linux-kernel - [PATCH] tracing: eliminate const char[] auto variables

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Message-Id: <20181102205442.7758-1-linux@rasmusvillemoes.dk>
Date:   Fri,  2 Nov 2018 21:54:42 +0100
From:   Rasmus Villemoes <linux@...musvillemoes.dk>
To:     Steven Rostedt <rostedt@...dmis.org>,
        Ingo Molnar <mingo@...hat.com>
Cc:     Rasmus Villemoes <linux@...musvillemoes.dk>,
        linux-kernel@...r.kernel.org
Subject: [PATCH] tracing: eliminate const char[] auto variables

Automatic const char[] variables cause unnecessary code
generation. For example, the this_mod variable leads to

    3f04:       48 b8 5f 5f 74 68 69 73 5f 6d   movabs $0x6d5f736968745f5f,%rax # __this_m
    3f0e:       4c 8d 44 24 02                  lea    0x2(%rsp),%r8
    3f13:       48 8d 7c 24 10                  lea    0x10(%rsp),%rdi
    3f18:       48 89 44 24 02                  mov    %rax,0x2(%rsp)
    3f1d:       4c 89 e9                        mov    %r13,%rcx
    3f20:       b8 65 00 00 00                  mov    $0x65,%eax # e
    3f25:       48 c7 c2 00 00 00 00            mov    $0x0,%rdx
                        3f28: R_X86_64_32S      .rodata.str1.1+0x18d
    3f2c:       be 48 00 00 00                  mov    $0x48,%esi
    3f31:       c7 44 24 0a 6f 64 75 6c         movl   $0x6c75646f,0xa(%rsp) # odul
    3f39:       66 89 44 24 0e                  mov    %ax,0xe(%rsp)

i.e., the string gets built on the stack at runtime. Similar code can be
found for the other instances I'm replacing here. Putting the string
in .rodata reduces the combined .text+.rodata size and saves time and
stack space at runtime.

The simplest fix, and what I've done for the this_mod case, is to just
make the variable static.

However, for the "<faulted>" case where the same string is used twice,
that prevents the linker from merging those two literals, so instead use
a macro - that's also slightly cleaner, since FAULTED_SIZE otherwise has
no direct relation to the faulted variable used in
tracing_mark_raw_write().

Finally, for the two runs of spaces, just use variables initialized with
string literals; the linker (at least for x86) will reuse the tail of
the longer for the shorter string.

Signed-off-by: Rasmus Villemoes <linux@...musvillemoes.dk>
---
 kernel/trace/ftrace.c |  2 +-
 kernel/trace/trace.c  | 13 ++++++-------
 2 files changed, 7 insertions(+), 8 deletions(-)

diff --git a/kernel/trace/ftrace.c b/kernel/trace/ftrace.c
index f536f601bd46..4b79d4ae9635 100644
--- a/kernel/trace/ftrace.c
+++ b/kernel/trace/ftrace.c
@@ -3862,7 +3862,7 @@ static int ftrace_hash_move_and_update_ops(struct ftrace_ops *ops,
 static bool module_exists(const char *module)
 {
 	/* All modules have the symbol __this_module */
-	const char this_mod[] = "__this_module";
+	static const char this_mod[] = "__this_module";
 	char modname[MAX_PARAM_PREFIX_LEN + sizeof(this_mod) + 2];
 	unsigned long val;
 	int n;
diff --git a/kernel/trace/trace.c b/kernel/trace/trace.c
index ff1c4b20cd0a..ffcceb33a9b2 100644
--- a/kernel/trace/trace.c
+++ b/kernel/trace/trace.c
@@ -3381,8 +3381,8 @@ static void print_func_help_header_irq(struct trace_buffer *buf, struct seq_file
 				       unsigned int flags)
 {
 	bool tgid = flags & TRACE_ITER_RECORD_TGID;
-	const char tgid_space[] = "          ";
-	const char space[] = "  ";
+	const char *tgid_space = "          ";
+	const char *space = "  ";
 
 	seq_printf(m, "#                          %s  _-----=> irqs-off\n",
 		   tgid ? tgid_space : space);
@@ -6089,13 +6089,13 @@ tracing_mark_write(struct file *filp, const char __user *ubuf,
 	struct ring_buffer *buffer;
 	struct print_entry *entry;
 	unsigned long irq_flags;
-	const char faulted[] = "<faulted>";
 	ssize_t written;
 	int size;
 	int len;
 
 /* Used in tracing_mark_raw_write() as well */
-#define FAULTED_SIZE (sizeof(faulted) - 1) /* '\0' is already accounted for */
+#define FAULTED_STR "<faulted>"
+#define FAULTED_SIZE (sizeof(FAULTED_STR) - 1) /* '\0' is already accounted for */
 
 	if (tracing_disabled)
 		return -EINVAL;
@@ -6127,7 +6127,7 @@ tracing_mark_write(struct file *filp, const char __user *ubuf,
 
 	len = __copy_from_user_inatomic(&entry->buf, ubuf, cnt);
 	if (len) {
-		memcpy(&entry->buf, faulted, FAULTED_SIZE);
+		memcpy(&entry->buf, FAULTED_STR, FAULTED_SIZE);
 		cnt = FAULTED_SIZE;
 		written = -EFAULT;
 	} else
@@ -6168,7 +6168,6 @@ tracing_mark_raw_write(struct file *filp, const char __user *ubuf,
 	struct ring_buffer_event *event;
 	struct ring_buffer *buffer;
 	struct raw_data_entry *entry;
-	const char faulted[] = "<faulted>";
 	unsigned long irq_flags;
 	ssize_t written;
 	int size;
@@ -6208,7 +6207,7 @@ tracing_mark_raw_write(struct file *filp, const char __user *ubuf,
 	len = __copy_from_user_inatomic(&entry->id, ubuf, cnt);
 	if (len) {
 		entry->id = -1;
-		memcpy(&entry->buf, faulted, FAULTED_SIZE);
+		memcpy(&entry->buf, FAULTED_STR, FAULTED_SIZE);
 		written = -EFAULT;
 	} else
 		written = cnt;
-- 
2.19.1.6.gbde171bbf5