proc: calculate end pointer for /proc/*/* lookup at compile time
diff mbox series

Message ID 20190114200422.GB9680@avx2
State In Next
Commit f6e3521a4c5b9ba3bc2b33081bf4082f784e92af
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Series
  • proc: calculate end pointer for /proc/*/* lookup at compile time
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Commit Message

Alexey Dobriyan Jan. 14, 2019, 8:04 p.m. UTC
Compilers like to transform loops like

	for (i = 0; i < n; i++) {
		[use p[i]]
	}

into
	for (p = p0; p < end; p++) {
		...
	}

Do it by hand, so that it results in overall simpler loop
and smaller code.

Space savings:

	$ ./scripts/bloat-o-meter ../vmlinux-001 ../obj/vmlinux
	add/remove: 0/0 grow/shrink: 2/1 up/down: 4/-9 (-5)
	Function                                     old     new   delta
	proc_tid_base_lookup                          17      19      +2
	proc_tgid_base_lookup                         17      19      +2
	proc_pident_lookup                           179     170      -9

Note: this trick bloats readdir, so don't do it :-\

Signed-off-by: Alexey Dobriyan <adobriyan@gmail.com>
---

 fs/proc/base.c |   17 +++++++++--------
 1 file changed, 9 insertions(+), 8 deletions(-)

Comments

Andrew Morton Jan. 29, 2019, 10:18 p.m. UTC | #1
On Mon, 14 Jan 2019 23:04:23 +0300 Alexey Dobriyan <adobriyan@gmail.com> wrote:

> Compilers like to transform loops like
> 
> 	for (i = 0; i < n; i++) {
> 		[use p[i]]
> 	}
> 
> into
> 	for (p = p0; p < end; p++) {
> 		...
> 	}
> 
> Do it by hand, so that it results in overall simpler loop
> and smaller code.
> 
> Space savings:
> 
> 	$ ./scripts/bloat-o-meter ../vmlinux-001 ../obj/vmlinux
> 	add/remove: 0/0 grow/shrink: 2/1 up/down: 4/-9 (-5)
> 	Function                                     old     new   delta
> 	proc_tid_base_lookup                          17      19      +2
> 	proc_tgid_base_lookup                         17      19      +2
> 	proc_pident_lookup                           179     170      -9
> 
> Note: this trick bloats readdir, so don't do it :-\

I don't understand the Note:.  Can you please expand?
Alexey Dobriyan Jan. 30, 2019, 5:55 a.m. UTC | #2
On Tue, Jan 29, 2019 at 02:18:48PM -0800, Andrew Morton wrote:
> On Mon, 14 Jan 2019 23:04:23 +0300 Alexey Dobriyan <adobriyan@gmail.com> wrote:
> 
> > Compilers like to transform loops like
> > 
> > 	for (i = 0; i < n; i++) {
> > 		[use p[i]]
> > 	}
> > 
> > into
> > 	for (p = p0; p < end; p++) {
> > 		...
> > 	}
> > 
> > Do it by hand, so that it results in overall simpler loop
> > and smaller code.
> > 
> > Space savings:
> > 
> > 	$ ./scripts/bloat-o-meter ../vmlinux-001 ../obj/vmlinux
> > 	add/remove: 0/0 grow/shrink: 2/1 up/down: 4/-9 (-5)
> > 	Function                                     old     new   delta
> > 	proc_tid_base_lookup                          17      19      +2
> > 	proc_tgid_base_lookup                         17      19      +2
> > 	proc_pident_lookup                           179     170      -9
> > 
> > Note: this trick bloats readdir, so don't do it :-\
> 
> I don't understand the Note:.  Can you please expand?

The same could be done to proc_pident_readdir(), but the code becomes
bigger for some reason.

Patch
diff mbox series

--- a/fs/proc/base.c
+++ b/fs/proc/base.c
@@ -2459,11 +2459,10 @@  static struct dentry *proc_pident_instantiate(struct dentry *dentry,
 
 static struct dentry *proc_pident_lookup(struct inode *dir, 
 					 struct dentry *dentry,
-					 const struct pid_entry *ents,
-					 unsigned int nents)
+					 const struct pid_entry *p,
+					 const struct pid_entry *end)
 {
 	struct task_struct *task = get_proc_task(dir);
-	const struct pid_entry *p, *last;
 	struct dentry *res = ERR_PTR(-ENOENT);
 
 	if (!task)
@@ -2473,8 +2472,7 @@  static struct dentry *proc_pident_lookup(struct inode *dir,
 	 * Yes, it does not scale. And it should not. Don't add
 	 * new entries into /proc/<tgid>/ without very good reasons.
 	 */
-	last = &ents[nents];
-	for (p = ents; p < last; p++) {
+	for (; p < end; p++) {
 		if (p->len != dentry->d_name.len)
 			continue;
 		if (!memcmp(dentry->d_name.name, p->name, p->len)) {
@@ -2613,7 +2611,8 @@  static struct dentry *proc_attr_dir_lookup(struct inode *dir,
 				struct dentry *dentry, unsigned int flags)
 {
 	return proc_pident_lookup(dir, dentry,
-				  attr_dir_stuff, ARRAY_SIZE(attr_dir_stuff));
+				  attr_dir_stuff,
+				  attr_dir_stuff + ARRAY_SIZE(attr_dir_stuff));
 }
 
 static const struct inode_operations proc_attr_dir_inode_operations = {
@@ -3049,7 +3048,8 @@  static const struct file_operations proc_tgid_base_operations = {
 static struct dentry *proc_tgid_base_lookup(struct inode *dir, struct dentry *dentry, unsigned int flags)
 {
 	return proc_pident_lookup(dir, dentry,
-				  tgid_base_stuff, ARRAY_SIZE(tgid_base_stuff));
+				  tgid_base_stuff,
+				  tgid_base_stuff + ARRAY_SIZE(tgid_base_stuff));
 }
 
 static const struct inode_operations proc_tgid_base_inode_operations = {
@@ -3421,7 +3421,8 @@  static int proc_tid_base_readdir(struct file *file, struct dir_context *ctx)
 static struct dentry *proc_tid_base_lookup(struct inode *dir, struct dentry *dentry, unsigned int flags)
 {
 	return proc_pident_lookup(dir, dentry,
-				  tid_base_stuff, ARRAY_SIZE(tid_base_stuff));
+				  tid_base_stuff,
+				  tid_base_stuff + ARRAY_SIZE(tid_base_stuff));
 }
 
 static const struct file_operations proc_tid_base_operations = {