mm/damon/core: introduce nr_snapshots damos stat

Patch series "mm/damon: introduce {,max_}nr_snapshots and tracepoint for
damos stats".

Introduce three changes for improving DAMOS stat's provided information,
deterministic control, and reading usability.

DAMOS provides stats that are important for understanding its behavior.
It lacks information about how many DAMON-generated monitoring output
snapshots it has worked on.  Add a new stat, nr_snapshots, to show the
information.

Users can control DAMOS schemes in multiple ways.  Using the online
parameters commit feature, they can install and uninstall DAMOS schemes
whenever they want while keeping DAMON runs.  DAMOS quotas and watermarks
can be used for manually or automatically turning on/off or adjusting the
aggressiveness of the scheme.  DAMOS filters can be used for applying the
scheme to specific memory entities based on their types and locations.
Some users want their DAMOS scheme to be applied to only specific number
of DAMON snapshots, for more deterministic control.  One example use case
is tracepoint based snapshot reading.  Add a new knob, max_nr_snapshots,
to support this.  If the nr_snapshots parameter becomes same to or greater
than the value of this parameter, the scheme is deactivated.

Users can read DAMOS stats via DAMON's sysfs interface.  For deep level
investigations on environments having advanced tools like perf and
bpftrace, exposing the stats via a tracepoint can be useful.  Implement a
new tracepoint, namely damon:damos_stat_after_apply_interval.

First five patches (patches 1-5) of this series implement the new stat,
nr_snapshots, on the core layer (patch 1), expose on DAMON sysfs user
interface (patch 2), and update documents (patches 3-5).

Following six patches (patches 6-11) are for the new stat based DAMOS
deactivation (max_nr_snapshots).  The first one (patch 6) of this group
updates a kernel-doc comment before making further changes.  Then an
implementation of it on the core layer (patch 7), an introduction of a new
DAMON sysfs interface file for users of the feature (patch 8), and three
updates of the documents (patches 9-11) follow.

The final one (patch 12) introduces the new tracepoint that exposes the
DAMOS stat values for each scheme apply interval.

This patch (of 12):

DAMON generates monitoring results snapshots for every sampling interval.
DAMOS applies given schemes on the regions of the snapshots, for every
apply interval of the scheme.

DAMOS stat informs a given scheme has tried to how many memory entities
and applied, in the region and byte level.  In some use cases including
user-space oriented tuning and investigations, it is useful to know that
in the DAMON-snapshot level.  Introduce a new stat, namely nr_snapshots
for DAMON core API callers.

[sj@kernel.org: fix wrong list_is_last() call in damons_is_last_region()]
Link: https://lkml.kernel.org/r/20260114152049.99727-1-sj@kernel.org
Link: https://lkml.kernel.org/r/20251216080128.42991-1-sj@kernel.org
Link: https://lkml.kernel.org/r/20251216080128.42991-2-sj@kernel.org
Signed-off-by: SeongJae Park <sj@kernel.org>
Cc: Jonathan Corbet <corbet@lwn.net>
Cc: Liam Howlett <liam.howlett@oracle.com>
Cc: Lorenzo Stoakes <lorenzo.stoakes@oracle.com>
Cc: "Masami Hiramatsu (Google)" <mhiramat@kernel.org>
Cc: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Cc: Michal Hocko <mhocko@suse.com>
Cc: Mike Rapoport <rppt@kernel.org>
Cc: Steven Rostedt <rostedt@goodmis.org>
Cc: Suren Baghdasaryan <surenb@google.com>
Cc: Vlastimil Babka <vbabka@suse.cz>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

diff --git a/include/linux/damon.h b/include/linux/damon.h
index 3813373a9200c44d300154419d2b11dd92a9094e..1d8a1515e75ad7484cdbac11d1cb4bdb03c34891 100644 (file)
--- a/include/linux/damon.h
+++ b/include/linux/damon.h
@@ -330,6 +330,8 @@ struct damos_watermarks {
  * @sz_ops_filter_passed:
  *             Total bytes that passed ops layer-handled DAMOS filters.
  * @qt_exceeds: Total number of times the quota of the scheme has exceeded.
+ * @nr_snapshots:
+ *             Total number of DAMON snapshots that the scheme has tried.
  *
  * "Tried an action to a region" in this context means the DAMOS core logic
  * determined the region as eligible to apply the action.  The access pattern
@@ -355,6 +357,7 @@ struct damos_stat {
        unsigned long sz_applied;
        unsigned long sz_ops_filter_passed;
        unsigned long qt_exceeds;
+       unsigned long nr_snapshots;
 };
 
 /**

diff --git a/mm/damon/core.c b/mm/damon/core.c
index 2379a07c2f877b320fcc0d2e8710de1e6a64be53..9d5be7e9b8e013c26e47f3c470a303838b46f497 100644 (file)
--- a/mm/damon/core.c
+++ b/mm/damon/core.c
@@ -157,6 +157,12 @@ void damon_destroy_region(struct damon_region *r, struct damon_target *t)
        damon_free_region(r);
 }
 
+static bool damon_is_last_region(struct damon_region *r,
+               struct damon_target *t)
+{
+       return list_is_last(&r->list, &t->regions_list);
+}
+
 /*
  * Check whether a region is intersecting an address range
  *
@@ -1978,10 +1984,11 @@ static void damon_do_apply_schemes(struct damon_ctx *c,
                if (damos_skip_charged_region(t, &r, s, c->min_sz_region))
                        continue;
 
-               if (!damos_valid_target(c, t, r, s))
-                       continue;
+               if (damos_valid_target(c, t, r, s))
+                       damos_apply_scheme(c, t, r, s);
 
-               damos_apply_scheme(c, t, r, s);
+               if (damon_is_last_region(r, t))
+                       s->stat.nr_snapshots++;
        }
 }