root/mm/thrash.c

DEFINITIONS

1. grab_swap_token
2. __put_swap_token

/*
 * mm/thrash.c
 *
 * Copyright (C) 2004, Red Hat, Inc.
 * Copyright (C) 2004, Rik van Riel <riel@redhat.com>
 * Released under the GPL, see the file COPYING for details.
 *
 * Simple token based thrashing protection, using the algorithm
 * described in:  http://www.cs.wm.edu/~sjiang/token.pdf
 *
 * Sep 2006, Ashwin Chaugule <ashwin.chaugule@celunite.com>
 * Improved algorithm to pass token:
 * Each task has a priority which is incremented if it contended
 * for the token in an interval less than its previous attempt.
 * If the token is acquired, that task's priority is boosted to prevent
 * the token from bouncing around too often and to let the task make
 * some progress in its execution.
 */

#include <linux/jiffies.h>
#include <linux/mm.h>
#include <linux/sched.h>
#include <linux/swap.h>

static DEFINE_SPINLOCK(swap_token_lock);
struct mm_struct *swap_token_mm;
static unsigned int global_faults;

void grab_swap_token(void)
{
        int current_interval;

        global_faults++;

        current_interval = global_faults - current->mm->faultstamp;

        if (!spin_trylock(&swap_token_lock))
                return;

        /* First come first served */
        if (swap_token_mm == NULL) {
                current->mm->token_priority = current->mm->token_priority + 2;
                swap_token_mm = current->mm;
                goto out;
        }

        if (current->mm != swap_token_mm) {
                if (current_interval < current->mm->last_interval)
                        current->mm->token_priority++;
                else {
                        if (likely(current->mm->token_priority > 0))
                                current->mm->token_priority--;
                }
                /* Check if we deserve the token */
                if (current->mm->token_priority >
                                swap_token_mm->token_priority) {
                        current->mm->token_priority += 2;
                        swap_token_mm = current->mm;
                }
        } else {
                /* Token holder came in again! */
                current->mm->token_priority += 2;
        }

out:
        current->mm->faultstamp = global_faults;
        current->mm->last_interval = current_interval;
        spin_unlock(&swap_token_lock);
return;
}

/* Called on process exit. */
void __put_swap_token(struct mm_struct *mm)
{
        spin_lock(&swap_token_lock);
        if (likely(mm == swap_token_mm))
                swap_token_mm = NULL;
        spin_unlock(&swap_token_lock);
}