[PATCH] mm/memcontrol: export memcg.swap watermark via sysfs for v2 memcg

["Lars R. Damerow" <lars@pixar.com>]

This patch is similar to commit 8e20d4b33266 ("mm/memcontrol: export
memcg->watermark via sysfs for v2 memcg"), but exports the swap counter's
watermark.

We allocate jobs to our compute farm using heuristics determined by memory
and swap usage from previous jobs. Tracking the peak swap usage for new
jobs is important for determining when jobs are exceeding their expected
bounds, or when our baseline metrics are getting outdated.

Our toolset was written to use the "memory.memsw.max_usage_in_bytes" file
in cgroups v1, and altering it to poll cgroups v2's "memory.swap.current"
would give less accurate results as well as add complication to the code.
Having this watermark exposed in sysfs is much preferred.

Signed-off-by: Lars R. Damerow <lars@pixar.com>
---
 Documentation/admin-guide/cgroup-v2.rst |  7 +++++++
 mm/memcontrol.c                         | 13 +++++++++++++
 2 files changed, 20 insertions(+)

diff --git a/Documentation/admin-guide/cgroup-v2.rst b/Documentation/admin-guide/cgroup-v2.rst
index f67c0829350b..1ffe019483ac 100644
--- a/Documentation/admin-guide/cgroup-v2.rst
+++ b/Documentation/admin-guide/cgroup-v2.rst
@@ -1582,6 +1582,13 @@ PAGE_SIZE multiple when read back.
 
 	Healthy workloads are not expected to reach this limit.
 
+  memory.swap.peak
+	A read-only single value file which exists on non-root
+	cgroups.
+
+	The max swap usage recorded for the cgroup and its
+	descendants since the creation of the cgroup.
+
   memory.swap.max
 	A read-write single value file which exists on non-root
 	cgroups.  The default is "max".
diff --git a/mm/memcontrol.c b/mm/memcontrol.c
index 4b27e245a055..1862fee15274 100644
--- a/mm/memcontrol.c
+++ b/mm/memcontrol.c
@@ -7656,6 +7656,14 @@ static u64 swap_current_read(struct cgroup_subsys_state *css,
 	return (u64)page_counter_read(&memcg->swap) * PAGE_SIZE;
 }
 
+static u64 swap_peak_read(struct cgroup_subsys_state *css,
+			  struct cftype *cft)
+{
+	struct mem_cgroup *memcg = mem_cgroup_from_css(css);
+
+	return (u64)memcg->swap.watermark * PAGE_SIZE;
+}
+
 static int swap_high_show(struct seq_file *m, void *v)
 {
 	return seq_puts_memcg_tunable(m,
@@ -7734,6 +7742,11 @@ static struct cftype swap_files[] = {
 		.seq_show = swap_max_show,
 		.write = swap_max_write,
 	},
+	{
+		.name = "swap.peak",
+		.flags = CFTYPE_NOT_ON_ROOT,
+		.read_u64 = swap_peak_read,
+	},
 	{
 		.name = "swap.events",
 		.flags = CFTYPE_NOT_ON_ROOT,

base-commit: 9d646009f65d62d32815f376465a3b92d8d9b046
-- 
2.39.2