[PATCH v4] memcg: event control at vmpressure.

[Hyunhee Kim <hyunhee.kim@xxxxxxxxxxx>]

In the original vmpressure, events are triggered whenever there is a reclaim
activity. This becomes overheads to user space module and also increases
power consumption if there is somebody to listen to it. This patch provides
options to trigger events only when the pressure level changes.
This trigger option can be set when registering each event by writing
a trigger option, "edge" or "always", next to the string of levels.
"edge" means that the event is triggered only when the pressure level is changed.
"always" means that events are triggered whenever there is a reclaim process.
To keep backward compatibility, "always" is set by default if nothing is input
as an option. Each event can have different option. For example,
"low" level uses "always" trigger option to see reclaim activity at user space
while "medium"/"critical" uses "edge" to do an important job
like killing tasks only once.

Signed-off-by: Hyunhee Kim <hyunhee.kim@xxxxxxxxxxx>
Signed-off-by: Kyungmin Park <kyungmin.park@xxxxxxxxxxx>
---
 Documentation/cgroups/memory.txt |   12 ++++++++++--
 mm/vmpressure.c                  |   32 ++++++++++++++++++++++++++++----
 2 files changed, 38 insertions(+), 6 deletions(-)

diff --git a/Documentation/cgroups/memory.txt b/Documentation/cgroups/memory.txt
index ddf4f93..181a11f 100644
--- a/Documentation/cgroups/memory.txt
+++ b/Documentation/cgroups/memory.txt
@@ -791,6 +791,13 @@ way to trigger. Applications should do whatever they can to help the
 system. It might be too late to consult with vmstat or any other
 statistics, so it's advisable to take an immediate action.
 
+Events can be triggered whenever there is a reclaim activity or
+only when the pressure level changes. Trigger option is decided
+by writing it next to level. The event whose trigger option is "always"
+is triggered whenever there is a reclaim process. If "edge" is set,
+the event is triggered only when the level is changed.
+If the trigger option is not set, "always" is set by default.
+
 The events are propagated upward until the event is handled, i.e. the
 events are not pass-through. Here is what this means: for example you have
 three cgroups: A->B->C. Now you set up an event listener on cgroups A, B
@@ -807,7 +814,8 @@ register a notification, an application must:
 
 - create an eventfd using eventfd(2);
 - open memory.pressure_level;
-- write string like "<event_fd> <fd of memory.pressure_level> <level>"
+- write string like
+	"<event_fd> <fd of memory.pressure_level> <level> <trigger_option>"
   to cgroup.event_control.
 
 Application will be notified through eventfd when memory pressure is at
@@ -823,7 +831,7 @@ Test:
    # cd /sys/fs/cgroup/memory/
    # mkdir foo
    # cd foo
-   # cgroup_event_listener memory.pressure_level low &
+   # cgroup_event_listener memory.pressure_level low edge &
    # echo 8000000 > memory.limit_in_bytes
    # echo 8000000 > memory.memsw.limit_in_bytes
    # echo $$ > tasks
diff --git a/mm/vmpressure.c b/mm/vmpressure.c
index 736a601..4f676b8 100644
--- a/mm/vmpressure.c
+++ b/mm/vmpressure.c
@@ -137,6 +137,8 @@ static enum vmpressure_levels vmpressure_calc_level(unsigned long scanned,
 struct vmpressure_event {
 	struct eventfd_ctx *efd;
 	enum vmpressure_levels level;
+	int last_level;
+	bool edge_trigger;
 	struct list_head node;
 };
 
@@ -153,11 +155,14 @@ static bool vmpressure_event(struct vmpressure *vmpr,
 
 	list_for_each_entry(ev, &vmpr->events, node) {
 		if (level >= ev->level) {
+			if (ev->edge_trigger && level == ev->last_level)
+				continue;
+
 			eventfd_signal(ev->efd, 1);
 			signalled = true;
 		}
+		ev->last_level = level;
 	}
-
 	mutex_unlock(&vmpr->events_lock);
 
 	return signalled;
@@ -290,9 +295,11 @@ void vmpressure_prio(gfp_t gfp, struct mem_cgroup *memcg, int prio)
  *
  * This function associates eventfd context with the vmpressure
  * infrastructure, so that the notifications will be delivered to the
- * @eventfd. The @args parameter is a string that denotes pressure level
+ * @eventfd. The @args parameters are a string that denotes pressure level
  * threshold (one of vmpressure_str_levels, i.e. "low", "medium", or
- * "critical").
+ * "critical") and a trigger option that decides whether events are triggered
+ * continuously or only on edge ("always" or "edge" if "edge", events
+ * are triggered when the pressure level changes.
  *
  * This function should not be used directly, just pass it to (struct
  * cftype).register_event, and then cgroup core will handle everything by
@@ -303,10 +310,21 @@ int vmpressure_register_event(struct cgroup *cg, struct cftype *cft,
 {
 	struct vmpressure *vmpr = cg_to_vmpressure(cg);
 	struct vmpressure_event *ev;
+	char *strlevel = NULL, *strtrigger = NULL, *p = NULL;
 	int level;
 
+	p = strchr(args, ' ');
+
+	if (p) {
+		strtrigger = p + 1;
+		*p = '\0';
+		strlevel = (char *)args;
+	} else {
+		strlevel = (char *)args;
+	}
+
 	for (level = 0; level < VMPRESSURE_NUM_LEVELS; level++) {
-		if (!strcmp(vmpressure_str_levels[level], args))
+		if (!strcmp(vmpressure_str_levels[level], strlevel))
 			break;
 	}
 
@@ -319,6 +337,12 @@ int vmpressure_register_event(struct cgroup *cg, struct cftype *cft,
 
 	ev->efd = eventfd;
 	ev->level = level;
+	ev->last_level = -1;
+
+	if (strtrigger && !strcmp(strtrigger, "edge"))
+		ev->edge_trigger = true;
+	else
+		ev->edge_trigger = false;
 
 	mutex_lock(&vmpr->events_lock);
 	list_add(&ev->node, &vmpr->events);
-- 
1.7.9.5


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