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Subject[RFC PATCH v2 1/8] mm, memcontrol.c: make memcg lru stats thread-safe without lru_lock
Date
lru_lock needs to be held to update memcg LRU statistics.  This
requirement arises fairly naturally based on when the stats are updated
because callers are holding lru_lock already.

In preparation for allowing concurrent adds and removes from the LRU,
however, make concurrent updates to these statistics safe without
lru_lock. The lock continues to be held until later in the series, when
it is replaced with a rwlock that also disables preemption, maintaining
the assumption of __mod_lru_zone_size, which is introduced here.

Follow the existing pattern for statistics in memcontrol.h by using a
combination of per-cpu counters and atomics.

Remove the negative statistics warning from ca707239e8a7 ("mm:
update_lru_size warn and reset bad lru_size"). Although an earlier
version of this patch updated the warning to account for the error
introduced by the per-cpu counters, Hugh says this warning has not been
seen in the wild and that for simplicity's sake it should probably just
be removed.

Signed-off-by: Daniel Jordan <daniel.m.jordan@oracle.com>
---
include/linux/memcontrol.h | 43 +++++++++++++++++++++++++++++---------
mm/memcontrol.c | 29 +++++++------------------
2 files changed, 40 insertions(+), 32 deletions(-)

diff --git a/include/linux/memcontrol.h b/include/linux/memcontrol.h
index d99b71bc2c66..6377dc76dc41 100644
--- a/include/linux/memcontrol.h
+++ b/include/linux/memcontrol.h
@@ -99,7 +99,8 @@ struct mem_cgroup_reclaim_iter {
};

struct lruvec_stat {
- long count[NR_VM_NODE_STAT_ITEMS];
+ long node[NR_VM_NODE_STAT_ITEMS];
+ long lru_zone_size[MAX_NR_ZONES][NR_LRU_LISTS];
};

/*
@@ -109,9 +110,8 @@ struct mem_cgroup_per_node {
struct lruvec lruvec;

struct lruvec_stat __percpu *lruvec_stat_cpu;
- atomic_long_t lruvec_stat[NR_VM_NODE_STAT_ITEMS];
-
- unsigned long lru_zone_size[MAX_NR_ZONES][NR_LRU_LISTS];
+ atomic_long_t node_stat[NR_VM_NODE_STAT_ITEMS];
+ atomic_long_t lru_zone_size[MAX_NR_ZONES][NR_LRU_LISTS];

struct mem_cgroup_reclaim_iter iter[DEF_PRIORITY + 1];

@@ -446,7 +446,7 @@ unsigned long mem_cgroup_get_lru_size(struct lruvec *lruvec, enum lru_list lru)

mz = container_of(lruvec, struct mem_cgroup_per_node, lruvec);
for (zid = 0; zid < MAX_NR_ZONES; zid++)
- nr_pages += mz->lru_zone_size[zid][lru];
+ nr_pages += atomic64_read(&mz->lru_zone_size[zid][lru]);
return nr_pages;
}

@@ -457,7 +457,7 @@ unsigned long mem_cgroup_get_zone_lru_size(struct lruvec *lruvec,
struct mem_cgroup_per_node *mz;

mz = container_of(lruvec, struct mem_cgroup_per_node, lruvec);
- return mz->lru_zone_size[zone_idx][lru];
+ return atomic64_read(&mz->lru_zone_size[zone_idx][lru]);
}

void mem_cgroup_handle_over_high(void);
@@ -575,7 +575,7 @@ static inline unsigned long lruvec_page_state(struct lruvec *lruvec,
return node_page_state(lruvec_pgdat(lruvec), idx);

pn = container_of(lruvec, struct mem_cgroup_per_node, lruvec);
- x = atomic_long_read(&pn->lruvec_stat[idx]);
+ x = atomic_long_read(&pn->node_stat[idx]);
#ifdef CONFIG_SMP
if (x < 0)
x = 0;
@@ -601,12 +601,12 @@ static inline void __mod_lruvec_state(struct lruvec *lruvec,
__mod_memcg_state(pn->memcg, idx, val);

/* Update lruvec */
- x = val + __this_cpu_read(pn->lruvec_stat_cpu->count[idx]);
+ x = val + __this_cpu_read(pn->lruvec_stat_cpu->node[idx]);
if (unlikely(abs(x) > MEMCG_CHARGE_BATCH)) {
- atomic_long_add(x, &pn->lruvec_stat[idx]);
+ atomic_long_add(x, &pn->node_stat[idx]);
x = 0;
}
- __this_cpu_write(pn->lruvec_stat_cpu->count[idx], x);
+ __this_cpu_write(pn->lruvec_stat_cpu->node[idx], x);
}

static inline void mod_lruvec_state(struct lruvec *lruvec,
@@ -619,6 +619,29 @@ static inline void mod_lruvec_state(struct lruvec *lruvec,
local_irq_restore(flags);
}

+/**
+ * __mod_lru_zone_size - update memcg lru statistics in batches
+ *
+ * Updates memcg lru statistics using per-cpu counters that spill into atomics
+ * above a threshold.
+ *
+ * Assumes that the caller has disabled preemption. IRQs may be enabled
+ * because this function is not called from irq context.
+ */
+static inline void __mod_lru_zone_size(struct mem_cgroup_per_node *pn,
+ enum lru_list lru, int zid, int val)
+{
+ long x;
+ struct lruvec_stat __percpu *lruvec_stat_cpu = pn->lruvec_stat_cpu;
+
+ x = val + __this_cpu_read(lruvec_stat_cpu->lru_zone_size[zid][lru]);
+ if (unlikely(abs(x) > MEMCG_CHARGE_BATCH)) {
+ atomic_long_add(x, &pn->lru_zone_size[zid][lru]);
+ x = 0;
+ }
+ __this_cpu_write(lruvec_stat_cpu->lru_zone_size[zid][lru], x);
+}
+
static inline void __mod_lruvec_page_state(struct page *page,
enum node_stat_item idx, int val)
{
diff --git a/mm/memcontrol.c b/mm/memcontrol.c
index 2bd3df3d101a..5463ad160e10 100644
--- a/mm/memcontrol.c
+++ b/mm/memcontrol.c
@@ -962,36 +962,20 @@ struct lruvec *mem_cgroup_page_lruvec(struct page *page, struct pglist_data *pgd
* @zid: zone id of the accounted pages
* @nr_pages: positive when adding or negative when removing
*
- * This function must be called under lru_lock, just before a page is added
- * to or just after a page is removed from an lru list (that ordering being
- * so as to allow it to check that lru_size 0 is consistent with list_empty).
+ * This function must be called just before a page is added to, or just after a
+ * page is removed from, an lru list. Callers aren't required to hold lru_lock
+ * because these statistics use per-cpu counters and atomics.
*/
void mem_cgroup_update_lru_size(struct lruvec *lruvec, enum lru_list lru,
int zid, int nr_pages)
{
struct mem_cgroup_per_node *mz;
- unsigned long *lru_size;
- long size;

if (mem_cgroup_disabled())
return;

mz = container_of(lruvec, struct mem_cgroup_per_node, lruvec);
- lru_size = &mz->lru_zone_size[zid][lru];
-
- if (nr_pages < 0)
- *lru_size += nr_pages;
-
- size = *lru_size;
- if (WARN_ONCE(size < 0,
- "%s(%p, %d, %d): lru_size %ld\n",
- __func__, lruvec, lru, nr_pages, size)) {
- VM_BUG_ON(1);
- *lru_size = 0;
- }
-
- if (nr_pages > 0)
- *lru_size += nr_pages;
+ __mod_lru_zone_size(mz, lru, zid, nr_pages);
}

bool task_in_mem_cgroup(struct task_struct *task, struct mem_cgroup *memcg)
@@ -1833,9 +1817,10 @@ static int memcg_hotplug_cpu_dead(unsigned int cpu)
struct mem_cgroup_per_node *pn;

pn = mem_cgroup_nodeinfo(memcg, nid);
- x = this_cpu_xchg(pn->lruvec_stat_cpu->count[i], 0);
+ x = this_cpu_xchg(pn->lruvec_stat_cpu->node[i],
+ 0);
if (x)
- atomic_long_add(x, &pn->lruvec_stat[i]);
+ atomic_long_add(x, &pn->node_stat[i]);
}
}

--
2.18.0
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