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 also keeps the two instances automatically in
sync (instead of only the compile-time strlen expression).
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.
x86-64 defconfig + CONFIG_FUNCTION_TRACER:
$ scripts/stackdelta /tmp/stackusage.{0,1}
./kernel/trace/ftrace.c ftrace_mod_callback 152 136 -16
./kernel/trace/trace.c trace_default_header 56 32 -24
./kernel/trace/trace.c tracing_mark_raw_write 96 72 -24
./kernel/trace/trace.c tracing_mark_write 104 80 -24
bloat-o-meter
add/remove: 1/0 grow/shrink: 0/4 up/down: 14/-258 (-244)
Function old new delta
this_mod - 14 +14
ftrace_mod_callback 577 542 -35
tracing_mark_raw_write 444 374 -70
tracing_mark_write 616 540 -76
trace_default_header 600 523 -77
Signed-off-by: Rasmus Villemoes <li...@rasmusvillemoes.dk>
---
Hi Steven
I didn't get any response to this; just let me know if you don't want
this kind of microoptimization patches.
v2: update change log with stack and code delta info.
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 fa79323331b2..e232dd31cfae 100644
--- a/kernel/trace/ftrace.c
+++ b/kernel/trace/ftrace.c
@@ -3879,7 +3879,7 @@ 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 21153e64bf1c..a7f7d141c902 100644
--- a/kernel/trace/trace.c
+++ b/kernel/trace/trace.c
@@ -3556,8 +3556,8 @@ 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 = " ";
print_event_info(buf, m);
@@ -6304,13 +6304,13 @@ 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;
@@ -6342,7 +6342,7 @@ 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
@@ -6383,7 +6383,6 @@ 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;
@@ -6423,7 +6422,7 @@ 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;
--
2.20.1
