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Subject[PATCH 4/6] memcg: fix shrinking memory to return -EBUSY by fixing retry algorithm
From: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>

As pointed out, shrinking memcg's limit should return -EBUSY
after reasonable retries. This patch tries to fix the current behavior
of shrink_usage.

Before looking into "shrink should return -EBUSY" problem, we should fix
hierarchical reclaim code. It compares current usage and current limit,
but it only makes sense when the kernel reclaims memory because hit limits.
This is also a problem.

What this patch does are.

1. add new argument "shrink" to hierarchical reclaim. If "shrink==true",
hierarchical reclaim returns immediately and the caller checks the kernel
should shrink more or not.
(At shrinking memory, usage is always smaller than limit. So check for
usage < limit is useless.)

2. For adjusting to above change, 2 changes in "shrink"'s retry path.
2-a. retry_count depends on # of children because the kernel visits
the children under hierarchy one by one.
2-b. rather than checking return value of hierarchical_reclaim's progress,
compares usage-before-shrink and usage-after-shrink.
If usage-before-shrink <= usage-after-shrink, retry_count is
decremented.

Reported-by: Li Zefan <lizf@cn.fujitsu.com>
Signed-off-by: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
---
Index: mmotm-2.6.29-Feb02/mm/memcontrol.c
===================================================================
--- mmotm-2.6.29-Feb02.orig/mm/memcontrol.c
+++ mmotm-2.6.29-Feb02/mm/memcontrol.c
@@ -702,6 +702,23 @@ static unsigned int get_swappiness(struc
return swappiness;
}

+static int mem_cgroup_count_children_cb(struct mem_cgroup *mem, void *data)
+{
+ int *val = data;
+ (*val)++;
+ return 0;
+}
+/*
+ * This function returns the number of memcg under hierarchy tree. Returns
+ * 1(self count) if no children.
+ */
+static int mem_cgroup_count_children(struct mem_cgroup *mem)
+{
+ int num = 0;
+ mem_cgroup_walk_tree(mem, &num, mem_cgroup_count_children_cb);
+ return num;
+}
+
/*
* Visit the first child (need not be the first child as per the ordering
* of the cgroup list, since we track last_scanned_child) of @mem and use
@@ -750,9 +767,11 @@ mem_cgroup_select_victim(struct mem_cgro
*
* We give up and return to the caller when we visit root_mem twice.
* (other groups can be removed while we're walking....)
+ *
+ * If shrink==true, for avoiding to free too much, this returns immedieately.
*/
static int mem_cgroup_hierarchical_reclaim(struct mem_cgroup *root_mem,
- gfp_t gfp_mask, bool noswap)
+ gfp_t gfp_mask, bool noswap, bool shrink)
{
struct mem_cgroup *victim;
int ret, total = 0;
@@ -771,6 +790,13 @@ static int mem_cgroup_hierarchical_recla
ret = try_to_free_mem_cgroup_pages(victim, gfp_mask, noswap,
get_swappiness(victim));
css_put(&victim->css);
+ /*
+ * At shrinking usage, we can't check we should stop here or
+ * reclaim more. It's depends on callers. last_scanned_child
+ * will work enough for keeping fairness under tree.
+ */
+ if (shrink)
+ return ret;
total += ret;
if (mem_cgroup_check_under_limit(root_mem))
return 1 + total;
@@ -856,7 +882,7 @@ static int __mem_cgroup_try_charge(struc
goto nomem;

ret = mem_cgroup_hierarchical_reclaim(mem_over_limit, gfp_mask,
- noswap);
+ noswap, false);
if (ret)
continue;

@@ -1489,7 +1515,8 @@ int mem_cgroup_shrink_usage(struct page
return 0;

do {
- progress = mem_cgroup_hierarchical_reclaim(mem, gfp_mask, true);
+ progress = mem_cgroup_hierarchical_reclaim(mem,
+ gfp_mask, true, false);
progress += mem_cgroup_check_under_limit(mem);
} while (!progress && --retry);

@@ -1504,11 +1531,21 @@ static DEFINE_MUTEX(set_limit_mutex);
static int mem_cgroup_resize_limit(struct mem_cgroup *memcg,
unsigned long long val)
{
-
- int retry_count = MEM_CGROUP_RECLAIM_RETRIES;
+ int retry_count;
int progress;
u64 memswlimit;
int ret = 0;
+ int children = mem_cgroup_count_children(memcg);
+ u64 curusage, oldusage;
+
+ /*
+ * For keeping hierarchical_reclaim simple, how long we should retry
+ * is depends on callers. We set our retry-count to be function
+ * of # of children which we should visit in this loop.
+ */
+ retry_count = MEM_CGROUP_RECLAIM_RETRIES * children;
+
+ oldusage = res_counter_read_u64(&memcg->res, RES_USAGE);

while (retry_count) {
if (signal_pending(current)) {
@@ -1534,8 +1571,13 @@ static int mem_cgroup_resize_limit(struc
break;

progress = mem_cgroup_hierarchical_reclaim(memcg, GFP_KERNEL,
- false);
- if (!progress) retry_count--;
+ false, true);
+ curusage = res_counter_read_u64(&memcg->res, RES_USAGE);
+ /* Usage is reduced ? */
+ if (curusage >= oldusage)
+ retry_count--;
+ else
+ oldusage = curusage;
}

return ret;
@@ -1544,13 +1586,16 @@ static int mem_cgroup_resize_limit(struc
int mem_cgroup_resize_memsw_limit(struct mem_cgroup *memcg,
unsigned long long val)
{
- int retry_count = MEM_CGROUP_RECLAIM_RETRIES;
+ int retry_count;
u64 memlimit, oldusage, curusage;
- int ret;
+ int children = mem_cgroup_count_children(memcg);
+ int ret = -EBUSY;

if (!do_swap_account)
return -EINVAL;
-
+ /* see mem_cgroup_resize_res_limit */
+ retry_count = children * MEM_CGROUP_RECLAIM_RETRIES;
+ oldusage = res_counter_read_u64(&memcg->memsw, RES_USAGE);
while (retry_count) {
if (signal_pending(current)) {
ret = -EINTR;
@@ -1574,11 +1619,13 @@ int mem_cgroup_resize_memsw_limit(struct
if (!ret)
break;

- oldusage = res_counter_read_u64(&memcg->memsw, RES_USAGE);
- mem_cgroup_hierarchical_reclaim(memcg, GFP_KERNEL, true);
+ mem_cgroup_hierarchical_reclaim(memcg, GFP_KERNEL, true, true);
curusage = res_counter_read_u64(&memcg->memsw, RES_USAGE);
+ /* Usage is reduced ? */
if (curusage >= oldusage)
retry_count--;
+ else
+ oldusage = curusage;
}
return ret;
}


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