1/*
2 * mm/thrash.c
3 *
4 * Copyright (C) 2004, Red Hat, Inc.
5 * Copyright (C) 2004, Rik van Riel <riel@redhat.com>
6 * Released under the GPL, see the file COPYING for details.
7 *
8 * Simple token based thrashing protection, using the algorithm
9 * described in:  http://www.cs.wm.edu/~sjiang/token.pdf
10 *
11 * Sep 2006, Ashwin Chaugule <ashwin.chaugule@celunite.com>
12 * Improved algorithm to pass token:
13 * Each task has a priority which is incremented if it contended
14 * for the token in an interval less than its previous attempt.
15 * If the token is acquired, that task's priority is boosted to prevent
16 * the token from bouncing around too often and to let the task make
17 * some progress in its execution.
18 */
19
20#include <linux/jiffies.h>
21#include <linux/mm.h>
22#include <linux/sched.h>
23#include <linux/swap.h>
24
25static DEFINE_SPINLOCK(swap_token_lock);
26struct mm_struct *swap_token_mm;
27static unsigned int global_faults;
28
29void grab_swap_token(struct mm_struct *mm)
30{
31	int current_interval;
32
33	global_faults++;
34
35	current_interval = global_faults - mm->faultstamp;
36
37	if (!spin_trylock(&swap_token_lock))
38		return;
39
40	/* First come first served */
41	if (swap_token_mm == NULL) {
42		mm->token_priority = mm->token_priority + 2;
43		swap_token_mm = mm;
44		goto out;
45	}
46
47	if (mm != swap_token_mm) {
48		if (current_interval < mm->last_interval)
49			mm->token_priority++;
50		else {
51			if (likely(mm->token_priority > 0))
52				mm->token_priority--;
53		}
54		/* Check if we deserve the token */
55		if (mm->token_priority > swap_token_mm->token_priority) {
56			mm->token_priority += 2;
57			swap_token_mm = mm;
58		}
59	} else {
60		/* Token holder came in again! */
61		mm->token_priority += 2;
62	}
63
64out:
65	mm->faultstamp = global_faults;
66	mm->last_interval = current_interval;
67	spin_unlock(&swap_token_lock);
68}
69
70/* Called on process exit. */
71void __put_swap_token(struct mm_struct *mm)
72{
73	spin_lock(&swap_token_lock);
74	if (likely(mm == swap_token_mm))
75		swap_token_mm = NULL;
76	spin_unlock(&swap_token_lock);
77}
78