oom: document obsolete oom_adj tunable

/proc/pid/oom_adj was deprecated in August 2010 with the introduction of
the new oom killer heuristic.

This patch copies the Documentation/feature-removal-schedule.txt entry for
this tunable to the Documentation/ABI/obsolete directory so nobody misses
it.

Signed-off-by: David Rientjes <rientjes@google.com>
Reported-by: Alan Cox <alan@lxorguk.ukuu.org.uk>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

1 files changed, 22 insertions, 0 deletions

diff --git a/Documentation/ABI/obsolete/proc-pid-oom_adj b/Documentation/ABI/obsolete/proc-pid-oom_adj
new file mode 100644
index 000000000000..cf63f264ce0f
--- /dev/null
+++ b/Documentation/ABI/obsolete/proc-pid-oom_adj
@@ -0,0 +1,22 @@
+What:   /proc/<pid>/oom_adj
+When:   August 2012
+Why:    /proc/<pid>/oom_adj allows userspace to influence the oom killer's
+        badness heuristic used to determine which task to kill when the kernel
+        is out of memory.
+
+        The badness heuristic has since been rewritten since the introduction of
+        this tunable such that its meaning is deprecated.  The value was
+        implemented as a bitshift on a score generated by the badness()
+        function that did not have any precise units of measure.  With the
+        rewrite, the score is given as a proportion of available memory to the
+        task allocating pages, so using a bitshift which grows the score
+        exponentially is, thus, impossible to tune with fine granularity.
+
+        A much more powerful interface, /proc/<pid>/oom_score_adj, was
+        introduced with the oom killer rewrite that allows users to increase or
+        decrease the badness() score linearly.  This interface will replace
+        /proc/<pid>/oom_adj.
+
+        A warning will be emitted to the kernel log if an application uses this
+        deprecated interface.  After it is printed once, future warnings will be
+        suppressed until the kernel is rebooted.