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Subject[PATCH 2/2] memcg: coalescing charges per cpu
From: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>

This is a patch for coalescing access to res_counter at charging by
percpu caching. At charge, memcg charges 64pages and remember it in
percpu cache. Because it's cache, drain/flush is done if necessary.

This version uses public percpu area.
2 benefits of using public percpu area.
1. Sum of stocked charge in the system is limited to # of cpus
not to the number of memcg. This shows better synchonization.
2. drain code for flush/cpuhotplug is very easy (and quick)

The most important point of this patch is that we never touch res_counter
in fast path. The res_counter is system-wide shared counter which is modified
very frequently. We shouldn't touch it as far as we can for avoiding
false sharing.

Changelog (new):
- rabased onto the latest mmotm
Changelog (old):
- moved charge size check before __GFP_WAIT check for avoiding unnecesary
- added asynchronous flush routine.
- fixed bugs pointed out by Nishimura-san.

Signed-off-by: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
---
mm/memcontrol.c | 126 +++++++++++++++++++++++++++++++++++++++++++++++++++++---
1 file changed, 120 insertions(+), 6 deletions(-)
Index: mmotm-2.6.31-Sep28/mm/memcontrol.c
===================================================================
--- mmotm-2.6.31-Sep28.orig/mm/memcontrol.c
+++ mmotm-2.6.31-Sep28/mm/memcontrol.c
@@ -38,6 +38,7 @@
#include <linux/vmalloc.h>
#include <linux/mm_inline.h>
#include <linux/page_cgroup.h>
+#include <linux/cpu.h>
#include "internal.h"

#include <asm/uaccess.h>
@@ -275,6 +276,7 @@ enum charge_type {
static void mem_cgroup_get(struct mem_cgroup *mem);
static void mem_cgroup_put(struct mem_cgroup *mem);
static struct mem_cgroup *parent_mem_cgroup(struct mem_cgroup *mem);
+static void drain_all_stock_async(void);

static struct mem_cgroup_per_zone *
mem_cgroup_zoneinfo(struct mem_cgroup *mem, int nid, int zid)
@@ -1137,6 +1139,8 @@ static int mem_cgroup_hierarchical_recla
victim = mem_cgroup_select_victim(root_mem);
if (victim == root_mem) {
loop++;
+ if (loop >= 1)
+ drain_all_stock_async();
if (loop >= 2) {
/*
* If we have not been able to reclaim
@@ -1258,6 +1262,103 @@ done:
unlock_page_cgroup(pc);
}

+/* size of first charge trial. "32" comes from vmscan.c's magic value */
+#define CHARGE_SIZE (32 * PAGE_SIZE)
+struct memcg_stock_pcp {
+ struct mem_cgroup *cached;
+ int charge;
+ struct work_struct work;
+};
+static DEFINE_PER_CPU(struct memcg_stock_pcp, memcg_stock);
+static DEFINE_MUTEX(memcg_drain_mutex);
+
+static bool consume_stock(struct mem_cgroup *mem)
+{
+ struct memcg_stock_pcp *stock;
+ bool ret = true;
+
+ stock = &get_cpu_var(memcg_stock);
+ if (mem == stock->cached && stock->charge)
+ stock->charge -= PAGE_SIZE;
+ else
+ ret = false;
+ put_cpu_var(memcg_stock);
+ return ret;
+}
+
+static void drain_stock(struct memcg_stock_pcp *stock)
+{
+ struct mem_cgroup *old = stock->cached;
+
+ if (stock->charge) {
+ res_counter_uncharge(&old->res, stock->charge);
+ if (do_swap_account)
+ res_counter_uncharge(&old->memsw, stock->charge);
+ }
+ stock->cached = NULL;
+ stock->charge = 0;
+}
+
+static void drain_local_stock(struct work_struct *dummy)
+{
+ struct memcg_stock_pcp *stock = &get_cpu_var(memcg_stock);
+ drain_stock(stock);
+ put_cpu_var(memcg_stock);
+}
+
+static void refill_stock(struct mem_cgroup *mem, int val)
+{
+ struct memcg_stock_pcp *stock = &get_cpu_var(memcg_stock);
+
+ if (stock->cached != mem) {
+ drain_stock(stock);
+ stock->cached = mem;
+ }
+ stock->charge += val;
+ put_cpu_var(memcg_stock);
+}
+
+static void drain_all_stock_async(void)
+{
+ int cpu;
+ /* Contention means someone tries to flush. */
+ if (!mutex_trylock(&memcg_drain_mutex))
+ return;
+ for_each_online_cpu(cpu) {
+ struct memcg_stock_pcp *stock = &per_cpu(memcg_stock, cpu);
+ if (work_pending(&stock->work))
+ continue;
+ INIT_WORK(&stock->work, drain_local_stock);
+ schedule_work_on(cpu, &stock->work);
+ }
+ mutex_unlock(&memcg_drain_mutex);
+ /* We don't wait for flush_work */
+}
+
+static void drain_all_stock_sync(void)
+{
+ /* called when force_empty is called */
+ mutex_lock(&memcg_drain_mutex);
+ schedule_on_each_cpu(drain_local_stock);
+ mutex_unlock(&memcg_drain_mutex);
+}
+
+static int __cpuinit memcg_stock_cpu_callback(struct notifier_block *nb,
+ unsigned long action,
+ void *hcpu)
+{
+#ifdef CONFIG_HOTPLUG_CPU
+ int cpu = (unsigned long)hcpu;
+ struct memcg_stock_pcp *stock;
+
+ if (action != CPU_DEAD)
+ return NOTIFY_OK;
+ stock = &per_cpu(memcg_stock, cpu);
+ drain_stock(stock);
+#endif
+ return NOTIFY_OK;
+}
+
/*
* Unlike exported interface, "oom" parameter is added. if oom==true,
* oom-killer can be invoked.
@@ -1269,6 +1370,7 @@ static int __mem_cgroup_try_charge(struc
struct mem_cgroup *mem, *mem_over_limit;
int nr_retries = MEM_CGROUP_RECLAIM_RETRIES;
struct res_counter *fail_res;
+ int csize = CHARGE_SIZE;

if (unlikely(test_thread_flag(TIF_MEMDIE))) {
/* Don't account this! */
@@ -1293,23 +1395,25 @@ static int __mem_cgroup_try_charge(struc
return 0;

VM_BUG_ON(css_is_removed(&mem->css));
+ if (mem_cgroup_is_root(mem))
+ goto done;

while (1) {
int ret = 0;
unsigned long flags = 0;

- if (mem_cgroup_is_root(mem))
- goto done;
- ret = res_counter_charge(&mem->res, PAGE_SIZE, &fail_res);
+ if (consume_stock(mem))
+ goto charged;
+
+ ret = res_counter_charge(&mem->res, csize, &fail_res);
if (likely(!ret)) {
if (!do_swap_account)
break;
- ret = res_counter_charge(&mem->memsw, PAGE_SIZE,
- &fail_res);
+ ret = res_counter_charge(&mem->memsw, csize, &fail_res);
if (likely(!ret))
break;
/* mem+swap counter fails */
- res_counter_uncharge(&mem->res, PAGE_SIZE);
+ res_counter_uncharge(&mem->res, csize);
flags |= MEM_CGROUP_RECLAIM_NOSWAP;
mem_over_limit = mem_cgroup_from_res_counter(fail_res,
memsw);
@@ -1318,6 +1422,11 @@ static int __mem_cgroup_try_charge(struc
mem_over_limit = mem_cgroup_from_res_counter(fail_res,
res);

+ /* reduce request size and retry */
+ if (csize > PAGE_SIZE) {
+ csize = PAGE_SIZE;
+ continue;
+ }
if (!(gfp_mask & __GFP_WAIT))
goto nomem;

@@ -1347,6 +1456,9 @@ static int __mem_cgroup_try_charge(struc
goto nomem;
}
}
+ if (csize > PAGE_SIZE)
+ refill_stock(mem, csize - PAGE_SIZE);
+charged:
/*
* Insert ancestor (and ancestor's ancestors), to softlimit RB-tree.
* if they exceeds softlimit.
@@ -2463,6 +2575,7 @@ move_account:
goto out;
/* This is for making all *used* pages to be on LRU. */
lru_add_drain_all();
+ drain_all_stock_sync();
ret = 0;
for_each_node_state(node, N_HIGH_MEMORY) {
for (zid = 0; !ret && zid < MAX_NR_ZONES; zid++) {
@@ -3181,6 +3294,7 @@ mem_cgroup_create(struct cgroup_subsys *
root_mem_cgroup = mem;
if (mem_cgroup_soft_limit_tree_init())
goto free_out;
+ hotcpu_notifier(memcg_stock_cpu_callback, 0);

} else {
parent = mem_cgroup_from_cont(cont->parent);


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