alinux: doc: alibaba: Add priority oom descriptions

Add "memory.priority" and "memory.use_priority_oom" descriptions.
Signed-off-by: NWenwei Tao <wenwei.tao@linux.alibaba.com>
Reviewed-by: NYang Shi <yang.shi@linux.alibaba.com>

Documentation/alibaba/interfaces.rst

@@ -122,3 +122,48 @@ zombie memcgs reaper
     - /sys/kernel/mm/memcg_reaper/reap
 
       users write "1" to trigger one round of zombie memcg reaping.
+
+memory.priority
+===============
+    Under memory pressure, reclaim and oom would happen, with multiple
+    cgroups exist in one system, we might want some of the cgroups's memory
+    or tasks survived the reclaim and oom while there are other candidates.
+
+    The "memory.low" and "memory.min" make that happen during reclaim, this
+    "memory.priority" introduces a priority oom to meet above requirement
+    in oom.
+
+    The priority value is from 0 to 12, the higher number the higher priority.
+    The priority is among siblings, it is not global priority, by this we can
+    map these 13 priorities to the tens of thousands of memcgs.
+
+    When oom happens it first chooses the lowest priority memcg as victim then
+    uses the kernel default algorithm(see function oom_evaluate_task()) to select
+    bad process from the victim memcg.
+
+    For example::
+
+     The following hierarchy:
+                    root
+                    /  \
+                   A    B
+                  / \  /  \
+                 C   D E   F
+
+     priority:
+        A: 10, B: 8
+        C: 5, D: 6, E: 7, F: 8
+
+    When oom happens in root, it first iterates its two children A and B, and selects
+    B as next iteration root since B's priority is lower than A, subsequent victim
+    selection is limit in the B's subtree. E is selected as victim memcg finally, since
+    its priority is lower than its sibling.
+
+    This priority oom works both for memcg and global oom. For global oom the root is
+    root memcg.
+
+memory.use_priority_oom
+=======================
+    This file is used to enable/disable priority oom.
+
+    Write "1" to enable the priority oom and "0" to disable it.