linux-kernel - [PATCH] perf: Improve startup time by reducing unnecessary stat() calls

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Message-ID: <20250206113314.335376-2-krzysztof.m.lopatowski@gmail.com>
Date: Thu,  6 Feb 2025 12:33:15 +0100
From: Krzysztof Łopatowski <krzysztof.m.lopatowski@...il.com>
To: namhyung@...nel.org,
	acme@...nel.org,
	irogers@...gle.com
Cc: linux-perf-users@...r.kernel.org,
	linux-kernel@...r.kernel.org,
	Krzysztof Łopatowski <krzysztof.m.lopatowski@...il.com>
Subject: [PATCH] perf: Improve startup time by reducing unnecessary stat() calls

When testing perf trace on NixOS, I noticed significant startup delays:
- `ls`: ~2ms
- `strace ls`: ~10ms
- `perf trace ls`: ~550ms

Profiling showed that 51% of the time is spent reading files,
26% in loading BPF programs, and 11% in `newfstatat`.

This patch optimizes module path exploration by avoiding `stat()` calls
unless necessary. For filesystems that do not implement `d_type`
(DT_UNKNOWN), it falls back to the old behavior.
See `readdir(3)` for details.

This reduces `perf trace ls` time to ~500ms.

A more thorough startup optimization based on command parameters would
be ideal, but that is a larger effort.

Signed-off-by: Krzysztof Łopatowski <krzysztof.m.lopatowski@...il.com>
---
 tools/perf/util/machine.c | 18 ++++++++++++------
 1 file changed, 12 insertions(+), 6 deletions(-)

diff --git a/tools/perf/util/machine.c b/tools/perf/util/machine.c
index 2d51badfbf2e..76b857fd752b 100644
--- a/tools/perf/util/machine.c
+++ b/tools/perf/util/machine.c
@@ -1354,7 +1354,7 @@ static int maps__set_module_path(struct maps *maps, const char *path, struct kmo
 
 static int maps__set_modules_path_dir(struct maps *maps, const char *dir_name, int depth)
 {
-	struct dirent *dent;
+	const struct dirent *dent;
 	DIR *dir = opendir(dir_name);
 	int ret = 0;
 
@@ -1365,14 +1365,20 @@ static int maps__set_modules_path_dir(struct maps *maps, const char *dir_name, i
 
 	while ((dent = readdir(dir)) != NULL) {
 		char path[PATH_MAX];
-		struct stat st;
+		unsigned char d_type = dent->d_type;
 
-		/*sshfs might return bad dent->d_type, so we have to stat*/
 		path__join(path, sizeof(path), dir_name, dent->d_name);
-		if (stat(path, &st))
-			continue;
 
-		if (S_ISDIR(st.st_mode)) {
+		if (d_type == DT_UNKNOWN) {
+			struct stat st;
+
+			if (stat(path, &st))
+				continue;
+			if (S_ISDIR(st.st_mode))
+				d_type = DT_DIR;
+		}
+
+		if (d_type == DT_DIR) {
 			if (!strcmp(dent->d_name, ".") ||
 			    !strcmp(dent->d_name, ".."))
 				continue;
-- 
2.47.2