proc: speed up /proc/stat handling

On a typical 16 cpus machine, "cat /proc/stat" gives more than 4096 bytes,
and is slow :

  # strace -T -o /tmp/STRACE cat /proc/stat | wc -c
  5826
  # grep "cpu " /tmp/STRACE
  read(0, "cpu  1949310 19 2144714 12117253"..., 32768) = 5826 <0.001504>

Thats partly because show_stat() must be called twice since initial
buffer size is too small (4096 bytes for less than 32 possible cpus)

Fix this by :

 1) Taking into account nr_irqs in the initial buffer sizing.

 2) Using ksize() to allow better filling of initial buffer.

Signed-off-by: Eric Dumazet <eric.dumazet@gmail.com>
Cc: Glauber Costa <glommer@parallels.com>
Cc: Russell King - ARM Linux <linux@arm.linux.org.uk>
Cc: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
Cc: Paul Turner <pjt@google.com>
Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
Cc: Ingo Molnar <mingo@elte.hu>
Cc: Alexey Dobriyan <adobriyan@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

fs/proc/stat.c

@@ -157,11 +157,14 @@ static int show_stat(struct seq_file *p, void *v)
 
 static int stat_open(struct inode *inode, struct file *file)
 {
-	unsigned size = 4096 * (1 + num_possible_cpus() / 32);
+	unsigned size = 1024 + 128 * num_possible_cpus();
 	char *buf;
 	struct seq_file *m;
 	int res;
 
+	/* minimum size to display an interrupt count : 2 bytes */
+	size += 2 * nr_irqs;
+
 	/* don't ask for more than the kmalloc() max size */
 	if (size > KMALLOC_MAX_SIZE)
 		size = KMALLOC_MAX_SIZE;
@@ -173,7 +176,7 @@ static int stat_open(struct inode *inode, struct file *file)
 	if (!res) {
 		m = file->private_data;
 		m->buf = buf;
-		m->size = size;
+		m->size = ksize(buf);
 	} else
 		kfree(buf);
 	return res;