Re: [mm] 42a3003535: will-it-scale.per_process_ops -25.9% regression

Johannes Weiner
2019-05-20T21:53:33Z

Hello,

On Mon, May 20, 2019 at 02:35:34PM +0800, kernel test robot wrote:
> Greeting,
>
> FYI, we noticed a -25.9% regression of will-it-scale.per_process_ops due to commit:
>
>
> commit: 42a300353577ccc17ecc627b8570a89fa1678bec ("mm: memcontrol: fix recursive statistics correctness & scalabilty")
> https://git.kernel.org/cgit/linux/kernel/git/torvalds/linux.git master
>
> in testcase: will-it-scale
> on test machine: 192 threads Skylake-SP with 256G memory
> with following parameters:

Ouch. That has to be the additional cache footprint of the split
local/recursive stat counters, rather than the extra instructions.

Could you please try re-running the test on that host with the below
patch applied?

Also CCing Aaron Lu, who has previously investigated the cache layout
in the struct memcg stat counters.

> nr_task: 100%
> mode: process
> test: page_fault3
> cpufreq_governor: performance

From c9ed8f78dfa25c4d29adf5a09cf9adeeb43e8bdd Mon Sep 17 00:00:00 2001
From: Johannes Weiner <hannes@cmpxchg.org>
Date: Mon, 20 May 2019 14:18:26 -0400
Subject: [PATCH] mm: memcontrol: don't batch updates of local VM stats and
events

The kernel test robot noticed a 26% will-it-scale pagefault regression
from commit 42a300353577 ("mm: memcontrol: fix recursive statistics
correctness & scalabilty"). This appears to be caused by bouncing the
additional cachelines from the new hierarchical statistics counters.

We can fix this by getting rid of the batched local counters instead.

Originally, there were *only* group-local counters, and they were
fully maintained per cpu. A reader of a stats file high up in the
cgroup tree would have to walk the entire subtree and collect each
level's per-cpu counters to get the recursive view. This was
prohibitively expensive, and so we switched to per-cpu batched updates
of the local counters during a983b5ebee57 ("mm: memcontrol: fix
excessive complexity in memory.stat reporting"), reducing the
complexity from nr_subgroups * nr_cpus to nr_subgroups.

With growing machines and cgroup trees, the tree walk itself became
too expensive for monitoring top-level groups, and this is when the
culprit patch added hierarchy counters on each cgroup level. When the
per-cpu batch size would be reached, both the local and the hierarchy
counters would get batch-updated from the per-cpu delta simultaneously.

This makes local and hierarchical counter reads blazingly fast, but it
unfortunately makes the write-side too cache line intense.

Since local counter reads were never a problem - we only centralized
them to accelerate the hierarchy walk - and use of the local counters
are becoming rarer due to replacement with hierarchical views (ongoing
rework in the page reclaim and workingset code), we can make those
local counters unbatched per-cpu counters again.

The scheme will then be as such:

when a memcg statistic changes, the writer will:
- update the local counter (per-cpu)
- update the batch counter (per-cpu). If the batch is full:
- spill the batch into the group's atomic_t
- spill the batch into all ancestors' atomic_ts
- empty out the batch counter (per-cpu)

when a local memcg counter is read, the reader will:
- collect the local counter from all cpus

when a hiearchy memcg counter is read, the reader will:
- read the atomic_t

We might be able to simplify this further and make the recursive
counters unbatched per-cpu counters as well (batch upward propagation,
but leave per-cpu collection to the readers), but that will require a
more in-depth analysis and testing of all the callsites. Deal with the
immediate regression for now.

Fixes: 42a300353577 ("mm: memcontrol: fix recursive statistics correctness & scalabilty")
Reported-by: kernel test robot <rong.a.chen@intel.com>
Signed-off-by: Johannes Weiner <hannes@cmpxchg.org>
---
include/linux/memcontrol.h | 26 ++++++++++++++++--------
mm/memcontrol.c | 41 ++++++++++++++++++++++++++------------
2 files changed, 46 insertions(+), 21 deletions(-)

diff --git a/include/linux/memcontrol.h b/include/linux/memcontrol.h
index bc74d6a4407c..2d23ae7bd36d 100644
--- a/include/linux/memcontrol.h
+++ b/include/linux/memcontrol.h
@@ -126,9 +126,12 @@ struct memcg_shrinker_map {
struct mem_cgroup_per_node {
struct lruvec lruvec;

+ /* Legacy local VM stats */
+ struct lruvec_stat __percpu *lruvec_stat_local;
+
+ /* Subtree VM stats (batched updates) */
struct lruvec_stat __percpu *lruvec_stat_cpu;
atomic_long_t lruvec_stat[NR_VM_NODE_STAT_ITEMS];
- atomic_long_t lruvec_stat_local[NR_VM_NODE_STAT_ITEMS];

unsigned long lru_zone_size[MAX_NR_ZONES][NR_LRU_LISTS];

@@ -274,17 +277,18 @@ struct mem_cgroup {
atomic_t moving_account;
struct task_struct *move_lock_task;

- /* memory.stat */
+ /* Legacy local VM stats and events */
+ struct memcg_vmstats_percpu __percpu *vmstats_local;
+
+ /* Subtree VM stats and events (batched updates) */
struct memcg_vmstats_percpu __percpu *vmstats_percpu;

MEMCG_PADDING(_pad2_);

atomic_long_t vmstats[MEMCG_NR_STAT];
- atomic_long_t vmstats_local[MEMCG_NR_STAT];
-
atomic_long_t vmevents[NR_VM_EVENT_ITEMS];
- atomic_long_t vmevents_local[NR_VM_EVENT_ITEMS];

+ /* memory.events */
atomic_long_t memory_events[MEMCG_NR_MEMORY_EVENTS];

unsigned long socket_pressure;
@@ -576,7 +580,11 @@ static inline unsigned long memcg_page_state(struct mem_cgroup *memcg, int idx)
static inline unsigned long memcg_page_state_local(struct mem_cgroup *memcg,
int idx)
{
- long x = atomic_long_read(&memcg->vmstats_local[idx]);
+ long x = 0;
+ int cpu;
+
+ for_each_possible_cpu(cpu)
+ x += per_cpu(memcg->vmstats_local->stat[idx], cpu);
#ifdef CONFIG_SMP
if (x < 0)
x = 0;
@@ -650,13 +658,15 @@ static inline unsigned long lruvec_page_state_local(struct lruvec *lruvec,
enum node_stat_item idx)
{
struct mem_cgroup_per_node *pn;
- long x;
+ long x = 0;
+ int cpu;

if (mem_cgroup_disabled())
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_local[idx]);
+ for_each_possible_cpu(cpu)
+ x += per_cpu(pn->lruvec_stat_local->count[idx], cpu);
#ifdef CONFIG_SMP
if (x < 0)
x = 0;
diff --git a/mm/memcontrol.c b/mm/memcontrol.c
index e50a2db5b4ff..8d42e5c7bf37 100644
--- a/mm/memcontrol.c
+++ b/mm/memcontrol.c
@@ -700,11 +700,12 @@ void __mod_memcg_state(struct mem_cgroup *memcg, int idx, int val)
if (mem_cgroup_disabled())
return;

+ __this_cpu_add(memcg->vmstats_local->stat[idx], val);
+
x = val + __this_cpu_read(memcg->vmstats_percpu->stat[idx]);
if (unlikely(abs(x) > MEMCG_CHARGE_BATCH)) {
struct mem_cgroup *mi;

- atomic_long_add(x, &memcg->vmstats_local[idx]);
for (mi = memcg; mi; mi = parent_mem_cgroup(mi))
atomic_long_add(x, &mi->vmstats[idx]);
x = 0;
@@ -754,11 +755,12 @@ void __mod_lruvec_state(struct lruvec *lruvec, enum node_stat_item idx,
__mod_memcg_state(memcg, idx, val);

/* Update lruvec */
+ __this_cpu_add(pn->lruvec_stat_local->count[idx], val);
+
x = val + __this_cpu_read(pn->lruvec_stat_cpu->count[idx]);
if (unlikely(abs(x) > MEMCG_CHARGE_BATCH)) {
struct mem_cgroup_per_node *pi;

- atomic_long_add(x, &pn->lruvec_stat_local[idx]);
for (pi = pn; pi; pi = parent_nodeinfo(pi, pgdat->node_id))
atomic_long_add(x, &pi->lruvec_stat[idx]);
x = 0;
@@ -780,11 +782,12 @@ void __count_memcg_events(struct mem_cgroup *memcg, enum vm_event_item idx,
if (mem_cgroup_disabled())
return;

+ __this_cpu_add(memcg->vmstats_local->events[idx], count);
+
x = count + __this_cpu_read(memcg->vmstats_percpu->events[idx]);
if (unlikely(x > MEMCG_CHARGE_BATCH)) {
struct mem_cgroup *mi;

- atomic_long_add(x, &memcg->vmevents_local[idx]);
for (mi = memcg; mi; mi = parent_mem_cgroup(mi))
atomic_long_add(x, &mi->vmevents[idx]);
x = 0;
@@ -799,7 +802,12 @@ static unsigned long memcg_events(struct mem_cgroup *memcg, int event)

static unsigned long memcg_events_local(struct mem_cgroup *memcg, int event)
{
- return atomic_long_read(&memcg->vmevents_local[event]);
+ long x = 0;
+ int cpu;
+
+ for_each_possible_cpu(cpu)
+ x += per_cpu(memcg->vmstats_local->events[event], cpu);
+ return x;
}

static void mem_cgroup_charge_statistics(struct mem_cgroup *memcg,
@@ -2200,11 +2208,9 @@ static int memcg_hotplug_cpu_dead(unsigned int cpu)
long x;

x = this_cpu_xchg(memcg->vmstats_percpu->stat[i], 0);
- if (x) {
- atomic_long_add(x, &memcg->vmstats_local[i]);
+ if (x)
for (mi = memcg; mi; mi = parent_mem_cgroup(mi))
atomic_long_add(x, &memcg->vmstats[i]);
- }

if (i >= NR_VM_NODE_STAT_ITEMS)
continue;
@@ -2214,12 +2220,10 @@ static int memcg_hotplug_cpu_dead(unsigned int cpu)

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_local[i]);
+ if (x)
do {
atomic_long_add(x, &pn->lruvec_stat[i]);
} while ((pn = parent_nodeinfo(pn, nid)));
- }
}
}

@@ -2227,11 +2231,9 @@ static int memcg_hotplug_cpu_dead(unsigned int cpu)
long x;

x = this_cpu_xchg(memcg->vmstats_percpu->events[i], 0);
- if (x) {
- atomic_long_add(x, &memcg->vmevents_local[i]);
+ if (x)
for (mi = memcg; mi; mi = parent_mem_cgroup(mi))
atomic_long_add(x, &memcg->vmevents[i]);
- }
}
}

@@ -4492,8 +4494,15 @@ static int alloc_mem_cgroup_per_node_info(struct mem_cgroup *memcg, int node)
if (!pn)
return 1;

+ pn->lruvec_stat_local = alloc_percpu(struct lruvec_stat);
+ if (!pn->lruvec_stat_local) {
+ kfree(pn);
+ return 1;
+ }
+
pn->lruvec_stat_cpu = alloc_percpu(struct lruvec_stat);
if (!pn->lruvec_stat_cpu) {
+ free_percpu(pn->lruvec_stat_local);
kfree(pn);
return 1;
}
@@ -4515,6 +4524,7 @@ static void free_mem_cgroup_per_node_info(struct mem_cgroup *memcg, int node)
return;

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

@@ -4525,6 +4535,7 @@ static void __mem_cgroup_free(struct mem_cgroup *memcg)
for_each_node(node)
free_mem_cgroup_per_node_info(memcg, node);
free_percpu(memcg->vmstats_percpu);
+ free_percpu(memcg->vmstats_local);
kfree(memcg);
}

@@ -4553,6 +4564,10 @@ static struct mem_cgroup *mem_cgroup_alloc(void)
if (memcg->id.id < 0)
goto fail;

+ memcg->vmstats_local = alloc_percpu(struct memcg_vmstats_percpu);
+ if (!memcg->vmstats_local)
+ goto fail;
+
memcg->vmstats_percpu = alloc_percpu(struct memcg_vmstats_percpu);
if (!memcg->vmstats_percpu)
goto fail;
--
2.21.0