[patch v2 -mm] mm, oom: prefer thread group leaders for display purposes

[David Rientjes <rientjes@google.com>]

When two threads have the same badness score, it's preferable to kill the 
thread group leader so that the actual process name is printed to the 
kernel log rather than the thread group name which may be shared amongst 
several processes.

This was the behavior when select_bad_process() used to do 
for_each_process(), but it now iterates threads instead and leads to 
ambiguity.

Signed-off-by: David Rientjes <rientjes@google.com>
---
 v2: fixes missing get_task_struct() found by Johannes.

 mm/memcontrol.c | 19 ++++++++++++-------
 mm/oom_kill.c   | 12 ++++++++----
 2 files changed, 20 insertions(+), 11 deletions(-)

diff --git a/mm/memcontrol.c b/mm/memcontrol.c
index a815686..d69c4b3 100644
--- a/mm/memcontrol.c
+++ b/mm/memcontrol.c
@@ -1841,13 +1841,18 @@ static void mem_cgroup_out_of_memory(struct mem_cgroup *memcg, gfp_t gfp_mask,
 				break;
 			};
 			points = oom_badness(task, memcg, NULL, totalpages);
-			if (points > chosen_points) {
-				if (chosen)
-					put_task_struct(chosen);
-				chosen = task;
-				chosen_points = points;
-				get_task_struct(chosen);
-			}
+			if (points < chosen_points)
+				continue;
+			/* Prefer thread group leaders for display purposes */
+			if (points == chosen_points &&
+			    thread_group_leader(chosen))
+				continue;
+
+			if (chosen)
+				put_task_struct(chosen);
+			chosen = task;
+			chosen_points = points;
+			get_task_struct(chosen);
 		}
 		css_task_iter_end(&it);
 	}
diff --git a/mm/oom_kill.c b/mm/oom_kill.c
index 054ff47..1dca3d8 100644
--- a/mm/oom_kill.c
+++ b/mm/oom_kill.c
@@ -327,10 +327,14 @@ static struct task_struct *select_bad_process(unsigned int *ppoints,
 			break;
 		};
 		points = oom_badness(p, NULL, nodemask, totalpages);
-		if (points > chosen_points) {
-			chosen = p;
-			chosen_points = points;
-		}
+		if (points < chosen_points)
+			continue;
+		/* Prefer thread group leaders for display purposes */
+		if (points == chosen_points && thread_group_leader(chosen))
+			continue;
+
+		chosen = p;
+		chosen_points = points;
 	}
 	if (chosen)
 		get_task_struct(chosen);

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