[PATCH v23 02/15] mm/damon/core: Implement region-based sampling

[SeongJae Park]

From: SeongJae Park <sjpark@xxxxxxxxx>

To avoid the unbounded increase of the overhead, DAMON groups adjacent
pages that assumed to have the same access frequencies into a region.
As long as the assumption (pages in a region have the same access
frequencies) is kept, only one page in the region is required to be
checked.  Thus, for each ``sampling interval``,

 1. the 'prepare_access_checks' primitive picks one page in each region,
 2. waits for one ``sampling interval``,
 3. checks whether the page is accessed meanwhile, and
 4. increases the access frequency of the region if so.

Therefore, the monitoring overhead is controllable by adjusting the
number of regions.  DAMON allows both the underlying primitives and user
callbacks adjust regions for the trade-off.  In other words, this commit
makes DAMON to use not only time-based sampling but also space-based
sampling.

This scheme, however, cannot preserve the quality of the output if the
assumption is not guaranteed.  Next commit will address this problem.

Another problem of this region abstraction is additional memory space
overhead for the regions metadata.  For example, suppose page
granularity monitoring that doesn't want to know fine-grained access
frequency but only if each page accessed or not.  Then, we can do that
by directly resetting and reading the PG_Idle flags and/or the PTE
Accessed bits.  The metadata for the region abstraction is only burden
in the case.  For the reason, this commit makes DAMON to support the
user-defined arbitrary target, which could be stored in a void pointer
of the monitoring context with specific target type.

Signed-off-by: SeongJae Park <sjpark@xxxxxxxxx>
Reviewed-by: Leonard Foerster <foersleo@xxxxxxxxx>
---
 include/linux/damon.h | 109 ++++++++++++++++++++++++++++++--
 mm/damon/core.c       | 142 +++++++++++++++++++++++++++++++++++++++++-
 2 files changed, 243 insertions(+), 8 deletions(-)

diff --git a/include/linux/damon.h b/include/linux/damon.h
index 387fa4399fc8..7d4685adc8a9 100644
--- a/include/linux/damon.h
+++ b/include/linux/damon.h
@@ -12,6 +12,48 @@
 #include <linux/time64.h>
 #include <linux/types.h>
 
+/**
+ * struct damon_addr_range - Represents an address region of [@start, @end).
+ * @start:	Start address of the region (inclusive).
+ * @end:	End address of the region (exclusive).
+ */
+struct damon_addr_range {
+	unsigned long start;
+	unsigned long end;
+};
+
+/**
+ * struct damon_region - Represents a monitoring target region.
+ * @ar:			The address range of the region.
+ * @sampling_addr:	Address of the sample for the next access check.
+ * @nr_accesses:	Access frequency of this region.
+ * @list:		List head for siblings.
+ */
+struct damon_region {
+	struct damon_addr_range ar;
+	unsigned long sampling_addr;
+	unsigned int nr_accesses;
+	struct list_head list;
+};
+
+/**
+ * struct damon_target - Represents a monitoring target.
+ * @id:			Unique identifier for this target.
+ * @regions_list:	Head of the monitoring target regions of this target.
+ * @list:		List head for siblings.
+ *
+ * Each monitoring context could have multiple targets.  For example, a context
+ * for virtual memory address spaces could have multiple target processes.  The
+ * @id of each target should be unique among the targets of the context.  For
+ * example, in the virtual address monitoring context, it could be a pidfd or
+ * an address of an mm_struct.
+ */
+struct damon_target {
+	unsigned long id;
+	struct list_head regions_list;
+	struct list_head list;
+};
+
 struct damon_ctx;
 
 /**
@@ -36,7 +78,8 @@ struct damon_ctx;
  *
  * @init_target_regions should construct proper monitoring target regions and
  * link those to the DAMON context struct.  The regions should be defined by
- * user and saved in @damon_ctx.target.
+ * user and saved in @damon_ctx.arbitrary_target if @damon_ctx.target_type is
+ * &DAMON_ARBITRARY_TARGET.  Otherwise, &struct damon_region should be used.
  * @update_target_regions should update the monitoring target regions for
  * current status.
  * @prepare_access_checks should manipulate the monitoring regions to be
@@ -46,7 +89,8 @@ struct damon_ctx;
  * @reset_aggregated should reset the access monitoring results that aggregated
  * by @check_accesses.
  * @target_valid should check whether the target is still valid for the
- * monitoring.
+ * monitoring.  It receives &damon_ctx.arbitrary_target or &struct damon_target
+ * pointer depends on &damon_ctx.target_type.
  * @cleanup is called from @kdamond just before its termination.  After this
  * call, only @kdamond_lock and @kdamond will be touched.
  */
@@ -91,6 +135,17 @@ struct damon_callback {
 	int (*before_terminate)(struct damon_ctx *context);
 };
 
+/**
+ * enum damon_target_type - Represents the type of the monitoring target.
+ *
+ * @DAMON_REGION_SAMPLING_TARGET:	Region based sampling target.
+ * @DAMON_ARBITRARY_TARGET:		User-defined arbitrary type target.
+ */
+enum damon_target_type {
+	DAMON_REGION_SAMPLING_TARGET,
+	DAMON_ARBITRARY_TARGET,
+};
+
 /**
  * struct damon_ctx - Represents a context for each monitoring.  This is the
  * main interface that allows users to set the attributes and get the results
@@ -130,7 +185,15 @@ struct damon_callback {
  * @primitive:	Set of monitoring primitives for given use cases.
  * @callback:	Set of callbacks for monitoring events notifications.
  *
- * @target:	Pointer to the user-defined monitoring target.
+ * @target_type:	Type of the monitoring target.
+ *
+ * @region_targets:	Head of monitoring targets (&damon_target) list.
+ *
+ * @arbitrary_target:	Pointer to arbitrary type target.
+ *
+ * @region_targets are valid only if @target_type is
+ * DAMON_REGION_SAMPLING_TARGET.  @arbitrary_target is valid only if
+ * @target_type is DAMON_ARBITRARY_TARGET.
  */
 struct damon_ctx {
 	unsigned long sample_interval;
@@ -149,12 +212,48 @@ struct damon_ctx {
 	struct damon_primitive primitive;
 	struct damon_callback callback;
 
-	void *target;
+	enum damon_target_type target_type;
+	union {
+		/* DAMON_REGION_SAMPLING_TARGET */
+		struct list_head region_targets;
+
+		/* DAMON_ARBITRARY_TARGET */
+		void *arbitrary_target;
+	};
 };
 
+#define damon_next_region(r) \
+	(container_of(r->list.next, struct damon_region, list))
+
+#define damon_prev_region(r) \
+	(container_of(r->list.prev, struct damon_region, list))
+
+#define damon_for_each_region(r, t) \
+	list_for_each_entry(r, &t->regions_list, list)
+
+#define damon_for_each_region_safe(r, next, t) \
+	list_for_each_entry_safe(r, next, &t->regions_list, list)
+
+#define damon_for_each_target(t, ctx) \
+	list_for_each_entry(t, &(ctx)->region_targets, list)
+
+#define damon_for_each_target_safe(t, next, ctx)	\
+	list_for_each_entry_safe(t, next, &(ctx)->region_targets, list)
+
 #ifdef CONFIG_DAMON
 
-struct damon_ctx *damon_new_ctx(void);
+struct damon_region *damon_new_region(unsigned long start, unsigned long end);
+inline void damon_insert_region(struct damon_region *r,
+		struct damon_region *prev, struct damon_region *next);
+void damon_add_region(struct damon_region *r, struct damon_target *t);
+void damon_destroy_region(struct damon_region *r);
+
+struct damon_target *damon_new_target(unsigned long id);
+void damon_add_target(struct damon_ctx *ctx, struct damon_target *t);
+void damon_free_target(struct damon_target *t);
+void damon_destroy_target(struct damon_target *t);
+
+struct damon_ctx *damon_new_ctx(enum damon_target_type type);
 void damon_destroy_ctx(struct damon_ctx *ctx);
 int damon_set_attrs(struct damon_ctx *ctx, unsigned long sample_int,
 		unsigned long aggr_int, unsigned long regions_update_int);
diff --git a/mm/damon/core.c b/mm/damon/core.c
index 8963804efdf9..167487e75737 100644
--- a/mm/damon/core.c
+++ b/mm/damon/core.c
@@ -15,7 +15,102 @@
 static DEFINE_MUTEX(damon_lock);
 static int nr_running_ctxs;
 
-struct damon_ctx *damon_new_ctx(void)
+/*
+ * Construct a damon_region struct
+ *
+ * Returns the pointer to the new struct if success, or NULL otherwise
+ */
+struct damon_region *damon_new_region(unsigned long start, unsigned long end)
+{
+	struct damon_region *region;
+
+	region = kmalloc(sizeof(*region), GFP_KERNEL);
+	if (!region)
+		return NULL;
+
+	region->ar.start = start;
+	region->ar.end = end;
+	region->nr_accesses = 0;
+	INIT_LIST_HEAD(&region->list);
+
+	return region;
+}
+
+/*
+ * Add a region between two other regions
+ */
+inline void damon_insert_region(struct damon_region *r,
+		struct damon_region *prev, struct damon_region *next)
+{
+	__list_add(&r->list, &prev->list, &next->list);
+}
+
+void damon_add_region(struct damon_region *r, struct damon_target *t)
+{
+	list_add_tail(&r->list, &t->regions_list);
+}
+
+static void damon_del_region(struct damon_region *r)
+{
+	list_del(&r->list);
+}
+
+static void damon_free_region(struct damon_region *r)
+{
+	kfree(r);
+}
+
+void damon_destroy_region(struct damon_region *r)
+{
+	damon_del_region(r);
+	damon_free_region(r);
+}
+
+/*
+ * Construct a damon_target struct
+ *
+ * Returns the pointer to the new struct if success, or NULL otherwise
+ */
+struct damon_target *damon_new_target(unsigned long id)
+{
+	struct damon_target *t;
+
+	t = kmalloc(sizeof(*t), GFP_KERNEL);
+	if (!t)
+		return NULL;
+
+	t->id = id;
+	INIT_LIST_HEAD(&t->regions_list);
+
+	return t;
+}
+
+void damon_add_target(struct damon_ctx *ctx, struct damon_target *t)
+{
+	list_add_tail(&t->list, &ctx->region_targets);
+}
+
+static void damon_del_target(struct damon_target *t)
+{
+	list_del(&t->list);
+}
+
+void damon_free_target(struct damon_target *t)
+{
+	struct damon_region *r, *next;
+
+	damon_for_each_region_safe(r, next, t)
+		damon_free_region(r);
+	kfree(t);
+}
+
+void damon_destroy_target(struct damon_target *t)
+{
+	damon_del_target(t);
+	damon_free_target(t);
+}
+
+struct damon_ctx *damon_new_ctx(enum damon_target_type type)
 {
 	struct damon_ctx *ctx;
 
@@ -32,13 +127,20 @@ struct damon_ctx *damon_new_ctx(void)
 
 	mutex_init(&ctx->kdamond_lock);
 
-	ctx->target = NULL;
+	ctx->target_type = type;
+	if (type != DAMON_ARBITRARY_TARGET)
+		INIT_LIST_HEAD(&ctx->region_targets);
 
 	return ctx;
 }
 
 void damon_destroy_ctx(struct damon_ctx *ctx)
 {
+	struct damon_target *t, *next_t;
+
+	damon_for_each_target_safe(t, next_t, ctx)
+		damon_destroy_target(t);
+
 	kfree(ctx);
 }
 
@@ -215,6 +317,21 @@ static bool kdamond_aggregate_interval_passed(struct damon_ctx *ctx)
 			ctx->aggr_interval);
 }
 
+/*
+ * Reset the aggregated monitoring results ('nr_accesses' of each region).
+ */
+static void kdamond_reset_aggregated(struct damon_ctx *c)
+{
+	struct damon_target *t;
+
+	damon_for_each_target(t, c) {
+		struct damon_region *r;
+
+		damon_for_each_region(r, t)
+			r->nr_accesses = 0;
+	}
+}
+
 /*
  * Check whether it is time to check and apply the target monitoring regions
  *
@@ -236,6 +353,7 @@ static bool kdamond_need_update_regions(struct damon_ctx *ctx)
  */
 static bool kdamond_need_stop(struct damon_ctx *ctx)
 {
+	struct damon_target *t;
 	bool stop;
 
 	mutex_lock(&ctx->kdamond_lock);
@@ -247,7 +365,15 @@ static bool kdamond_need_stop(struct damon_ctx *ctx)
 	if (!ctx->primitive.target_valid)
 		return false;
 
-	return !ctx->primitive.target_valid(ctx->target);
+	if (ctx->target_type == DAMON_ARBITRARY_TARGET)
+		return !ctx->primitive.target_valid(ctx->arbitrary_target);
+
+	damon_for_each_target(t, ctx) {
+		if (ctx->primitive.target_valid(t))
+			return false;
+	}
+
+	return true;
 }
 
 static void set_kdamond_stop(struct damon_ctx *ctx)
@@ -263,6 +389,8 @@ static void set_kdamond_stop(struct damon_ctx *ctx)
 static int kdamond_fn(void *data)
 {
 	struct damon_ctx *ctx = (struct damon_ctx *)data;
+	struct damon_target *t;
+	struct damon_region *r, *next;
 
 	pr_info("kdamond (%d) starts\n", ctx->kdamond->pid);
 
@@ -287,6 +415,8 @@ static int kdamond_fn(void *data)
 			if (ctx->callback.after_aggregation &&
 					ctx->callback.after_aggregation(ctx))
 				set_kdamond_stop(ctx);
+			if (ctx->target_type != DAMON_ARBITRARY_TARGET)
+				kdamond_reset_aggregated(ctx);
 			if (ctx->primitive.reset_aggregated)
 				ctx->primitive.reset_aggregated(ctx);
 		}
@@ -296,6 +426,12 @@ static int kdamond_fn(void *data)
 				ctx->primitive.update_target_regions(ctx);
 		}
 	}
+	if (ctx->target_type != DAMON_ARBITRARY_TARGET) {
+		damon_for_each_target(t, ctx) {
+			damon_for_each_region_safe(r, next, t)
+				damon_destroy_region(r);
+		}
+	}
 
 	if (ctx->callback.before_terminate &&
 			ctx->callback.before_terminate(ctx))
-- 
2.17.1