For now, fgraph is used for the fprobe, even if we need trace the entry
only. However, the performance of ftrace is better than fgraph, and we
can use ftrace_ops for this case.

Then performance of kprobe-multi increases from 54M to 69M. Before this
commit:

  $ ./benchs/run_bench_trigger.sh kprobe-multi
  kprobe-multi   :   54.663 ± 0.493M/s

After this commit:

  $ ./benchs/run_bench_trigger.sh kprobe-multi
  kprobe-multi   :   69.447 ± 0.143M/s

Mitigation is disable during the bench testing above.

Signed-off-by: Menglong Dong <dongml2@chinatelecom.cn>
---
 kernel/trace/fprobe.c | 88 +++++++++++++++++++++++++++++++++++++++----
 1 file changed, 81 insertions(+), 7 deletions(-)

diff --git a/kernel/trace/fprobe.c b/kernel/trace/fprobe.c
index 1785fba367c9..de4ae075548d 100644
--- a/kernel/trace/fprobe.c
+++ b/kernel/trace/fprobe.c
@@ -292,7 +292,7 @@ static int fprobe_fgraph_entry(struct ftrace_graph_ent *trace, struct fgraph_ops
 				if (node->addr != func)
 					continue;
 				fp = READ_ONCE(node->fp);
-				if (fp && !fprobe_disabled(fp))
+				if (fp && !fprobe_disabled(fp) && fp->exit_handler)
 					fp->nmissed++;
 			}
 			return 0;
@@ -312,11 +312,11 @@ static int fprobe_fgraph_entry(struct ftrace_graph_ent *trace, struct fgraph_ops
 		if (node->addr != func)
 			continue;
 		fp = READ_ONCE(node->fp);
-		if (!fp || fprobe_disabled(fp))
+		if (unlikely(!fp || fprobe_disabled(fp) || !fp->exit_handler))
 			continue;
 
 		data_size = fp->entry_data_size;
-		if (data_size && fp->exit_handler)
+		if (data_size)
 			data = fgraph_data + used + FPROBE_HEADER_SIZE_IN_LONG;
 		else
 			data = NULL;
@@ -327,7 +327,7 @@ static int fprobe_fgraph_entry(struct ftrace_graph_ent *trace, struct fgraph_ops
 			ret = __fprobe_handler(func, ret_ip, fp, fregs, data);
 
 		/* If entry_handler returns !0, nmissed is not counted but skips exit_handler. */
-		if (!ret && fp->exit_handler) {
+		if (!ret) {
 			int size_words = SIZE_IN_LONG(data_size);
 
 			if (write_fprobe_header(&fgraph_data[used], fp, size_words))
@@ -384,6 +384,70 @@ static struct fgraph_ops fprobe_graph_ops = {
 };
 static int fprobe_graph_active;
 
+/* ftrace_ops backend (entry-only) */
+static void fprobe_ftrace_entry(unsigned long ip, unsigned long parent_ip,
+	struct ftrace_ops *ops, struct ftrace_regs *fregs)
+{
+	struct fprobe_hlist_node *node;
+	struct rhlist_head *head, *pos;
+	struct fprobe *fp;
+
+	guard(rcu)();
+	head = rhltable_lookup(&fprobe_ip_table, &ip, fprobe_rht_params);
+
+	rhl_for_each_entry_rcu(node, pos, head, hlist) {
+		if (node->addr != ip)
+			break;
+		fp = READ_ONCE(node->fp);
+		if (unlikely(!fp || fprobe_disabled(fp) || fp->exit_handler))
+			continue;
+		/* entry-only path: no exit_handler nor per-call data */
+		if (fprobe_shared_with_kprobes(fp))
+			__fprobe_kprobe_handler(ip, parent_ip, fp, fregs, NULL);
+		else
+			__fprobe_handler(ip, parent_ip, fp, fregs, NULL);
+	}
+}
+NOKPROBE_SYMBOL(fprobe_ftrace_entry);
+
+static struct ftrace_ops fprobe_ftrace_ops = {
+	.func	= fprobe_ftrace_entry,
+	.flags	= FTRACE_OPS_FL_SAVE_REGS,
+};
+static int fprobe_ftrace_active;
+
+static int fprobe_ftrace_add_ips(unsigned long *addrs, int num)
+{
+	int ret;
+
+	lockdep_assert_held(&fprobe_mutex);
+
+	ret = ftrace_set_filter_ips(&fprobe_ftrace_ops, addrs, num, 0, 0);
+	if (ret)
+		return ret;
+
+	if (!fprobe_ftrace_active) {
+		ret = register_ftrace_function(&fprobe_ftrace_ops);
+		if (ret) {
+			ftrace_free_filter(&fprobe_ftrace_ops);
+			return ret;
+		}
+	}
+	fprobe_ftrace_active++;
+	return 0;
+}
+
+static void fprobe_ftrace_remove_ips(unsigned long *addrs, int num)
+{
+	lockdep_assert_held(&fprobe_mutex);
+
+	fprobe_ftrace_active--;
+	if (!fprobe_ftrace_active)
+		unregister_ftrace_function(&fprobe_ftrace_ops);
+	if (num)
+		ftrace_set_filter_ips(&fprobe_ftrace_ops, addrs, num, 1, 0);
+}
+
 /* Add @addrs to the ftrace filter and register fgraph if needed. */
 static int fprobe_graph_add_ips(unsigned long *addrs, int num)
 {
@@ -500,9 +564,12 @@ static int fprobe_module_callback(struct notifier_block *nb,
 	} while (node == ERR_PTR(-EAGAIN));
 	rhashtable_walk_exit(&iter);
 
-	if (alist.index < alist.size && alist.index > 0)
+	if (alist.index < alist.size && alist.index > 0) {
 		ftrace_set_filter_ips(&fprobe_graph_ops.ops,
 				      alist.addrs, alist.index, 1, 0);
+		ftrace_set_filter_ips(&fprobe_ftrace_ops,
+				      alist.addrs, alist.index, 1, 0);
+	}
 	mutex_unlock(&fprobe_mutex);
 
 	kfree(alist.addrs);
@@ -735,7 +802,11 @@ int register_fprobe_ips(struct fprobe *fp, unsigned long *addrs, int num)
 	mutex_lock(&fprobe_mutex);
 
 	hlist_array = fp->hlist_array;
-	ret = fprobe_graph_add_ips(addrs, num);
+	if (fp->exit_handler)
+		ret = fprobe_graph_add_ips(addrs, num);
+	else
+		ret = fprobe_ftrace_add_ips(addrs, num);
+
 	if (!ret) {
 		add_fprobe_hash(fp);
 		for (i = 0; i < hlist_array->size; i++) {
@@ -831,7 +902,10 @@ int unregister_fprobe(struct fprobe *fp)
 	}
 	del_fprobe_hash(fp);
 
-	fprobe_graph_remove_ips(addrs, count);
+	if (fp->exit_handler)
+		fprobe_graph_remove_ips(addrs, count);
+	else
+		fprobe_ftrace_remove_ips(addrs, count);
 
 	kfree_rcu(hlist_array, rcu);
 	fp->hlist_array = NULL;
-- 
2.51.0