[PATCH] memcg: provide reclaim stats via 'memory.reclaim'

[Vaibhav Jain <vaibhav@linux.ibm.com>]

[1] Provides a way for user-space to trigger proactive reclaim by introducing
a write-only memcg file 'memory.reclaim'. However reclaim stats like number
of pages scanned and reclaimed is still not directly available to the
user-space.

This patch proposes to extend [1] to make the memcg file 'memory.reclaim'
readable which returns the number of pages scanned / reclaimed during the
reclaim process from 'struct vmpressure' associated with each memcg. This should
let user-space asses how successful proactive reclaim triggered from memcg
'memory.reclaim' was ?

With the patch following command flow is expected:

 # echo "1M" > memory.reclaim

 # cat memory.reclaim
   scanned 76
   reclaimed 32

[1]:  https://lore.kernel.org/r/20220425190040.2475377-1-yosryahmed@google.com

Cc: Shakeel Butt <shakeelb@google.com>
Cc: Yosry Ahmed <yosryahmed@google.com>
Signed-off-by: Vaibhav Jain <vaibhav@linux.ibm.com>
---
 Documentation/admin-guide/cgroup-v2.rst | 15 ++++++++++++---
 mm/memcontrol.c                         | 14 ++++++++++++++
 2 files changed, 26 insertions(+), 3 deletions(-)

diff --git a/Documentation/admin-guide/cgroup-v2.rst b/Documentation/admin-guide/cgroup-v2.rst
index 27ebef2485a3..44610165261d 100644
--- a/Documentation/admin-guide/cgroup-v2.rst
+++ b/Documentation/admin-guide/cgroup-v2.rst
@@ -1209,18 +1209,27 @@ PAGE_SIZE multiple when read back.
 	utility is limited to providing the final safety net.
 
   memory.reclaim
-	A write-only nested-keyed file which exists for all cgroups.
+	A nested-keyed file which exists for all cgroups.
 
-	This is a simple interface to trigger memory reclaim in the
-	target cgroup.
+	This is a simple interface to trigger memory reclaim and retrieve
+	reclaim stats in the target cgroup.
 
 	This file accepts a single key, the number of bytes to reclaim.
 	No nested keys are currently supported.
 
+	Reading the file returns number of pages scanned and number of
+	pages reclaimed from the memcg. This information fetched from
+	vmpressure info associated with each cgroup.
+
 	Example::
 
 	  echo "1G" > memory.reclaim
 
+	  cat memory.reclaim
+
+	  scanned 78
+	  reclaimed 30
+
 	The interface can be later extended with nested keys to
 	configure the reclaim behavior. For example, specify the
 	type of memory to reclaim from (anon, file, ..).
diff --git a/mm/memcontrol.c b/mm/memcontrol.c
index 2e2bfbed4717..9e43580a8726 100644
--- a/mm/memcontrol.c
+++ b/mm/memcontrol.c
@@ -6423,6 +6423,19 @@ static ssize_t memory_oom_group_write(struct kernfs_open_file *of,
 	return nbytes;
 }
 
+static int memory_reclaim_show(struct seq_file *m, void *v)
+{
+	struct mem_cgroup *memcg = mem_cgroup_from_seq(m);
+	struct vmpressure *vmpr = memcg_to_vmpressure(memcg);
+
+	spin_lock(&vmpr->sr_lock);
+	seq_printf(m, "scanned %lu\nreclaimed %lu\n",
+		   vmpr->scanned, vmpr->reclaimed);
+	spin_unlock(&vmpr->sr_lock);
+
+	return 0;
+}
+
 static ssize_t memory_reclaim(struct kernfs_open_file *of, char *buf,
 			      size_t nbytes, loff_t off)
 {
@@ -6525,6 +6538,7 @@ static struct cftype memory_files[] = {
 		.name = "reclaim",
 		.flags = CFTYPE_NS_DELEGATABLE,
 		.write = memory_reclaim,
+		.seq_show  = memory_reclaim_show,
 	},
 	{ }	/* terminate */
 };
-- 
2.35.1