On Sun, 6 Jun 2010 15:34:28 -0700 (PDT)
David Rientjes <rientjes@xxxxxxxxxx> wrote:
> When a task is chosen for oom kill, the oom killer first attempts to
> sacrifice a child not sharing its parent's memory instead. Unfortunately,
> this often kills in a seemingly random fashion based on the ordering of
> the selected task's child list. Additionally, it is not guaranteed at all
> to free a large amount of memory that we need to prevent additional oom
> killing in the very near future.
>
> Instead, we now only attempt to sacrifice the worst child not sharing its
> parent's memory, if one exists. The worst child is indicated with the
> highest badness() score. This serves two advantages: we kill a
> memory-hogging task more often, and we allow the configurable
> /proc/pid/oom_adj value to be considered as a factor in which child to
> kill.
>
> Reviewers may observe that the previous implementation would iterate
> through the children and attempt to kill each until one was successful and
> then the parent if none were found while the new code simply kills the
> most memory-hogging task or the parent. Note that the only time
> oom_kill_task() fails, however, is when a child does not have an mm or has
> a /proc/pid/oom_adj of OOM_DISABLE. badness() returns 0 for both cases,
> so the final oom_kill_task() will always succeed.
>
> Acked-by: Rik van Riel <riel@xxxxxxxxxx>
> Acked-by: Nick Piggin <npiggin@xxxxxxx>
> Acked-by: Balbir Singh <balbir@xxxxxxxxxxxxxxxxxx>
> Acked-by: KOSAKI Motohiro <kosaki.motohiro@xxxxxxxxxxxxxx>
> Reviewed-by: KAMEZAWA Hiroyuki <kamezawa.hiroyu@xxxxxxxxxxxxxx>
> Reviewed-by: KOSAKI Motohiro <kosaki.motohiro@xxxxxxxxxxxxxx>
> Signed-off-by: David Rientjes <rientjes@xxxxxxxxxx>
> ---
> mm/oom_kill.c | 23 +++++++++++++++++------
> 1 files changed, 17 insertions(+), 6 deletions(-)
>
> diff --git a/mm/oom_kill.c b/mm/oom_kill.c
> --- a/mm/oom_kill.c
> +++ b/mm/oom_kill.c
> @@ -441,8 +441,11 @@ static int oom_kill_process(struct task_struct *p, gfp_t gfp_mask, int order,
> unsigned long points, struct mem_cgroup *mem,
> const char *message)
> {
> + struct task_struct *victim = p;
> struct task_struct *c;
> struct task_struct *t = p;
> + unsigned long victim_points = 0;
> + struct timespec uptime;
>
> if (printk_ratelimit())
> dump_header(p, gfp_mask, order, mem);
> @@ -456,22 +459,30 @@ static int oom_kill_process(struct task_struct *p, gfp_t gfp_mask, int order,
> return 0;
> }
>
> - printk(KERN_ERR "%s: kill process %d (%s) score %li or a child\n",
> - message, task_pid_nr(p), p->comm, points);
> + pr_err("%s: Kill process %d (%s) score %lu or sacrifice child\n",
> + message, task_pid_nr(p), p->comm, points);
fyi, access to another task's ->comm is racy against prctl(). Fixable
with get_task_comm(). But that takes task_lock(), which is risky in
this code. The world wouldn't end if we didn't fix this ;)
> - /* Try to kill a child first */
> + /* Try to sacrifice the worst child first */
> + do_posix_clock_monotonic_gettime(&uptime);
> do {
> + unsigned long cpoints;
This could be local to the list_for_each_entry() block.
What does "cpoints" mean?
> list_for_each_entry(c, &t->children, sibling) {
I'm surprised we don't have a sched.h helper for this. Maybe it's not
a very common thing to do.
> if (c->mm == p->mm)
> continue;
> if (mem && !task_in_mem_cgroup(c, mem))
> continue;
> - if (!oom_kill_task(c))
> - return 0;
> +
> + /* badness() returns 0 if the thread is unkillable */
> + cpoints = badness(c, uptime.tv_sec);
> + if (cpoints > victim_points) {
> + victim = c;
> + victim_points = cpoints;
> + }
> }
> } while_each_thread(p, t);
>
> - return oom_kill_task(p);
> + return oom_kill_task(victim);
> }
And this function is secretly called under tasklist_lock, which is what
pins *victim, yes?
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