[PATCH 2/2] procfs: optimize seq_pad() to speed up /proc/pid/maps

[Andrei Vagin <avagin@xxxxxxxxxx>]

seq_printf() is slow and it can be replaced by memset() in this case.

== test.py
num = 0
with open("/proc/1/maps") as f:
        while num < 10000 :
                data = f.read()
                f.seek(0, 0)
                num = num + 1
==

== Before patch ==
$  time python test.py

real	0m0.986s
user	0m0.279s
sys	0m0.707s

== After patch ==
$ time python test.py
real	0m0.932s
user	0m0.261s
sys	0m0.669s

$ perf record -g python test.py
== Before patch ==
-   47.35%     3.38%  python   [kernel.kallsyms] [k] show_map_vma.isra.23
   - 43.97% show_map_vma.isra.23
      + 20.84% seq_path
      - 15.73% show_vma_header_prefix
      + 6.96% seq_pad
   + 2.94% __GI___libc_read

== After patch ==
-   44.01%     0.34%  python   [kernel.kallsyms] [k] show_pid_map
   - 43.67% show_pid_map
      - 42.91% show_map_vma.isra.23
         + 21.55% seq_path
         - 15.68% show_vma_header_prefix
         + 2.08% seq_pad
        0.55% seq_putc

Cc: Alexey Dobriyan <adobriyan@xxxxxxxxx>
Cc: KAMEZAWA Hiroyuki <kamezawa.hiroyu@xxxxxxxxxxxxxx>
Signed-off-by: Andrei Vagin <avagin@xxxxxxxxxx>
---
 fs/seq_file.c | 10 ++++++++--
 1 file changed, 8 insertions(+), 2 deletions(-)

diff --git a/fs/seq_file.c b/fs/seq_file.c
index fb37ec42fae2..0ee52a372b66 100644
--- a/fs/seq_file.c
+++ b/fs/seq_file.c
@@ -829,8 +829,14 @@ EXPORT_SYMBOL(seq_write);
 void seq_pad(struct seq_file *m, char c)
 {
 	int size = m->pad_until - m->count;
-	if (size > 0)
-		seq_printf(m, "%*s", size, "");
+	if (size > 0) {
+		if (size + m->count > m->size) {
+			seq_set_overflow(m);
+			return;
+		}
+		memset(m->buf + m->count, ' ', size);
+		m->count += size;
+	}
 	if (c)
 		seq_putc(m, c);
 }
-- 
2.13.6