[PATCH 3/3] mm: memcontrol: fix excessive complexity in memory.stat reporting

From: Johannes Weiner
Date: Fri Nov 03 2017 - 11:34:08 EST

We've seen memory.stat reads in top-level cgroups take up to fourteen
seconds during a userspace bug that created tens of thousands of ghost
cgroups pinned by lingering page cache.

Even with a more reasonable number of cgroups, aggregating memory.stat
is unnecessarily heavy. The complexity is this:

nr_cgroups * nr_stat_items * nr_possible_cpus

where the stat items are ~70 at this point. With 128 cgroups and 128
CPUs - decent, not enormous setups - reading the top-level memory.stat
has to aggregate over a million per-cpu counters. This doesn't scale.

Instead of spreading the source of truth across all CPUs, use the
per-cpu counters merely to batch updates to shared atomic counters.

This is the same as the per-cpu stocks we use for charging memory to
the shared atomic page_counters, and also the way the global vmstat
counters are implemented.

Vmstat has elaborate spilling thresholds that depend on the number of
CPUs, amount of memory, and memory pressure - carefully balancing the
cost of counter updates with the amount of per-cpu error. That's
because the vmstat counters are system-wide, but also used for
decisions inside the kernel (e.g. NR_FREE_PAGES in the
allocator). Neither is true for the memory controller.

Use the same static batch size we already use for page_counter updates
during charging. The per-cpu error in the stats will be 128k, which is
an acceptable ratio of cores to memory accounting granularity.

Signed-off-by: Johannes Weiner <hannes@xxxxxxxxxxx>
---
include/linux/memcontrol.h | 96 +++++++++++++++++++++++++++---------------
mm/memcontrol.c | 101 +++++++++++++++++++++++----------------------
2 files changed, 113 insertions(+), 84 deletions(-)

diff --git a/include/linux/memcontrol.h b/include/linux/memcontrol.h
index 1ffc54ac4cc9..882046863581 100644
--- a/include/linux/memcontrol.h
+++ b/include/linux/memcontrol.h
@@ -108,7 +108,10 @@ struct lruvec_stat {
*/
struct mem_cgroup_per_node {
struct lruvec lruvec;
- struct lruvec_stat __percpu *lruvec_stat;
+
+ 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];

struct mem_cgroup_reclaim_iter iter[DEF_PRIORITY + 1];
@@ -227,10 +230,10 @@ struct mem_cgroup {
spinlock_t move_lock;
struct task_struct *move_lock_task;
unsigned long move_lock_flags;
- /*
- * percpu counter.
- */
- struct mem_cgroup_stat_cpu __percpu *stat;
+
+ struct mem_cgroup_stat_cpu __percpu *stat_cpu;
+ atomic_long_t stat[MEMCG_NR_STAT];
+ atomic_long_t events[MEMCG_NR_EVENTS];

unsigned long socket_pressure;

@@ -265,6 +268,12 @@ struct mem_cgroup {
/* WARNING: nodeinfo must be the last member here */
};

+/*
+ * size of first charge trial. "32" comes from vmscan.c's magic value.
+ * TODO: maybe necessary to use big numbers in big irons.
+ */
+#define MEMCG_CHARGE_BATCH 32U
+
extern struct mem_cgroup *root_mem_cgroup;

static inline bool mem_cgroup_disabled(void)
@@ -485,32 +494,38 @@ void unlock_page_memcg(struct page *page);
static inline unsigned long memcg_page_state(struct mem_cgroup *memcg,
int idx)
{
- long val = 0;
- int cpu;
-
- for_each_possible_cpu(cpu)
- val += per_cpu(memcg->stat->count[idx], cpu);
-
- if (val < 0)
- val = 0;
-
- return val;
+ long x = atomic_long_read(&memcg->stat[idx]);
+#ifdef CONFIG_SMP
+ if (x < 0)
+ x = 0;
+#endif
+ return x;
}

/* idx can be of type enum memcg_stat_item or node_stat_item */
static inline void __mod_memcg_state(struct mem_cgroup *memcg,
int idx, int val)
{
- if (!mem_cgroup_disabled())
- __this_cpu_add(memcg->stat->count[idx], val);
+ long x;
+
+ if (mem_cgroup_disabled())
+ return;
+
+ x = val + __this_cpu_read(memcg->stat_cpu->count[idx]);
+ if (unlikely(abs(x) > MEMCG_CHARGE_BATCH)) {
+ atomic_long_add(x, &memcg->stat[idx]);
+ x = 0;
+ }
+ __this_cpu_write(memcg->stat_cpu->count[idx], x);
}

/* idx can be of type enum memcg_stat_item or node_stat_item */
static inline void mod_memcg_state(struct mem_cgroup *memcg,
int idx, int val)
{
- if (!mem_cgroup_disabled())
- this_cpu_add(memcg->stat->count[idx], val);
+ preempt_disable();
+ __mod_memcg_state(memcg, idx, val);
+ preempt_enable();
}

/**
@@ -548,26 +563,25 @@ static inline unsigned long lruvec_page_state(struct lruvec *lruvec,
enum node_stat_item idx)
{
struct mem_cgroup_per_node *pn;
- long val = 0;
- int cpu;
+ long x;

if (mem_cgroup_disabled())
return node_page_state(lruvec_pgdat(lruvec), idx);

pn = container_of(lruvec, struct mem_cgroup_per_node, lruvec);
- for_each_possible_cpu(cpu)
- val += per_cpu(pn->lruvec_stat->count[idx], cpu);
-
- if (val < 0)
- val = 0;
-
- return val;
+ x = atomic_long_read(&pn->lruvec_stat[idx]);
+#ifdef CONFIG_SMP
+ if (x < 0)
+ x = 0;
+#endif
+ return x;
}

static inline void __mod_lruvec_state(struct lruvec *lruvec,
enum node_stat_item idx, int val)
{
struct mem_cgroup_per_node *pn;
+ long x;

/* Update node */
__mod_node_page_state(lruvec_pgdat(lruvec), idx, val);
@@ -581,7 +595,12 @@ static inline void __mod_lruvec_state(struct lruvec *lruvec,
__mod_memcg_state(pn->memcg, idx, val);

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

static inline void mod_lruvec_state(struct lruvec *lruvec,
@@ -624,16 +643,25 @@ unsigned long mem_cgroup_soft_limit_reclaim(pg_data_t *pgdat, int order,
static inline void __count_memcg_events(struct mem_cgroup *memcg,
int idx, unsigned long count)
{
- if (!mem_cgroup_disabled())
- __this_cpu_add(memcg->stat->events[idx], count);
+ unsigned long x;
+
+ if (mem_cgroup_disabled())
+ return;
+
+ x = count + __this_cpu_read(memcg->stat_cpu->events[idx]);
+ if (unlikely(x > MEMCG_CHARGE_BATCH)) {
+ atomic_long_add(x, &memcg->events[idx]);
+ x = 0;
+ }
+ __this_cpu_write(memcg->stat_cpu->events[idx], x);
}

-/* idx can be of type enum memcg_event_item or vm_event_item */
static inline void count_memcg_events(struct mem_cgroup *memcg,
int idx, unsigned long count)
{
- if (!mem_cgroup_disabled())
- this_cpu_add(memcg->stat->events[idx], count);
+ preempt_disable();
+ __count_memcg_events(memcg, idx, count);
+ preempt_enable();
}

/* idx can be of type enum memcg_event_item or vm_event_item */
diff --git a/mm/memcontrol.c b/mm/memcontrol.c
index dabbc076e503..40d1ef65fbd2 100644
--- a/mm/memcontrol.c
+++ b/mm/memcontrol.c
@@ -542,39 +542,10 @@ mem_cgroup_largest_soft_limit_node(struct mem_cgroup_tree_per_node *mctz)
return mz;
}

-/*
- * Return page count for single (non recursive) @memcg.
- *
- * Implementation Note: reading percpu statistics for memcg.
- *
- * Both of vmstat[] and percpu_counter has threshold and do periodic
- * synchronization to implement "quick" read. There are trade-off between
- * reading cost and precision of value. Then, we may have a chance to implement
- * a periodic synchronization of counter in memcg's counter.
- *
- * But this _read() function is used for user interface now. The user accounts
- * memory usage by memory cgroup and he _always_ requires exact value because
- * he accounts memory. Even if we provide quick-and-fuzzy read, we always
- * have to visit all online cpus and make sum. So, for now, unnecessary
- * synchronization is not implemented. (just implemented for cpu hotplug)
- *
- * If there are kernel internal actions which can make use of some not-exact
- * value, and reading all cpu value can be performance bottleneck in some
- * common workload, threshold and synchronization as vmstat[] should be
- * implemented.
- *
- * The parameter idx can be of type enum memcg_event_item or vm_event_item.
- */
-
static unsigned long memcg_sum_events(struct mem_cgroup *memcg,
int event)
{
- unsigned long val = 0;
- int cpu;
-
- for_each_possible_cpu(cpu)
- val += per_cpu(memcg->stat->events[event], cpu);
- return val;
+ return atomic_long_read(&memcg->events[event]);
}

static void mem_cgroup_charge_statistics(struct mem_cgroup *memcg,
@@ -606,7 +577,7 @@ static void mem_cgroup_charge_statistics(struct mem_cgroup *memcg,
nr_pages = -nr_pages; /* for event */
}

- __this_cpu_add(memcg->stat->nr_page_events, nr_pages);
+ __this_cpu_add(memcg->stat_cpu->nr_page_events, nr_pages);
}

unsigned long mem_cgroup_node_nr_lru_pages(struct mem_cgroup *memcg,
@@ -642,8 +613,8 @@ static bool mem_cgroup_event_ratelimit(struct mem_cgroup *memcg,
{
unsigned long val, next;

- val = __this_cpu_read(memcg->stat->nr_page_events);
- next = __this_cpu_read(memcg->stat->targets[target]);
+ val = __this_cpu_read(memcg->stat_cpu->nr_page_events);
+ next = __this_cpu_read(memcg->stat_cpu->targets[target]);
/* from time_after() in jiffies.h */
if ((long)(next - val) < 0) {
switch (target) {
@@ -659,7 +630,7 @@ static bool mem_cgroup_event_ratelimit(struct mem_cgroup *memcg,
default:
break;
}
- __this_cpu_write(memcg->stat->targets[target], next);
+ __this_cpu_write(memcg->stat_cpu->targets[target], next);
return true;
}
return false;
@@ -1707,11 +1678,6 @@ void unlock_page_memcg(struct page *page)
}
EXPORT_SYMBOL(unlock_page_memcg);

-/*
- * size of first charge trial. "32" comes from vmscan.c's magic value.
- * TODO: maybe necessary to use big numbers in big irons.
- */
-#define CHARGE_BATCH 32U
struct memcg_stock_pcp {
struct mem_cgroup *cached; /* this never be root cgroup */
unsigned int nr_pages;
@@ -1739,7 +1705,7 @@ static bool consume_stock(struct mem_cgroup *memcg, unsigned int nr_pages)
unsigned long flags;
bool ret = false;

- if (nr_pages > CHARGE_BATCH)
+ if (nr_pages > MEMCG_CHARGE_BATCH)
return ret;

local_irq_save(flags);
@@ -1808,7 +1774,7 @@ static void refill_stock(struct mem_cgroup *memcg, unsigned int nr_pages)
}
stock->nr_pages += nr_pages;

- if (stock->nr_pages > CHARGE_BATCH)
+ if (stock->nr_pages > MEMCG_CHARGE_BATCH)
drain_stock(stock);

local_irq_restore(flags);
@@ -1858,9 +1824,44 @@ static void drain_all_stock(struct mem_cgroup *root_memcg)
static int memcg_hotplug_cpu_dead(unsigned int cpu)
{
struct memcg_stock_pcp *stock;
+ struct mem_cgroup *memcg;

stock = &per_cpu(memcg_stock, cpu);
drain_stock(stock);
+
+ for_each_mem_cgroup(memcg) {
+ int i;
+
+ for (i = 0; i < MEMCG_NR_STAT; i++) {
+ int nid;
+ long x;
+
+ x = __this_cpu_xchg(memcg->stat_cpu->count[i], 0);
+ if (x)
+ atomic_long_add(x, &memcg->stat[i]);
+
+ if (i >= NR_VM_NODE_STAT_ITEMS)
+ continue;
+
+ for_each_node(nid) {
+ struct mem_cgroup_per_node *pn;
+
+ pn = mem_cgroup_nodeinfo(memcg, nid);
+ x = __this_cpu_xchg(pn->lruvec_stat_cpu->count[i], 0);
+ if (x)
+ atomic_long_add(x, &pn->lruvec_stat[i]);
+ }
+ }
+
+ for (i = 0; i < MEMCG_NR_EVENTS; i++) {
+ long x;
+
+ x = __this_cpu_xchg(memcg->stat_cpu->events[i], 0);
+ if (x)
+ atomic_long_add(x, &memcg->events[i]);
+ }
+ }
+
return 0;
}

@@ -1881,7 +1882,7 @@ static void high_work_func(struct work_struct *work)
struct mem_cgroup *memcg;

memcg = container_of(work, struct mem_cgroup, high_work);
- reclaim_high(memcg, CHARGE_BATCH, GFP_KERNEL);
+ reclaim_high(memcg, MEMCG_CHARGE_BATCH, GFP_KERNEL);
}

/*
@@ -1905,7 +1906,7 @@ void mem_cgroup_handle_over_high(void)
static int try_charge(struct mem_cgroup *memcg, gfp_t gfp_mask,
unsigned int nr_pages)
{
- unsigned int batch = max(CHARGE_BATCH, nr_pages);
+ unsigned int batch = max(MEMCG_CHARGE_BATCH, nr_pages);
int nr_retries = MEM_CGROUP_RECLAIM_RETRIES;
struct mem_cgroup *mem_over_limit;
struct page_counter *counter;
@@ -4161,8 +4162,8 @@ static int alloc_mem_cgroup_per_node_info(struct mem_cgroup *memcg, int node)
if (!pn)
return 1;

- pn->lruvec_stat = alloc_percpu(struct lruvec_stat);
- if (!pn->lruvec_stat) {
+ pn->lruvec_stat_cpu = alloc_percpu(struct lruvec_stat);
+ if (!pn->lruvec_stat_cpu) {
kfree(pn);
return 1;
}
@@ -4180,7 +4181,7 @@ static void free_mem_cgroup_per_node_info(struct mem_cgroup *memcg, int node)
{
struct mem_cgroup_per_node *pn = memcg->nodeinfo[node];

- free_percpu(pn->lruvec_stat);
+ free_percpu(pn->lruvec_stat_cpu);
kfree(pn);
}

@@ -4190,7 +4191,7 @@ static void __mem_cgroup_free(struct mem_cgroup *memcg)

for_each_node(node)
free_mem_cgroup_per_node_info(memcg, node);
- free_percpu(memcg->stat);
+ free_percpu(memcg->stat_cpu);
kfree(memcg);
}

@@ -4219,8 +4220,8 @@ static struct mem_cgroup *mem_cgroup_alloc(void)
if (memcg->id.id < 0)
goto fail;

- memcg->stat = alloc_percpu(struct mem_cgroup_stat_cpu);
- if (!memcg->stat)
+ memcg->stat_cpu = alloc_percpu(struct mem_cgroup_stat_cpu);
+ if (!memcg->stat_cpu)
goto fail;

for_each_node(node)
@@ -5638,7 +5639,7 @@ static void uncharge_batch(const struct uncharge_gather *ug)
__mod_memcg_state(ug->memcg, MEMCG_RSS_HUGE, -ug->nr_huge);
__mod_memcg_state(ug->memcg, NR_SHMEM, -ug->nr_shmem);
__count_memcg_events(ug->memcg, PGPGOUT, ug->pgpgout);
- __this_cpu_add(ug->memcg->stat->nr_page_events, nr_pages);
+ __this_cpu_add(ug->memcg->stat_cpu->nr_page_events, nr_pages);
memcg_check_events(ug->memcg, ug->dummy_page);
local_irq_restore(flags);

--
2.15.0