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Subject[PATCH] mm/memcontrol.c: make mem_cgroup_read_stat() read all interested stat item in one go
Date
Currently, mem_cgroup_read_stat() is used for user interface. The
user accounts memory usage by memory cgroup and he _always_ requires
exact value because he accounts memory. So we don't use quick-and-fuzzy
-read-and-do-periodic-synchronization way. Thus, we iterate all cpus
for one read.

And we mem_cgroup_usage() and mem_cgroup_recursive_stat() both finally
call into mem_cgroup_read_stat().

However, these *stat snapshot* operations are implemented in a quite
coarse way: it takes M*N iteration for each stat item(M=nr_memcgs,
N=nr_possible_cpus). There are two deficiencies:

1. for every stat item, we have to iterate over all percpu value, which
is not so cache friendly.
2. for every stat item, we call mem_cgroup_read_stat() once, which
increase the probablity of contending on pcp_counter_lock.

So, this patch improve this a bit. Concretely, for all interested stat
items, mark them in a bitmap, and then make mem_cgroup_read_stat() read
them all in one go.

This is more efficient, and to some degree make it more like *stat snapshot*.

Signed-off-by: Jianyu Zhan <nasa4836@gmail.com>
---
mm/memcontrol.c | 91 +++++++++++++++++++++++++++++++++++++++------------------
1 file changed, 62 insertions(+), 29 deletions(-)

diff --git a/mm/memcontrol.c b/mm/memcontrol.c
index 29501f0..009357e 100644
--- a/mm/memcontrol.c
+++ b/mm/memcontrol.c
@@ -347,7 +347,7 @@ struct mem_cgroup {
struct mem_cgroup_stat_cpu __percpu *stat;
/*
* used when a cpu is offlined or other synchronizations
- * See mem_cgroup_read_stat().
+ * See mem_cgroup_read_stat_vec().
*/
struct mem_cgroup_stat_cpu nocpu_base;
spinlock_t pcp_counter_lock;
@@ -855,7 +855,13 @@ mem_cgroup_largest_soft_limit_node(struct mem_cgroup_tree_per_zone *mctz)
return mz;
}

-/*
+/**
+ * @memcg: the mem_cgroup to account for.
+ * @stat_bitmask: a bitmap record which stat items to read,
+ * each mem_cgroup_stat_index has its corresponding bit.
+ * @stat_vec: a stat vector to hold the stat value for returing, caller
+ * shall take care of initializing it.
+ *
* Implementation Note: reading percpu statistics for memcg.
*
* Both of vmstat[] and percpu_counter has threshold and do periodic
@@ -874,22 +880,25 @@ mem_cgroup_largest_soft_limit_node(struct mem_cgroup_tree_per_zone *mctz)
* common workload, threashold and synchonization as vmstat[] should be
* implemented.
*/
-static long mem_cgroup_read_stat(struct mem_cgroup *memcg,
- enum mem_cgroup_stat_index idx)
+static void mem_cgroup_read_stat_vec(struct mem_cgroup *memcg,
+ unsigned long *stat_bitmask,
+ long long *stat_vec)
{
- long val = 0;
int cpu;
+ int i;

get_online_cpus();
for_each_online_cpu(cpu)
- val += per_cpu(memcg->stat->count[idx], cpu);
+ for_each_set_bit(i, stat_bitmask, MEM_CGROUP_STAT_NSTATS)
+ stat_vec[i] += per_cpu(memcg->stat->count[i], cpu);
+
#ifdef CONFIG_HOTPLUG_CPU
spin_lock(&memcg->pcp_counter_lock);
- val += memcg->nocpu_base.count[idx];
+ for_each_set_bit(i, stat_bitmask, MEM_CGROUP_STAT_NSTATS)
+ stat_vec[i] += memcg->nocpu_base.count[i];
spin_unlock(&memcg->pcp_counter_lock);
#endif
put_online_cpus();
- return val;
}

static void mem_cgroup_swap_statistics(struct mem_cgroup *memcg,
@@ -1674,6 +1683,7 @@ void mem_cgroup_print_oom_info(struct mem_cgroup *memcg, struct task_struct *p)
static DEFINE_MUTEX(oom_info_lock);
struct mem_cgroup *iter;
unsigned int i;
+ DECLARE_BITMAP(stat_bitmask, MEM_CGROUP_STAT_NSTATS);

if (!p)
return;
@@ -1702,16 +1712,22 @@ void mem_cgroup_print_oom_info(struct mem_cgroup *memcg, struct task_struct *p)
res_counter_read_u64(&memcg->kmem, RES_LIMIT) >> 10,
res_counter_read_u64(&memcg->kmem, RES_FAILCNT));

+ bitmap_fill(stat_bitmask, MEM_CGROUP_STAT_NSTATS);
+ if (!do_swap_account)
+ clear_bit(MEM_CGROUP_STAT_SWAP, stat_bitmask);
for_each_mem_cgroup_tree(iter, memcg) {
+ long long stat_vec[MEM_CGROUP_STAT_NSTATS] = {0};
+
pr_info("Memory cgroup stats for ");
pr_cont_cgroup_path(iter->css.cgroup);
pr_cont(":");

+ mem_cgroup_read_stat_vec(iter, stat_bitmask, stat_vec);
for (i = 0; i < MEM_CGROUP_STAT_NSTATS; i++) {
if (i == MEM_CGROUP_STAT_SWAP && !do_swap_account)
continue;
- pr_cont(" %s:%ldKB", mem_cgroup_stat_names[i],
- K(mem_cgroup_read_stat(iter, i)));
+ pr_cont(" %s:%lldKB", mem_cgroup_stat_names[i],
+ K(stat_vec[i]));
}

for (i = 0; i < NR_LRU_LISTS; i++)
@@ -4940,25 +4956,28 @@ out:
return retval;
}

-
-static unsigned long mem_cgroup_recursive_stat(struct mem_cgroup *memcg,
- enum mem_cgroup_stat_index idx)
+/* Callers should take care of initialize stat_vec array */
+static void mem_cgroup_recursive_stat(struct mem_cgroup *memcg,
+ unsigned long *stat_bitmask,
+ long long *stat_vec)
{
struct mem_cgroup *iter;
- long val = 0;
+ int idx;

/* Per-cpu values can be negative, use a signed accumulator */
for_each_mem_cgroup_tree(iter, memcg)
- val += mem_cgroup_read_stat(iter, idx);
+ mem_cgroup_read_stat_vec(iter, stat_bitmask, stat_vec);

- if (val < 0) /* race ? */
- val = 0;
- return val;
+ for_each_set_bit(idx, stat_bitmask, MEM_CGROUP_STAT_NSTATS)
+ if (stat_vec[idx] < 0) /* race ? */
+ stat_vec[idx] = 0;
}

static inline u64 mem_cgroup_usage(struct mem_cgroup *memcg, bool swap)
{
u64 val;
+ DECLARE_BITMAP(stat_bitmask, MEM_CGROUP_STAT_NSTATS);
+ long long stat_vec[MEM_CGROUP_STAT_NSTATS] = {0};

if (!mem_cgroup_is_root(memcg)) {
if (!swap)
@@ -4967,15 +4986,21 @@ static inline u64 mem_cgroup_usage(struct mem_cgroup *memcg, bool swap)
return res_counter_read_u64(&memcg->memsw, RES_USAGE);
}

+
/*
* Transparent hugepages are still accounted for in MEM_CGROUP_STAT_RSS
* as well as in MEM_CGROUP_STAT_RSS_HUGE.
*/
- val = mem_cgroup_recursive_stat(memcg, MEM_CGROUP_STAT_CACHE);
- val += mem_cgroup_recursive_stat(memcg, MEM_CGROUP_STAT_RSS);
-
+ bitmap_zero(stat_bitmask, MEM_CGROUP_STAT_NSTATS);
+ set_bit(MEM_CGROUP_STAT_CACHE, stat_bitmask);
+ set_bit(MEM_CGROUP_STAT_RSS, stat_bitmask);
if (swap)
- val += mem_cgroup_recursive_stat(memcg, MEM_CGROUP_STAT_SWAP);
+ set_bit(MEM_CGROUP_STAT_SWAP, stat_bitmask);
+
+ mem_cgroup_recursive_stat(memcg, stat_bitmask, stat_vec);
+
+ val = stat_vec[MEM_CGROUP_STAT_CACHE] + stat_vec[MEM_CGROUP_STAT_RSS] +
+ (swap ? stat_vec[MEM_CGROUP_STAT_SWAP] : 0);

return val << PAGE_SHIFT;
}
@@ -5349,12 +5374,19 @@ static int memcg_stat_show(struct seq_file *m, void *v)
struct mem_cgroup *memcg = mem_cgroup_from_css(seq_css(m));
struct mem_cgroup *mi;
unsigned int i;
+ DECLARE_BITMAP(stat_bitmask, MEM_CGROUP_STAT_NSTATS);
+ long long stat_vec[MEM_CGROUP_STAT_NSTATS] = {0};
+
+ bitmap_fill(stat_bitmask, MEM_CGROUP_STAT_NSTATS);
+ if (!do_swap_account)
+ clear_bit(MEM_CGROUP_STAT_SWAP, stat_bitmask);
+ mem_cgroup_read_stat_vec(memcg, stat_bitmask, stat_vec);

for (i = 0; i < MEM_CGROUP_STAT_NSTATS; i++) {
if (i == MEM_CGROUP_STAT_SWAP && !do_swap_account)
continue;
- seq_printf(m, "%s %ld\n", mem_cgroup_stat_names[i],
- mem_cgroup_read_stat(memcg, i) * PAGE_SIZE);
+ seq_printf(m, "%s %lld\n", mem_cgroup_stat_names[i],
+ stat_vec[i] * PAGE_SIZE);
}

for (i = 0; i < MEM_CGROUP_EVENTS_NSTATS; i++)
@@ -5375,14 +5407,15 @@ static int memcg_stat_show(struct seq_file *m, void *v)
memsw_limit);
}

+ for (i = 0; i < MEM_CGROUP_STAT_NSTATS; i++)
+ stat_vec[i] = 0;
+ mem_cgroup_recursive_stat(memcg, stat_bitmask, stat_vec);
for (i = 0; i < MEM_CGROUP_STAT_NSTATS; i++) {
- long long val = 0;
-
if (i == MEM_CGROUP_STAT_SWAP && !do_swap_account)
continue;
- for_each_mem_cgroup_tree(mi, memcg)
- val += mem_cgroup_read_stat(mi, i) * PAGE_SIZE;
- seq_printf(m, "total_%s %lld\n", mem_cgroup_stat_names[i], val);
+
+ seq_printf(m, "total_%s %lld\n", mem_cgroup_stat_names[i],
+ stat_vec[i] * PAGE_SIZE);
}

for (i = 0; i < MEM_CGROUP_EVENTS_NSTATS; i++) {
--
1.9.0.GIT


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