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SubjectRe: [PATCH 2/5] oom: kill younger process first
On Fri, 20 May 2011, KOSAKI Motohiro wrote:

> 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)

Same response as when you initially proposed this patch: the comment needs
to explicitly state that it is not break-safe just like do_each_thread().
See http://marc.info/?l=linux-mm&m=130507027312785

A comment such as

/*
* Reverse of do_each_thread(); still not break-safe.
* Must hold tasklist_lock.
*/

would suffice. There are no "callers" to a macro.

After that:

Acked-by: David Rientjes <rientjes@google.com>


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