[PATCH 2/2] vmevent: Implement greater-than attribute and one-shot mode

[Anton Vorontsov <anton.vorontsov@linaro.org>]

This patch implements a new event type, it will trigger whenever a
value becomes greater than user-specified threshold, it complements
the 'less-then' trigger type.

Also, let's implement the one-shot mode for the events, when set,
userspace will only receive one notification per crossing the
boundaries.

Now when both LT and GT are set on the same level, the event type
works as a cross event type: it triggers whenever a value crosses
the threshold from a lesser values side to a greater values side,
and vice versa.

We use the event types in an userspace low-memory killer: we get a
notification when memory becomes low, so we start freeing memory by
killing unneeded processes, and we get notification when memory hits
the threshold from another side, so we know that we freed enough of
memory.

Signed-off-by: Anton Vorontsov <anton.vorontsov@linaro.org>
---
 include/linux/vmevent.h              |   13 ++++++++++
 mm/vmevent.c                         |   44 +++++++++++++++++++++++++++++-----
 tools/testing/vmevent/vmevent-test.c |   22 +++++++++++++----
 3 files changed, 68 insertions(+), 11 deletions(-)

diff --git a/include/linux/vmevent.h b/include/linux/vmevent.h
index 64357e4..ca97cf0 100644
--- a/include/linux/vmevent.h
+++ b/include/linux/vmevent.h
@@ -22,6 +22,19 @@ enum {
 	 * Sample value is less than user-specified value
 	 */
 	VMEVENT_ATTR_STATE_VALUE_LT	= (1UL << 0),
+	/*
+	 * Sample value is greater than user-specified value
+	 */
+	VMEVENT_ATTR_STATE_VALUE_GT	= (1UL << 1),
+	/*
+	 * One-shot mode.
+	 */
+	VMEVENT_ATTR_STATE_ONE_SHOT	= (1UL << 2),
+
+	/* Saved state, used internally by the kernel for one-shot mode. */
+	__VMEVENT_ATTR_STATE_VALUE_WAS_LT	= (1UL << 30),
+	/* Saved state, used internally by the kernel for one-shot mode. */
+	__VMEVENT_ATTR_STATE_VALUE_WAS_GT	= (1UL << 31),
 };
 
 struct vmevent_attr {
diff --git a/mm/vmevent.c b/mm/vmevent.c
index 9ed6aca..3cce215 100644
--- a/mm/vmevent.c
+++ b/mm/vmevent.c
@@ -1,5 +1,6 @@
 #include <linux/anon_inodes.h>
 #include <linux/atomic.h>
+#include <linux/compiler.h>
 #include <linux/vmevent.h>
 #include <linux/syscalls.h>
 #include <linux/timer.h>
@@ -83,16 +84,47 @@ static bool vmevent_match(struct vmevent_watch *watch)
 
 	for (i = 0; i < config->counter; i++) {
 		struct vmevent_attr *attr = &config->attrs[i];
-		u64 value;
+		u32 state = attr->state;
+		bool attr_lt = state & VMEVENT_ATTR_STATE_VALUE_LT;
+		bool attr_gt = state & VMEVENT_ATTR_STATE_VALUE_GT;
 
-		if (!attr->state)
+		if (!state)
 			continue;
 
-		value = vmevent_sample_attr(watch, attr);
-
-		if (attr->state & VMEVENT_ATTR_STATE_VALUE_LT) {
-			if (value < attr->value)
+		if (attr_lt || attr_gt) {
+			bool one_shot = state & VMEVENT_ATTR_STATE_ONE_SHOT;
+			u32 was_lt_mask = __VMEVENT_ATTR_STATE_VALUE_WAS_LT;
+			u32 was_gt_mask = __VMEVENT_ATTR_STATE_VALUE_WAS_GT;
+			u64 value = vmevent_sample_attr(watch, attr);
+			bool lt = value < attr->value;
+			bool gt = value > attr->value;
+			bool was_lt = state & was_lt_mask;
+			bool was_gt = state & was_gt_mask;
+			bool ret = false;
+
+			if ((lt || gt) && !one_shot)
 				return true;
+
+			if (attr_lt && lt && was_lt) {
+				return false;
+			} else if (attr_gt && gt && was_gt) {
+				return false;
+			} else if (lt) {
+				state |= was_lt_mask;
+				state &= ~was_gt_mask;
+				if (attr_lt)
+					ret = true;
+			} else if (gt) {
+				state |= was_gt_mask;
+				state &= ~was_lt_mask;
+				if (attr_gt)
+					ret = true;
+			} else {
+				state &= ~was_lt_mask;
+				state &= ~was_gt_mask;
+			}
+			attr->state = state;
+			return ret;
 		}
 	}
 
diff --git a/tools/testing/vmevent/vmevent-test.c b/tools/testing/vmevent/vmevent-test.c
index 534f827..fec7b57 100644
--- a/tools/testing/vmevent/vmevent-test.c
+++ b/tools/testing/vmevent/vmevent-test.c
@@ -33,20 +33,32 @@ int main(int argc, char *argv[])
 
 	config = (struct vmevent_config) {
 		.sample_period_ns	= 1000000000L,
-		.counter		= 4,
+		.counter		= 6,
 		.attrs			= {
-			[0]			= {
+			{
 				.type	= VMEVENT_ATTR_NR_FREE_PAGES,
 				.state	= VMEVENT_ATTR_STATE_VALUE_LT,
 				.value	= phys_pages,
 			},
-			[1]			= {
+			{
+				.type	= VMEVENT_ATTR_NR_FREE_PAGES,
+				.state	= VMEVENT_ATTR_STATE_VALUE_GT,
+				.value	= phys_pages,
+			},
+			{
+				.type	= VMEVENT_ATTR_NR_FREE_PAGES,
+				.state	= VMEVENT_ATTR_STATE_VALUE_LT |
+					  VMEVENT_ATTR_STATE_VALUE_GT |
+					  VMEVENT_ATTR_STATE_ONE_SHOT,
+				.value	= phys_pages / 2,
+			},
+			{
 				.type	= VMEVENT_ATTR_NR_AVAIL_PAGES,
 			},
-			[2]			= {
+			{
 				.type	= VMEVENT_ATTR_NR_SWAP_PAGES,
 			},
-			[3]			= {
+			{
 				.type	= 0xffff, /* invalid */
 			},
 		},
-- 
1.7.9.2

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