mm/damon/stat: calculate and expose estimated memory bandwidth

The raw form of DAMON's monitoring results captures many details of the
information.  However, not every bit of the information is always required
for understanding practical access patterns.  Especially on real world
production systems of high scale time and size, the raw form is difficult
to be aggregated and compared.

Convert the raw monitoring results into a single number metric, namely
estimated memory bandwidth and expose it to users as a read-only
DAMON_STAT parameter.  The metric represents access intensiveness
(hotness) of the system.  It can easily be aggregated and compared for
high level understanding of the access pattern on large systems.

Link: https://lkml.kernel.org/r/20250604183127.13968-3-sj@kernel.org
Signed-off-by: SeongJae Park <sj@kernel.org>
Cc: Jonathan Corbet <corbet@lwn.net>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

diff --git a/mm/damon/stat.c b/mm/damon/stat.c
index dd00f2384b95798add9cef2baba80bb7176fb2f7..3be86ca57c8b7f3dcb37b4f5425b8b43e7face59 100644 (file)
--- a/mm/damon/stat.c
+++ b/mm/damon/stat.c
@@ -29,8 +29,42 @@ static bool enabled __read_mostly = IS_ENABLED(
 module_param_cb(enabled, &enabled_param_ops, &enabled, 0600);
 MODULE_PARM_DESC(enabled, "Enable of disable DAMON_STAT");
 
+static unsigned long estimated_memory_bandwidth __read_mostly;
+module_param(estimated_memory_bandwidth, ulong, 0400);
+MODULE_PARM_DESC(estimated_memory_bandwidth,
+               "Estimated memory bandwidth usage in bytes per second");
+
 static struct damon_ctx *damon_stat_context;
 
+static void damon_stat_set_estimated_memory_bandwidth(struct damon_ctx *c)
+{
+       struct damon_target *t;
+       struct damon_region *r;
+       unsigned long access_bytes = 0;
+
+       damon_for_each_target(t, c) {
+               damon_for_each_region(r, t)
+                       access_bytes += (r->ar.end - r->ar.start) *
+                               r->nr_accesses;
+       }
+       estimated_memory_bandwidth = access_bytes * USEC_PER_MSEC *
+               MSEC_PER_SEC / c->attrs.aggr_interval;
+}
+
+static int damon_stat_after_aggregation(struct damon_ctx *c)
+{
+       static unsigned long last_refresh_jiffies;
+
+       /* avoid unnecessarily frequent stat update */
+       if (time_before_eq(jiffies, last_refresh_jiffies +
+                               msecs_to_jiffies(5 * MSEC_PER_SEC)))
+               return 0;
+       last_refresh_jiffies = jiffies;
+
+       damon_stat_set_estimated_memory_bandwidth(c);
+       return 0;
+}
+
 static struct damon_ctx *damon_stat_build_ctx(void)
 {
        struct damon_ctx *ctx;
@@ -76,6 +110,7 @@ static struct damon_ctx *damon_stat_build_ctx(void)
        damon_add_target(ctx, target);
        if (damon_set_region_biggest_system_ram_default(target, &start, &end))
                goto free_out;
+       ctx->callback.after_aggregation = damon_stat_after_aggregation;
        return ctx;
 free_out:
        damon_destroy_ctx(ctx);