mm,oom: speed up select_bad_process() loop

Since commit 3a5dda7a17cf ("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@suse.com: do not blow the signal_struct size]
Link: http://lkml.kernel.org/r/20160520075035.GF19172@dhcp22.suse.cz
Link: http://lkml.kernel.org/r/201605182230.IDC73435.MVSOHLFOQFOJtF@I-love.SAKURA.ne.jp
Signed-off-by: Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp>
Acked-by: Michal Hocko <mhocko@suse.com>
Cc: David Rientjes <rientjes@google.com>
Cc: Oleg Nesterov <oleg@redhat.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

diff --git a/include/linux/sched.h b/include/linux/sched.h
index 479e3ca..01fe1bb 100644 (file)
--- a/include/linux/sched.h
+++ b/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 --git a/mm/oom_kill.c b/mm/oom_kill.c
index c0e37dd..5bb2f76 100644 (file)
--- a/mm/oom_kill.c
+++ b/mm/oom_kill.c
@@ -283,12 +283,8 @@ enum oom_scan_t oom_scan_process_thread(struct oom_control *oc,
         * 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(struct oom_control *oc,
 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_process(struct oom_control *oc,
                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 *tsk)
        /* 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 *tsk)
 {
        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);