit free_area_init_
  #endif
  		zone->name = zone_names[j];
  		spin_lock_init(&zone->lock);
-		spin_lock_init(&zone->lru_lock);
  		zone_seqlock_init(zone);
  		zone->zone_pgdat = pgdat;

  		zone_pcp_init(zone);
  		zone->lruvec.zone = zone;
+		spin_lock_init(&zone->lruvec.lru_lock);
  		for_each_lru(lru)
  			INIT_LIST_HEAD(&zone->lruvec.lists[lru]);
  		zone->lruvec.reclaim_stat.recent_rotated[0] = 0;

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