/*
 *  linux/mm/oom_kill.c
 * 
 *  Copyright (C)  1998,2000  Rik van Riel
 *      Thanks go out to Claus Fischer for some serious inspiration and
 *      for goading me into coding this file...
 *
 *  The routines in this file are used to kill a process when
 *  we're seriously out of memory. This gets called from kswapd()
 *  in linux/mm/vmscan.c when we really run out of memory.
 *
 *  Since we won't call these routines often (on a well-configured
 *  machine) this file will double as a 'coding guide' and a signpost
 *  for newbie kernel hackers. It features several pointers to major
 *  kernel subsystems and hints as to where to find out what things do.
 */

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

/* #define DEBUG */

/**
 * int_sqrt - oom_kill.c internal function, rough approximation to sqrt
 * @x: integer of which to calculate the sqrt
 * 
 * A very rough approximation to the sqrt() function.
 */
static unsigned int int_sqrt(unsigned int x)
{
        unsigned int out = x;
        while (x & ~(unsigned int)1) x >>=2, out >>=1;
        if (x) out -= out >> 2;
        return (out ? out : 1);
}       

/**
 * oom_badness - calculate a numeric value for how bad this task has been
 * @p: task struct of which task we should calculate
 *
 * The formula used is relatively simple and documented inline in the
 * function. The main rationale is that we want to select a good task
 * to kill when we run out of memory.
 *
 * Good in this context means that:
 * 1) we lose the minimum amount of work done
 * 2) we recover a large amount of memory
 * 3) we don't kill anything innocent of eating tons of memory
 * 4) we want to kill the minimum amount of processes (one)
 * 5) we try to kill the process the user expects us to kill, this
 *    algorithm has been meticulously tuned to meet the priniciple
 *    of least surprise ... (be careful when you change it)
 */

static int badness(struct task_struct *p)
{
        int points, cpu_time, run_time;

        if (!p->mm)
                return 0;
        /*
         * The memory size of the process is the basis for the badness.
         */
        points = p->mm->total_vm;

        /*
         * CPU time is in seconds and run time is in minutes. There is no
         * particular reason for this other than that it turned out to work
         * very well in practice. This is not safe against jiffie wraps
         * but we don't care _that_ much...
         */
        cpu_time = (p->utime + p->stime) >> (SHIFT_HZ + 3);
        run_time = (jiffies - p->start_time) >> (SHIFT_HZ + 10);

        points /= int_sqrt(cpu_time);
        points /= int_sqrt(int_sqrt(run_time));

        /*
         * Niced processes are most likely less important, so double
         * their badness points.
         */
        if (task_nice(p) > 0)
                points *= 2;

        /*
         * Superuser processes are usually more important, so we make it
         * less likely that we kill those.
         */
        if (cap_t(p->cap_effective) & CAP_TO_MASK(CAP_SYS_ADMIN) ||
                                p->uid == 0 || p->euid == 0)
                points /= 4;

        /*
         * We don't want to kill a process with direct hardware access.
         * Not only could that mess up the hardware, but usually users
         * tend to only have this flag set on applications they think
         * of as important.
         */
        if (cap_t(p->cap_effective) & CAP_TO_MASK(CAP_SYS_RAWIO))
                points /= 4;
#ifdef DEBUG
        printk(KERN_DEBUG "OOMkill: task %d (%s) got %d points\n",
        p->pid, p->comm, points);
#endif
        return points;
}

/*
 * Simple selection loop. We chose the process with the highest
 * number of 'points'. We expect the caller will lock the tasklist.
 *
 * (not docbooked, we don't want this one cluttering up the manual)
 */
static struct task_struct * select_bad_process(void)
{
        int maxpoints = 0;
        struct task_struct *g, *p;
        struct task_struct *chosen = NULL;

        do_each_thread(g, p)
                if (p->pid) {
                        int points = badness(p);
                        if (points > maxpoints) {
                                chosen = p;
                                maxpoints = points;
                        }
                }
        while_each_thread(g, p);
        return chosen;
}

/**
 * We must be careful though to never send SIGKILL a process with
 * CAP_SYS_RAW_IO set, send SIGTERM instead (but it's unlikely that
 * we select a process with CAP_SYS_RAW_IO set).
 */
void oom_kill_task(struct task_struct *p)
{
        printk(KERN_ERR "Out of Memory: Killed process %d (%s).\n", p->pid, p->comm);

        /*
         * We give our sacrificial lamb high priority and access to
         * all the memory it needs. That way it should be able to
         * exit() and clear out its resources quickly...
         */
        p->time_slice = HZ;
        p->flags |= PF_MEMALLOC | PF_MEMDIE;

        /* This process has hardware access, be more careful. */
        if (cap_t(p->cap_effective) & CAP_TO_MASK(CAP_SYS_RAWIO)) {
                force_sig(SIGTERM, p);
        } else {
                force_sig(SIGKILL, p);
        }
}

/**
 * oom_kill - kill the "best" process when we run out of memory
 *
 * If we run out of memory, we have the choice between either
 * killing a random task (bad), letting the system crash (worse)
 * OR try to be smart about which process to kill. Note that we
 * don't have to be perfect here, we just have to be good.
 */
static void oom_kill(void)
{
        struct task_struct *g, *p, *q;
        
        read_lock(&tasklist_lock);
        p = select_bad_process();

        /* Found nothing?!?! Either we hang forever, or we panic. */
        if (p == NULL)
                panic("Out of memory and no killable processes...\n");

        /* kill all processes that share the ->mm (i.e. all threads) */
        do_each_thread(g, q)
                if (q->mm == p->mm)
                        oom_kill_task(q);
        while_each_thread(g, q);

        read_unlock(&tasklist_lock);

        /*
         * Make kswapd go out of the way, so "p" has a good chance of
         * killing itself before someone else gets the chance to ask
         * for more memory.
         */
        yield();
        return;
}

/**
 * out_of_memory - is the system out of memory?
 */
void out_of_memory(void)
{
        static unsigned long first, last, count, lastkill;
        unsigned long now, since;

        /*
         * Enough swap space left?  Not OOM.
         */
        if (nr_swap_pages > 0)
                return;

        now = jiffies;
        since = now - last;
        last = now;

        /*
         * If it's been a long time since last failure,
         * we're not oom.
         */
        last = now;
        if (since > 5*HZ)
                goto reset;

        /*
         * If we haven't tried for at least one second,
         * we're not really oom.
         */
        since = now - first;
        if (since < HZ)
                return;

        /*
         * If we have gotten only a few failures,
         * we're not really oom. 
         */
        if (++count < 10)
                return;

        /*
         * If we just killed a process, wait a while
         * to give that task a chance to exit. This
         * avoids killing multiple processes needlessly.
         */
        since = now - lastkill;
        if (since < HZ*5)
                return;

        /*
         * Ok, really out of memory. Kill something.
         */
        lastkill = now;
        oom_kill();

reset:
        first = now;
        count = 0;
}