[PATCH v2] mm,oom: add tracepoints for oom reaper-related events

[Roman Gushchin]

During the debugging of the problem described in
https://lkml.org/lkml/2017/5/17/542 and fixed by Tetsuo Handa
in https://lkml.org/lkml/2017/5/19/383 , I've found that
the existing debug output is not really useful to understand
issues related to the oom reaper.

So, I assume, that adding some tracepoints might help with
debugging of similar issues.

Trace the following events:
1) a process is marked as an oom victim,
2) a process is added to the oom reaper list,
3) the oom reaper starts reaping process's mm,
4) the oom reaper finished reaping,
5) the oom reaper skips reaping.

How it works in practice? Below is an example which show
how the problem mentioned above can be found: one process is added
twice to the oom_reaper list:

$ cd /sys/kernel/debug/tracing
$ echo "oom:mark_victim" > set_event
$ echo "oom:wake_reaper" >> set_event
$ echo "oom:skip_task_reaping" >> set_event
$ echo "oom:start_task_reaping" >> set_event
$ echo "oom:finish_task_reaping" >> set_event
$ cat trace_pipe
        allocate-502   [001] ....    91.836405: mark_victim: pid=502
        allocate-502   [001] .N..    91.837356: wake_reaper: pid=502
        allocate-502   [000] .N..    91.871149: wake_reaper: pid=502
      oom_reaper-23    [000] ....    91.871177: start_task_reaping: pid=502
      oom_reaper-23    [000] .N..    91.879511: finish_task_reaping: pid=502
      oom_reaper-23    [000] ....    91.879580: skip_task_reaping: pid=502

Signed-off-by: Roman Gushchin <guro@xxxxxx>
Cc: Michal Hocko <mhocko@xxxxxxxx>
Cc: Tetsuo Handa <penguin-kernel@xxxxxxxxxxxxxxxxxxx>
Cc: Johannes Weiner <hannes@xxxxxxxxxxx>
Cc: Vladimir Davydov <vdavydov.dev@xxxxxxxxx>
Cc: kernel-team@xxxxxx
Cc: linux-kernel@xxxxxxxxxxxxxxx
Cc: linux-mm@xxxxxxxxx
---
 include/trace/events/oom.h | 80 ++++++++++++++++++++++++++++++++++++++++++++++
 mm/oom_kill.c              |  7 ++++
 2 files changed, 87 insertions(+)

diff --git a/include/trace/events/oom.h b/include/trace/events/oom.h
index 38baeb2..c3c19d4 100644
--- a/include/trace/events/oom.h
+++ b/include/trace/events/oom.h
@@ -70,6 +70,86 @@ TRACE_EVENT(reclaim_retry_zone,
 			__entry->wmark_check)
 );
 
+TRACE_EVENT(mark_victim,
+	TP_PROTO(int pid),
+
+	TP_ARGS(pid),
+
+	TP_STRUCT__entry(
+		__field(int, pid)
+	),
+
+	TP_fast_assign(
+		__entry->pid = pid;
+	),
+
+	TP_printk("pid=%d", __entry->pid)
+);
+
+TRACE_EVENT(wake_reaper,
+	TP_PROTO(int pid),
+
+	TP_ARGS(pid),
+
+	TP_STRUCT__entry(
+		__field(int, pid)
+	),
+
+	TP_fast_assign(
+		__entry->pid = pid;
+	),
+
+	TP_printk("pid=%d", __entry->pid)
+);
+
+TRACE_EVENT(start_task_reaping,
+	TP_PROTO(int pid),
+
+	TP_ARGS(pid),
+
+	TP_STRUCT__entry(
+		__field(int, pid)
+	),
+
+	TP_fast_assign(
+		__entry->pid = pid;
+	),
+
+	TP_printk("pid=%d", __entry->pid)
+);
+
+TRACE_EVENT(finish_task_reaping,
+	TP_PROTO(int pid),
+
+	TP_ARGS(pid),
+
+	TP_STRUCT__entry(
+		__field(int, pid)
+	),
+
+	TP_fast_assign(
+		__entry->pid = pid;
+	),
+
+	TP_printk("pid=%d", __entry->pid)
+);
+
+TRACE_EVENT(skip_task_reaping,
+	TP_PROTO(int pid),
+
+	TP_ARGS(pid),
+
+	TP_STRUCT__entry(
+		__field(int, pid)
+	),
+
+	TP_fast_assign(
+		__entry->pid = pid;
+	),
+
+	TP_printk("pid=%d", __entry->pid)
+);
+
 #ifdef CONFIG_COMPACTION
 TRACE_EVENT(compact_retry,
 
diff --git a/mm/oom_kill.c b/mm/oom_kill.c
index 04c9143..409b685 100644
--- a/mm/oom_kill.c
+++ b/mm/oom_kill.c
@@ -490,6 +490,7 @@ static bool __oom_reap_task_mm(struct task_struct *tsk, struct mm_struct *mm)
 
 	if (!down_read_trylock(&mm->mmap_sem)) {
 		ret = false;
+		trace_skip_task_reaping(tsk->pid);
 		goto unlock_oom;
 	}
 
@@ -500,9 +501,12 @@ static bool __oom_reap_task_mm(struct task_struct *tsk, struct mm_struct *mm)
 	 */
 	if (!mmget_not_zero(mm)) {
 		up_read(&mm->mmap_sem);
+		trace_skip_task_reaping(tsk->pid);
 		goto unlock_oom;
 	}
 
+	trace_start_task_reaping(tsk->pid);
+
 	/*
 	 * Tell all users of get_user/copy_from_user etc... that the content
 	 * is no longer stable. No barriers really needed because unmapping
@@ -544,6 +548,7 @@ static bool __oom_reap_task_mm(struct task_struct *tsk, struct mm_struct *mm)
 	 * put the oom_reaper out of the way.
 	 */
 	mmput_async(mm);
+	trace_finish_task_reaping(tsk->pid);
 unlock_oom:
 	mutex_unlock(&oom_lock);
 	return ret;
@@ -615,6 +620,7 @@ static void wake_oom_reaper(struct task_struct *tsk)
 	tsk->oom_reaper_list = oom_reaper_list;
 	oom_reaper_list = tsk;
 	spin_unlock(&oom_reaper_lock);
+	trace_wake_reaper(tsk->pid);
 	wake_up(&oom_reaper_wait);
 }
 
@@ -666,6 +672,7 @@ static void mark_oom_victim(struct task_struct *tsk)
 	 */
 	__thaw_task(tsk);
 	atomic_inc(&oom_victims);
+	trace_mark_victim(tsk->pid);
 }
 
 /**
-- 
2.7.4