FreeBSD/Linux Kernel Cross Reference
sys/mm/oom_kill.c

     /*
      *  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/swapctl.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;
    
            if (p->flags & PF_MEMDIE)
                    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->times.tms_utime + p->times.tms_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 (p->nice > 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 *p = NULL;
           struct task_struct *chosen = NULL;
   
           for_each_task(p) {
                   if (p->pid) {
                           int points = badness(p);
                           if (points > maxpoints) {
                                   chosen = p;
                                   maxpoints = points;
                           }
                   }
           }
           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->counter = 5 * 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 *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) */
           for_each_task(q) {
                   if (q->mm == p->mm)
                           oom_kill_task(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;
   }

Cache object: 88e9948792d72311fb547b41f1ba7525