From: Bijan Tabatabai <bijantabatab@micron.com>

Any DAMON command that uses damos_walk() internally, such as
update_schemes_tried_bytes and update_schemes_tried_regions, waits until
each scheme in the DAMON context have finished. This can be useful, for
example, if a user wants to know when new scheme parameters they've
committed have been applied. Another use case could be if a user wants
to record the system state every time a scheme is applied for debug
purposes.

This patch adds a new command, wait_for_schemes_apply, which calls
damos_walk() without a walk function so that all it does is wait until
all schemes have been applied. The same functionality can be achieved
by using update_schemes_tried_bytes, for example, but having a separate
command for this avoid unnecessary work. It also makes user intent
clearer when used in scripts.

Signed-off-by: Bijan Tabatabai <bijantabatab@micron.com>
---
 mm/damon/core.c  |  3 ++-
 mm/damon/sysfs.c | 27 +++++++++++++++++++++++++++
 2 files changed, 29 insertions(+), 1 deletion(-)

diff --git a/mm/damon/core.c b/mm/damon/core.c
index 07b4fc5a9790..56a13d16e4d1 100644
--- a/mm/damon/core.c
+++ b/mm/damon/core.c
@@ -1731,7 +1731,8 @@ static void damos_walk_call_walk(struct damon_ctx *ctx, struct damon_target *t,
 	if (!control)
 		return;
 
-	control->walk_fn(control->data, ctx, t, r, s, sz_filter_passed);
+	if (control->walk_fn)
+		control->walk_fn(control->data, ctx, t, r, s, sz_filter_passed);
 }
 
 /*
diff --git a/mm/damon/sysfs.c b/mm/damon/sysfs.c
index c2b036abb25b..ded3f60e4e22 100644
--- a/mm/damon/sysfs.c
+++ b/mm/damon/sysfs.c
@@ -1223,6 +1223,11 @@ enum damon_sysfs_cmd {
 	 * intevals.
 	 */
 	DAMON_SYSFS_CMD_UPDATE_TUNED_INTERVALS,
+	/*
+	 * @DAMON_SYSFS_CMD_WAIT_FOR_SCHEMES_APPLY: Wait for all schemes to be
+	 * applied.
+	 */
+	DAMON_SYSFS_CMD_WAIT_FOR_SCHEMES_APPLY,
 	/*
 	 * @NR_DAMON_SYSFS_CMDS: Total number of DAMON sysfs commands.
 	 */
@@ -1242,6 +1247,7 @@ static const char * const damon_sysfs_cmd_strs[] = {
 	"clear_schemes_tried_regions",
 	"update_schemes_effective_quotas",
 	"update_tuned_intervals",
+	"wait_for_schemes_apply",
 };
 
 static ssize_t state_show(struct kobject *kobj, struct kobj_attribute *attr,
@@ -1643,6 +1649,25 @@ static int damon_sysfs_update_schemes_tried_regions(
 	return damos_walk(ctx, &control);
 }
 
+static int damon_sysfs_wait_for_schemes_apply(
+		struct damon_sysfs_kdamond *sysfs_kdamond)
+{
+	/*
+	 * damos_walk returns after the next time all of the schemes have been
+	 * applied. We don't need to do any actual work, so walk_fn is NULL.
+	 */
+	struct damos_walk_control control = {
+		.walk_fn = NULL,
+		.data = NULL,
+	};
+	struct damon_ctx *ctx = sysfs_kdamond->damon_ctx;
+
+	if (!ctx)
+		return -EINVAL;
+
+	return damos_walk(ctx, &control);
+}
+
 /*
  * damon_sysfs_handle_cmd() - Handle a command for a specific kdamond.
  * @cmd:	The command to handle.
@@ -1688,6 +1713,8 @@ static int damon_sysfs_handle_cmd(enum damon_sysfs_cmd cmd,
 	case DAMON_SYSFS_CMD_UPDATE_TUNED_INTERVALS:
 		return damon_sysfs_damon_call(
 				damon_sysfs_upd_tuned_intervals, kdamond);
+	case DAMON_SYSFS_CMD_WAIT_FOR_SCHEMES_APPLY:
+		return damon_sysfs_wait_for_schemes_apply(kdamond);
 	default:
 		return -EINVAL;
 	}
-- 
2.43.5