mm: OOM documentation update

values are in the range -16 to +15, plus the special value -17, which disables

oom-killing altogether for this process.

The process to be killed in an out-of-memory situation is selected among all others

based on its badness score. This value equals the original memory size of the process

and is then updated according to its CPU time (utime + stime) and the

run time (uptime - start time). The longer it runs the smaller is the score.

Badness score is divided by the square root of the CPU time and then by

the double square root of the run time.

Swapped out tasks are killed first. Half of each child's memory size is added to

the parent's score if they do not share the same memory. Thus forking servers

are the prime candidates to be killed. Having only one 'hungry' child will make

parent less preferable than the child.

/proc/<pid>/oom_score shows process' current badness score.

The following heuristics are then applied:

 * if the task was reniced, its score doubles

 * superuser or direct hardware access tasks (CAP_SYS_ADMIN, CAP_SYS_RESOURCE

 	or CAP_SYS_RAWIO) have their score divided by 4

 * if oom condition happened in one cpuset and checked task does not belong

 	to it, its score is divided by 8

 * the resulting score is multiplied by two to the power of oom_adj, i.e.

	points <<= oom_adj when it is positive and

	points >>= -(oom_adj) otherwise

The task with the highest badness score is then selected and its children

are killed, process itself will be killed in an OOM situation when it does

not have children or some of them disabled oom like described above.

2.13 /proc/<pid>/oom_score - Display current oom-killer score

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