bcachefs: Add a tracepoint for the btree cache shrinker

This is to help with diagnosing why the btree node can doesn't seem to
be shrinking - we've had issues in the past with granularity/batch size,
since btree nodes are so big.
Signed-off-by: Kent Overstreet <kent.overstreet@gmail.com>

fs/bcachefs/btree_cache.c

@@ -275,6 +275,7 @@ static unsigned long bch2_btree_cache_scan(struct shrinker *shrink,
 	unsigned long touched = 0;
 	unsigned long freed = 0;
 	unsigned i, flags;
+	unsigned long ret = SHRINK_STOP;
 
 	if (bch2_btree_shrinker_disabled)
 		return SHRINK_STOP;
@@ -283,7 +284,7 @@ static unsigned long bch2_btree_cache_scan(struct shrinker *shrink,
 	if (sc->gfp_mask & __GFP_FS)
 		mutex_lock(&bc->lock);
 	else if (!mutex_trylock(&bc->lock))
-		return -1;
+		goto out_norestore;
 
 	flags = memalloc_nofs_save();
 
@@ -358,8 +359,14 @@ static unsigned long bch2_btree_cache_scan(struct shrinker *shrink,
 
 	mutex_unlock(&bc->lock);
 out:
+	ret = (unsigned long) freed * btree_pages(c);
 	memalloc_nofs_restore(flags);
-	return (unsigned long) freed * btree_pages(c);
+out_norestore:
+	trace_btree_cache_scan(sc->nr_to_scan,
+			       sc->nr_to_scan / btree_pages(c),
+			       btree_cache_can_free(bc),
+			       ret);
+	return ret;
 }
 
 static unsigned long bch2_btree_cache_count(struct shrinker *shrink,

fs/bcachefs/trace.h

@@ -318,6 +318,34 @@ DEFINE_EVENT(btree_node, btree_set_root,
 	TP_ARGS(c, b)
 );
 
+TRACE_EVENT(btree_cache_scan,
+	TP_PROTO(unsigned long nr_to_scan_pages,
+		 unsigned long nr_to_scan_nodes,
+		 unsigned long can_free_nodes,
+		 long ret),
+	TP_ARGS(nr_to_scan_pages, nr_to_scan_nodes, can_free_nodes, ret),
+
+	TP_STRUCT__entry(
+		__field(unsigned long,	nr_to_scan_pages	)
+		__field(unsigned long,	nr_to_scan_nodes	)
+		__field(unsigned long,	can_free_nodes		)
+		__field(long,		ret			)
+	),
+
+	TP_fast_assign(
+		__entry->nr_to_scan_pages	= nr_to_scan_pages;
+		__entry->nr_to_scan_nodes	= nr_to_scan_nodes;
+		__entry->can_free_nodes		= can_free_nodes;
+		__entry->ret			= ret;
+	),
+
+	TP_printk("scanned for %lu pages, %lu nodes, can free %lu nodes, ret %li",
+		  __entry->nr_to_scan_pages,
+		  __entry->nr_to_scan_nodes,
+		  __entry->can_free_nodes,
+		  __entry->ret)
+);
+
 /* Garbage collection */
 
 DEFINE_EVENT(btree_node, btree_gc_rewrite_node,