[RFC 1/4] memcg: provide root figures from system totals

[Glauber Costa <glommer@parallels.com>]

For the root memcg, there is no need to rely on the res_counters.
The sum of all mem cgroups plus the tasks in root itself, is necessarily
the amount of memory used for the whole system. Since those figures are
already kept somewhere anyway, we can just return them here, without too
much hassle.

The limit can't be set for the root cgroup, so it is left at 0. Same is
true for failcnt, because its actual meaning is how many times we failed
allocations due to the limit being hit. We will fail allocations in the
root cgroup, but the limit will never the reason.

TODO includes figuring out what to do with the soft limit and max_usage.
Comments and suggestions appreciated.

Signed-off-by: Glauber Costa <glommer@parallels.com>
CC: Michal Hocko <mhocko@suse.cz>
CC: Kamezawa Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
CC: Johannes Weiner <hannes@cmpxchg.org>
CC: Mel Gorman <mgorman@suse.de>
CC: Andrew Morton <akpm@linux-foundation.org>
---
 mm/memcontrol.c | 45 +++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 45 insertions(+)

diff --git a/mm/memcontrol.c b/mm/memcontrol.c
index 82c5b8f..bac398b 100644
--- a/mm/memcontrol.c
+++ b/mm/memcontrol.c
@@ -4506,6 +4506,45 @@ static inline u64 mem_cgroup_usage(struct mem_cgroup *memcg, bool swap)
 	return val << PAGE_SHIFT;
 }
 
+static u64 mem_cgroup_read_root(struct mem_cgroup *memcg, enum res_type type, int name)
+{
+	struct sysinfo i;
+        unsigned long pages[NR_LRU_LISTS];
+        int lru;
+
+	si_meminfo(&i);
+	si_swapinfo(&i);
+
+	if (name == RES_LIMIT)
+		return RESOURCE_MAX;
+	if (name == RES_SOFT_LIMIT)
+		return 0;
+	if (name == RES_FAILCNT)
+		return 0;
+	if (name == RES_MAX_USAGE)
+		return 0;
+
+	if (WARN_ON_ONCE(name != RES_USAGE))
+		return 0;
+
+	for (lru = LRU_BASE; lru < NR_LRU_LISTS; lru++)
+		pages[lru] = global_page_state(NR_LRU_BASE + lru);
+
+	switch (type) {
+	case _MEM:
+                return pages[LRU_ACTIVE_ANON]   + pages[LRU_ACTIVE_FILE] +
+			pages[LRU_INACTIVE_ANON] + pages[LRU_INACTIVE_FILE];
+	case _MEMSWAP:
+                return pages[LRU_ACTIVE_ANON]   + pages[LRU_ACTIVE_FILE] +
+			pages[LRU_INACTIVE_ANON] + pages[LRU_INACTIVE_FILE] +
+			i.totalswap - i.freeswap;
+	case _KMEM:
+		return 0;
+	default:
+		BUG();
+	};
+}
+
 static ssize_t mem_cgroup_read(struct cgroup *cont, struct cftype *cft,
 			       struct file *file, char __user *buf,
 			       size_t nbytes, loff_t *ppos)
@@ -4522,6 +4561,11 @@ static ssize_t mem_cgroup_read(struct cgroup *cont, struct cftype *cft,
 	if (!do_swap_account && type == _MEMSWAP)
 		return -EOPNOTSUPP;
 
+	if (mem_cgroup_is_root(memcg)) {
+		val = mem_cgroup_read_root(memcg, type, name);
+		goto root_bypass;
+	}
+
 	switch (type) {
 	case _MEM:
 		if (name == RES_USAGE)
@@ -4542,6 +4586,7 @@ static ssize_t mem_cgroup_read(struct cgroup *cont, struct cftype *cft,
 		BUG();
 	}
 
+root_bypass:
 	len = scnprintf(str, sizeof(str), "%llu\n", (unsigned long long)val);
 	return simple_read_from_buffer(buf, nbytes, ppos, str, len);
 }
-- 
1.7.11.4

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