[RFC PATCH v2 4/9] mm/damon: add damos_node_eligible_mem_bp tracepoint

[Ravi Jonnalagadda]

Add a per-tick tracepoint for DAMOS_QUOTA_NODE_ELIGIBLE_MEM_BP goal
evaluation, exposing the goal target/current values for observability of
the node-eligible-memory bandwidth quota loop.  Bandwidth-feedback tiering
loops read this goal metric to detect convergence.

Signed-off-by: Ravi Jonnalagadda <ravis.opensrc@xxxxxxxxx>
---
 include/trace/events/damon.h | 32 ++++++++++++++++++++++++++++++++
 mm/damon/core.c              | 28 ++++++++++++++++++++++++++++
 2 files changed, 60 insertions(+)

diff --git a/include/trace/events/damon.h b/include/trace/events/damon.h
index 4dfc9467ea4b5..b2a472eb45d95 100644
--- a/include/trace/events/damon.h
+++ b/include/trace/events/damon.h
@@ -74,6 +74,38 @@ TRACE_EVENT(damos_esz,
 			__entry->esz)
 );
 
+/* Per-tick DAMOS_QUOTA_NODE_ELIGIBLE_MEM_BP goal evaluation. */
+TRACE_EVENT(damos_node_eligible_mem_bp,
+
+	TP_PROTO(unsigned int context_idx, unsigned int scheme_idx,
+		int nid,
+		unsigned long target_value, unsigned long current_value),
+
+	TP_ARGS(context_idx, scheme_idx, nid, target_value, current_value),
+
+	TP_STRUCT__entry(
+		__field(unsigned int, context_idx)
+		__field(unsigned int, scheme_idx)
+		__field(int, nid)
+		__field(unsigned long, target_value)
+		__field(unsigned long, current_value)
+	),
+
+	TP_fast_assign(
+		__entry->context_idx = context_idx;
+		__entry->scheme_idx = scheme_idx;
+		__entry->nid = nid;
+		__entry->target_value = target_value;
+		__entry->current_value = current_value;
+	),
+
+	TP_printk("ctx_idx=%u scheme_idx=%u nid=%d "
+			"target_value=%lu current_value=%lu",
+			__entry->context_idx, __entry->scheme_idx,
+			__entry->nid,
+			__entry->target_value, __entry->current_value)
+);
+
 TRACE_EVENT_CONDITION(damos_before_apply,
 
 	TP_PROTO(unsigned int context_idx, unsigned int scheme_idx,
diff --git a/mm/damon/core.c b/mm/damon/core.c
index 991ddb9a09414..6b56a3e961c4e 100644
--- a/mm/damon/core.c
+++ b/mm/damon/core.c
@@ -3461,6 +3461,34 @@ static unsigned long damos_quota_score(struct damon_ctx *c, struct damos *s)
 		highest_score = max(highest_score,
 				mult_frac(goal->current_value, 10000,
 					goal->target_value));
+
+		/*
+		 * Per-tick visibility of NODE_ELIGIBLE_MEM_BP goal evaluation
+		 * for userspace convergence-detection.
+		 */
+		if (goal->metric == DAMOS_QUOTA_NODE_ELIGIBLE_MEM_BP &&
+		    trace_damos_node_eligible_mem_bp_enabled()) {
+			/*
+			 * cidx is hardcoded 0 because DAMON sysfs currently
+			 * supports a single context per kdamond.  Other DAMOS
+			 * tracepoints (damos_before_apply, damos_esz,
+			 * damos_stat_after_apply_interval) follow the same
+			 * convention; once multi-ctx-per-kdamond lands the
+			 * field becomes meaningful.
+			 */
+			unsigned int cidx = 0, sidx = 0;
+			struct damos *siter;
+
+			damon_for_each_scheme(siter, c) {
+				if (siter == s)
+					break;
+				sidx++;
+			}
+			trace_damos_node_eligible_mem_bp(cidx, sidx,
+					goal->nid,
+					goal->target_value,
+					goal->current_value);
+		}
 	}
 
 	return highest_score;
-- 
2.43.0