proc: add seq_put_decimal_ull_width to speed up /proc/pid/smaps

seq_put_decimal_ull_w(m, str, val, width) prints a decimal number with a
specified minimal field width.

It is equivalent of seq_printf(m, "%s%*d", str, width, val), but it
works much faster.

== test_smaps.py
  num = 0
  with open("/proc/1/smaps") as f:
          for x in xrange(10000):
                  data = f.read()
                  f.seek(0, 0)
==

== Before patch ==
  $ time python test_smaps.py
  real    0m4.593s
  user    0m0.398s
  sys     0m4.158s

== After patch ==
  $ time python test_smaps.py
  real    0m3.828s
  user    0m0.413s
  sys     0m3.408s

$ perf -g record python test_smaps.py
== Before patch ==
-   79.01%     3.36%  python   [kernel.kallsyms]    [k] show_smap.isra.33
   - 75.65% show_smap.isra.33
      + 48.85% seq_printf
      + 15.75% __walk_page_range
      + 9.70% show_map_vma.isra.23
        0.61% seq_puts

== After patch ==
-   75.51%     4.62%  python   [kernel.kallsyms]    [k] show_smap.isra.33
   - 70.88% show_smap.isra.33
      + 24.82% seq_put_decimal_ull_w
      + 19.78% __walk_page_range
      + 12.74% seq_printf
      + 11.08% show_map_vma.isra.23
      + 1.68% seq_puts

[akpm@linux-foundation.org: fix drivers/of/unittest.c build]
Link: http://lkml.kernel.org/r/20180212074931.7227-1-avagin@openvz.org
Signed-off-by: Andrei Vagin <avagin@openvz.org>
Cc: Alexey Dobriyan <adobriyan@gmail.com>
Cc: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

fs/seq_file.c

@@ -673,15 +673,20 @@ void seq_puts(struct seq_file *m, const char *s)
 }
 EXPORT_SYMBOL(seq_puts);
 
-/*
+/**
  * A helper routine for putting decimal numbers without rich format of printf().
  * only 'unsigned long long' is supported.
- * This routine will put strlen(delimiter) + number into seq_file.
+ * @m: seq_file identifying the buffer to which data should be written
+ * @delimiter: a string which is printed before the number
+ * @num: the number
+ * @width: a minimum field width
+ *
+ * This routine will put strlen(delimiter) + number into seq_filed.
  * This routine is very quick when you show lots of numbers.
  * In usual cases, it will be better to use seq_printf(). It's easier to read.
  */
-void seq_put_decimal_ull(struct seq_file *m, const char *delimiter,
-			 unsigned long long num)
+void seq_put_decimal_ull_width(struct seq_file *m, const char *delimiter,
+			 unsigned long long num, unsigned int width)
 {
 	int len;
 
@@ -695,7 +700,10 @@ void seq_put_decimal_ull(struct seq_file *m, const char *delimiter,
 	memcpy(m->buf + m->count, delimiter, len);
 	m->count += len;
 
-	if (m->count + 1 >= m->size)
+	if (!width)
+		width = 1;
+
+	if (m->count + width >= m->size)
 		goto overflow;
 
 	if (num < 10) {
@@ -703,7 +711,7 @@ void seq_put_decimal_ull(struct seq_file *m, const char *delimiter,
 		return;
 	}
 
-	len = num_to_str(m->buf + m->count, m->size - m->count, num);
+	len = num_to_str(m->buf + m->count, m->size - m->count, num, width);
 	if (!len)
 		goto overflow;
 
@@ -713,6 +721,12 @@ void seq_put_decimal_ull(struct seq_file *m, const char *delimiter,
 overflow:
 	seq_set_overflow(m);
 }
+
+void seq_put_decimal_ull(struct seq_file *m, const char *delimiter,
+			 unsigned long long num)
+{
+	return seq_put_decimal_ull_width(m, delimiter, num, 0);
+}
 EXPORT_SYMBOL(seq_put_decimal_ull);
 
 /**
@@ -788,7 +802,7 @@ void seq_put_decimal_ll(struct seq_file *m, const char *delimiter, long long num
 		return;
 	}
 
-	len = num_to_str(m->buf + m->count, m->size - m->count, num);
+	len = num_to_str(m->buf + m->count, m->size - m->count, num, 0);
 	if (!len)
 		goto overflow;