From: Tetsuo Handa <penguin-kernel@xxxxxxxxxxxxxxxxxxx>
Subject: mm,oom: speed up select_bad_process() loop
Since commit 3a5dda7a17cf3706 ("oom: prevent unnecessary oom kills or
kernel panics"), select_bad_process() is using for_each_process_thread().
Since oom_unkillable_task() scans all threads in the caller's thread group
and oom_task_origin() scans signal_struct of the caller's thread group, we
don't need to call oom_unkillable_task() and oom_task_origin() on each
thread. Also, since !mm test will be done later at oom_badness(), we
don't need to do !mm test on each thread. Therefore, we only need to do
TIF_MEMDIE test on each thread.
Although the original code was correct it was quite inefficient because
each thread group was scanned num_threads times which can be a lot
especially with processes with many threads. Even though the OOM is
extremely cold path it is always good to be as effective as possible when
we are inside rcu_read_lock() - aka unpreemptible context.
If we track number of TIF_MEMDIE threads inside signal_struct, we don't
need to do TIF_MEMDIE test on each thread. This will allow
select_bad_process() to use for_each_process().
This patch adds a counter to signal_struct for tracking how many
TIF_MEMDIE threads are in a given thread group, and check it at
oom_scan_process_thread() so that select_bad_process() can use
for_each_process() rather than for_each_process_thread().
[mhocko@xxxxxxxx: do not blow the signal_struct size]
Link: http://lkml.kernel.org/r/20160520075035.GF19172@xxxxxxxxxxxxxx
Link: http://lkml.kernel.org/r/201605182230.IDC73435.MVSOHLFOQFOJtF@xxxxxxxxxxxxxxxxxxx
Signed-off-by: Tetsuo Handa <penguin-kernel@xxxxxxxxxxxxxxxxxxx>
Acked-by: Michal Hocko <mhocko@xxxxxxxx>
Cc: David Rientjes <rientjes@xxxxxxxxxx>
Cc: Oleg Nesterov <oleg@xxxxxxxxxx>
Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
---
include/linux/sched.h | 1 +
mm/oom_kill.c | 17 ++++++-----------
2 files changed, 7 insertions(+), 11 deletions(-)
diff -puN include/linux/sched.h~mmoom-speed-up-select_bad_process-loop include/linux/sched.h
--- a/include/linux/sched.h~mmoom-speed-up-select_bad_process-loop
+++ a/include/linux/sched.h
@@ -669,6 +669,7 @@ struct signal_struct {
atomic_t sigcnt;
atomic_t live;
int nr_threads;
+ atomic_t oom_victims; /* # of TIF_MEDIE threads in this thread group */
struct list_head thread_head;
wait_queue_head_t wait_chldexit; /* for wait4() */
diff -puN mm/oom_kill.c~mmoom-speed-up-select_bad_process-loop mm/oom_kill.c
--- a/mm/oom_kill.c~mmoom-speed-up-select_bad_process-loop
+++ a/mm/oom_kill.c
@@ -283,12 +283,8 @@ enum oom_scan_t oom_scan_process_thread(
* This task already has access to memory reserves and is being killed.
* Don't allow any other task to have access to the reserves.
*/
- if (test_tsk_thread_flag(task, TIF_MEMDIE)) {
- if (!is_sysrq_oom(oc))
- return OOM_SCAN_ABORT;
- }
- if (!task->mm)
- return OOM_SCAN_CONTINUE;
+ if (!is_sysrq_oom(oc) && atomic_read(&task->signal->oom_victims))
+ return OOM_SCAN_ABORT;
/*
* If task is allocating a lot of memory and has been marked to be
@@ -307,12 +303,12 @@ enum oom_scan_t oom_scan_process_thread(
static struct task_struct *select_bad_process(struct oom_control *oc,
unsigned int *ppoints, unsigned long totalpages)
{
- struct task_struct *g, *p;
+ struct task_struct *p;
struct task_struct *chosen = NULL;
unsigned long chosen_points = 0;
rcu_read_lock();
- for_each_process_thread(g, p) {
+ for_each_process(p) {
unsigned int points;
switch (oom_scan_process_thread(oc, p, totalpages)) {
@@ -331,9 +327,6 @@ static struct task_struct *select_bad_pr
points = oom_badness(p, NULL, oc->nodemask, totalpages);
if (!points || 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;
@@ -673,6 +666,7 @@ void mark_oom_victim(struct task_struct
/* OOM killer might race with memcg OOM */
if (test_and_set_tsk_thread_flag(tsk, TIF_MEMDIE))
return;
+ atomic_inc(&tsk->signal->oom_victims);
/*
* Make sure that the task is woken up from uninterruptible sleep
* if it is frozen because OOM killer wouldn't be able to free
@@ -690,6 +684,7 @@ void exit_oom_victim(struct task_struct
{
if (!test_and_clear_tsk_thread_flag(tsk, TIF_MEMDIE))
return;
+ atomic_dec(&tsk->signal->oom_victims);
if (!atomic_dec_return(&oom_victims))
wake_up_all(&oom_victims_wait);
_
--
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