[PATCH v1] TTY: tty flip buffer optimisation.

[Ilya Zykov]

Currently, free flip buffer (tty->buf.free) reserve memory for further used,
only if driver send to ldisc less 257 bytes in one time.
If driver send more, flip buffer reserve(kmalloc()) and then
free(kfree()) every chunk more 256 bytes every time,
even we have in free buffer enough space, because we have only 256 byte chunks,
and don't split data on 256 byte shunks.
Also we can't limit reserve memory size. In worse case we will have in free buffer:
(256 * 256 byte chunk) = (256 * 552 ) = 141312 byte unused memory.
Many program use "pty" for communicate and send many big chunk 
from/to master/slave. Instance: "pppd" with pty.

This patch allow reserve more than 256 bytes chunks in free buffer. 
And have self-regulation chunk size ability(very useful for pty).
Also we can limit reserve memory size, but then,
we will be use kmalloc()-kree() calls on intensive stream from the driver.

I think my patch clear, because if stream from driver very slow, 
we will be use only two 256 byte chunk, as before. But with pty, maybe, new drivers,
we could use flip buffer more effective(less memory fragmentation and TLB use).

Disscussed on linux-kernel@xxxxxxxxxxxxxxx "[PATCH] TTY: tty flip buffer change reserve memory strategy."
and not accepted. 
Although, I think it's clear.

diff -uprN last-orig/drivers/tty/tty_buffer.c tty_buffer.patched/drivers/tty/tty_buffer.c
--- last-orig/drivers/tty/tty_buffer.c	2011-10-18 13:57:32.000000000 +0400
+++ tty_buffer.patched/drivers/tty/tty_buffer.c	2011-10-22 09:45:09.000000000 +0400
@@ -58,7 +58,7 @@ static struct tty_buffer *tty_buffer_all
 {
 	struct tty_buffer *p;
 
-	if (tty->buf.memory_used + size > 65536)
+	if (tty->buf.memory_used + size > TTY_BUFFER_MAX)
 		return NULL;
 	p = kmalloc(sizeof(struct tty_buffer) + 2 * size, GFP_ATOMIC);
 	if (p == NULL)
@@ -91,10 +91,21 @@ static void tty_buffer_free(struct tty_s
 	tty->buf.memory_used -= b->size;
 	WARN_ON(tty->buf.memory_used < 0);
 
-	if (b->size >= 512)
-		kfree(b);
-	else {
-		b->next = tty->buf.free;
+	tty->buf.memory_reserve += b->size;
+	b->next = NULL;
+	if (tty->buf.free != NULL) {
+		struct tty_buffer *p = tty->buf.free;
+		while (p->next != NULL)
+			p = p->next;
+		p->next = b;
+		while (tty->buf.memory_reserve > TTY_BUFFER_RESERVE) {
+			p = tty->buf.free;
+			tty->buf.free = p->next;
+			tty->buf.memory_reserve -= p->size;
+			WARN_ON(tty->buf.memory_reserve < 0);
+			kfree(p);
+		}
+	} else {
 		tty->buf.free = b;
 	}
 }
@@ -175,6 +186,8 @@ static struct tty_buffer *tty_buffer_fin
 			t->commit = 0;
 			t->read = 0;
 			tty->buf.memory_used += t->size;
+			tty->buf.memory_reserve -= t->size;
+			WARN_ON(tty->buf.memory_reserve < 0);
 			return t;
 		}
 		tbh = &((*tbh)->next);
@@ -517,6 +530,7 @@ void tty_buffer_init(struct tty_struct *
 	tty->buf.tail = NULL;
 	tty->buf.free = NULL;
 	tty->buf.memory_used = 0;
+	tty->buf.memory_reserve = 0;
 	INIT_WORK(&tty->buf.work, flush_to_ldisc);
 }
 
diff -uprN last-orig/include/linux/tty.h tty_buffer.patched/include/linux/tty.h
--- last-orig/include/linux/tty.h	2011-05-19 08:06:34.000000000 +0400
+++ tty_buffer.patched/include/linux/tty.h	2011-10-22 13:48:46.000000000 +0400
@@ -79,7 +79,8 @@ struct tty_buffer {
  */
 
 #define TTY_BUFFER_PAGE	(((PAGE_SIZE - sizeof(struct tty_buffer)) / 2) & ~0xFF)
-
+#define TTY_BUFFER_MAX		65536
+#define TTY_BUFFER_RESERVE	TTY_BUFFER_MAX
 
 struct tty_bufhead {
 	struct work_struct work;
@@ -87,8 +88,8 @@ struct tty_bufhead {
 	struct tty_buffer *head;	/* Queue head */
 	struct tty_buffer *tail;	/* Active buffer */
 	struct tty_buffer *free;	/* Free queue head */
-	int memory_used;		/* Buffer space used excluding
-								free queue */
+	int memory_used;		/* Buffer space used excluding free queue */
+	int memory_reserve;		/* Free queue space */
 };
 /*
  * When a break, frame error, or parity error happens, these codes are
Signed-off-by: Ilya Zykov <ilya@xxxxxxx>

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