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    Subject[PATCH 2/5] oom: kill younger process first
    This patch introduces do_each_thread_reverse() and select_bad_process()
    uses it. The benefits are two, 1) oom-killer can kill younger process
    than older if they have a same oom score. Usually younger process is
    less important. 2) younger task often have PF_EXITING because shell
    script makes a lot of short lived processes. Reverse order search can
    detect it faster.

    Reported-by: CAI Qian <caiqian@redhat.com>
    Signed-off-by: KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com>
    Cc: Minchan Kim <minchan.kim@gmail.com>
    Cc: David Rientjes <rientjes@google.com>
    Cc: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
    ---
    include/linux/sched.h | 11 +++++++++++
    mm/oom_kill.c | 2 +-
    2 files changed, 12 insertions(+), 1 deletions(-)

    diff --git a/include/linux/sched.h b/include/linux/sched.h
    index 013314a..3698379 100644
    --- a/include/linux/sched.h
    +++ b/include/linux/sched.h
    @@ -2194,6 +2194,9 @@ static inline unsigned long wait_task_inactive(struct task_struct *p,
    #define next_task(p) \
    list_entry_rcu((p)->tasks.next, struct task_struct, tasks)

    +#define prev_task(p) \
    + list_entry((p)->tasks.prev, struct task_struct, tasks)
    +
    #define for_each_process(p) \
    for (p = &init_task ; (p = next_task(p)) != &init_task ; )

    @@ -2206,6 +2209,14 @@ extern bool current_is_single_threaded(void);
    #define do_each_thread(g, t) \
    for (g = t = &init_task ; (g = t = next_task(g)) != &init_task ; ) do

    +/*
    + * Similar to do_each_thread(). but two difference are there.
    + * - traverse tasks reverse order (i.e. younger to older)
    + * - caller must hold tasklist_lock. rcu_read_lock isn't enough
    +*/
    +#define do_each_thread_reverse(g, t) \
    + for (g = t = &init_task ; (g = t = prev_task(g)) != &init_task ; ) do
    +
    #define while_each_thread(g, t) \
    while ((t = next_thread(t)) != g)

    diff --git a/mm/oom_kill.c b/mm/oom_kill.c
    index 43d32ae..e6a6c6f 100644
    --- a/mm/oom_kill.c
    +++ b/mm/oom_kill.c
    @@ -282,7 +282,7 @@ static struct task_struct *select_bad_process(unsigned int *ppoints,
    struct task_struct *chosen = NULL;
    *ppoints = 0;

    - do_each_thread(g, p) {
    + do_each_thread_reverse(g, p) {
    unsigned int points;

    if (!p->mm)
    --
    1.7.3.1




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