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mm: memcontrol: fix excessive complexity in memory.stat reporting
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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.

[hannes@cmpxchg.org: fix warning in __this_cpu_xchg() calls]
  Link: http://lkml.kernel.org/r/20171201135750.GB8097@cmpxchg.org
Link: http://lkml.kernel.org/r/20171103153336.24044-3-hannes@cmpxchg.org
Signed-off-by: Johannes Weiner <hannes@cmpxchg.org>
Acked-by: Vladimir Davydov <vdavydov.dev@gmail.com>
Cc: Michal Hocko <mhocko@suse.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
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@hnaz @torvalds
hnaz authored and torvalds committed Feb 1, 2018
1 parent 2845426 commit a983b5e
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Showing 2 changed files with 113 additions and 84 deletions.
96 changes: 62 additions & 34 deletions include/linux/memcontrol.h
View file
Original file line number Diff line number Diff line change
Expand Up @@ -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];
Expand Down Expand Up @@ -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;

Expand Down Expand Up @@ -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)
Expand Down Expand Up @@ -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();
}

/**
Expand Down Expand Up @@ -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);
Expand All @@ -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,
Expand Down Expand Up @@ -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 */
Expand Down
101 changes: 51 additions & 50 deletions mm/memcontrol.c
View file
Original file line number Diff line number Diff line change
Expand Up @@ -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,
Expand Down Expand Up @@ -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,
Expand Down Expand Up @@ -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) {
Expand All @@ -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;
Expand Down Expand Up @@ -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;
Expand Down Expand Up @@ -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);
Expand Down Expand Up @@ -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);
Expand Down Expand Up @@ -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;
}

Expand All @@ -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);
}

/*
Expand All @@ -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;
Expand Down Expand Up @@ -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;
}
Expand All @@ -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);
}

Expand All @@ -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);
}

Expand Down Expand Up @@ -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)
Expand Down Expand Up @@ -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);

Expand Down

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