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SubjectRe: [PATCH v3 2/2] mm, oom: remove 'prefer children over parent' heuristic
On Mon, Jan 21, 2019 at 1:59 PM Shakeel Butt <shakeelb@google.com> wrote:
>
> From the start of the git history of Linux, the kernel after selecting
> the worst process to be oom-killed, prefer to kill its child (if the
> child does not share mm with the parent). Later it was changed to prefer
> to kill a child who is worst. If the parent is still the worst then the
> parent will be killed.
>
> This heuristic assumes that the children did less work than their parent
> and by killing one of them, the work lost will be less. However this is
> very workload dependent. If there is a workload which can benefit from
> this heuristic, can use oom_score_adj to prefer children to be killed
> before the parent.
>
> The select_bad_process() has already selected the worst process in the
> system/memcg. There is no need to recheck the badness of its children
> and hoping to find a worse candidate. That's a lot of unneeded racy
> work. Also the heuristic is dangerous because it make fork bomb like
> workloads to recover much later because we constantly pick and kill
> processes which are not memory hogs. So, let's remove this whole
> heuristic.
>
> Signed-off-by: Shakeel Butt <shakeelb@google.com>
> Acked-by: Michal Hocko <mhocko@suse.com>

Michal, though I have kept your Acked-by but I have made a couple of
changes in the code. Please let me know if you are ok with the
changes.

> Cc: Roman Gushchin <guro@fb.com>
> Cc: Andrew Morton <akpm@linux-foundation.org>
> Cc: David Rientjes <rientjes@google.com>
> Cc: Johannes Weiner <hannes@cmpxchg.org>
> Cc: Tetsuo Handa <penguin-kernel@i-love.sakura.ne.jp>
> Cc: linux-mm@kvack.org
> Cc: linux-kernel@vger.kernel.org
>
> ---
> Changelog since v2:
> - Propagate the message to __oom_kill_process().
>
> Changelog since v1:
> - Improved commit message based on mhocko's comment.
> - Replaced 'p' with 'victim'.
> - Removed extra pr_err message.
>
> ---
> mm/oom_kill.c | 78 ++++++++++++---------------------------------------
> 1 file changed, 18 insertions(+), 60 deletions(-)
>
> diff --git a/mm/oom_kill.c b/mm/oom_kill.c
> index 1a007dae1e8f..c90184fd48a3 100644
> --- a/mm/oom_kill.c
> +++ b/mm/oom_kill.c
> @@ -843,7 +843,7 @@ static bool task_will_free_mem(struct task_struct *task)
> return ret;
> }
>
> -static void __oom_kill_process(struct task_struct *victim)
> +static void __oom_kill_process(struct task_struct *victim, const char *message)
> {
> struct task_struct *p;
> struct mm_struct *mm;
> @@ -874,8 +874,9 @@ static void __oom_kill_process(struct task_struct *victim)
> */
> do_send_sig_info(SIGKILL, SEND_SIG_PRIV, victim, PIDTYPE_TGID);
> mark_oom_victim(victim);
> - pr_err("Killed process %d (%s) total-vm:%lukB, anon-rss:%lukB, file-rss:%lukB, shmem-rss:%lukB\n",
> - task_pid_nr(victim), victim->comm, K(victim->mm->total_vm),
> + pr_err("%s: Killed process %d (%s) total-vm:%lukB, anon-rss:%lukB, file-rss:%lukB, shmem-rss:%lukB\n",
> + message, task_pid_nr(victim), victim->comm,
> + K(victim->mm->total_vm),
> K(get_mm_counter(victim->mm, MM_ANONPAGES)),
> K(get_mm_counter(victim->mm, MM_FILEPAGES)),
> K(get_mm_counter(victim->mm, MM_SHMEMPAGES)));
> @@ -932,24 +933,19 @@ static void __oom_kill_process(struct task_struct *victim)
> * Kill provided task unless it's secured by setting
> * oom_score_adj to OOM_SCORE_ADJ_MIN.
> */
> -static int oom_kill_memcg_member(struct task_struct *task, void *unused)
> +static int oom_kill_memcg_member(struct task_struct *task, void *message)
> {
> if (task->signal->oom_score_adj != OOM_SCORE_ADJ_MIN) {
> get_task_struct(task);
> - __oom_kill_process(task);
> + __oom_kill_process(task, message);
> }
> return 0;
> }
>
> static void oom_kill_process(struct oom_control *oc, const char *message)
> {
> - struct task_struct *p = oc->chosen;
> - unsigned int points = oc->chosen_points;
> - struct task_struct *victim = p;
> - struct task_struct *child;
> - struct task_struct *t;
> + struct task_struct *victim = oc->chosen;
> struct mem_cgroup *oom_group;
> - unsigned int victim_points = 0;
> static DEFINE_RATELIMIT_STATE(oom_rs, DEFAULT_RATELIMIT_INTERVAL,
> DEFAULT_RATELIMIT_BURST);
>
> @@ -958,57 +954,18 @@ static void oom_kill_process(struct oom_control *oc, const char *message)
> * its children or threads, just give it access to memory reserves
> * so it can die quickly
> */
> - task_lock(p);
> - if (task_will_free_mem(p)) {
> - mark_oom_victim(p);
> - wake_oom_reaper(p);
> - task_unlock(p);
> - put_task_struct(p);
> + task_lock(victim);
> + if (task_will_free_mem(victim)) {
> + mark_oom_victim(victim);
> + wake_oom_reaper(victim);
> + task_unlock(victim);
> + put_task_struct(victim);
> return;
> }
> - task_unlock(p);
> + task_unlock(victim);
>
> if (__ratelimit(&oom_rs))
> - dump_header(oc, p);
> -
> - pr_err("%s: Kill process %d (%s) score %u or sacrifice child\n",
> - message, task_pid_nr(p), p->comm, points);
> -
> - /*
> - * If any of p's children has a different mm and is eligible for kill,
> - * the one with the highest oom_badness() score is sacrificed for its
> - * parent. This attempts to lose the minimal amount of work done while
> - * still freeing memory.
> - */
> - read_lock(&tasklist_lock);
> -
> - /*
> - * The task 'p' might have already exited before reaching here. The
> - * put_task_struct() will free task_struct 'p' while the loop still try
> - * to access the field of 'p', so, get an extra reference.
> - */
> - get_task_struct(p);
> - for_each_thread(p, t) {
> - list_for_each_entry(child, &t->children, sibling) {
> - unsigned int child_points;
> -
> - if (process_shares_mm(child, p->mm))
> - continue;
> - /*
> - * oom_badness() returns 0 if the thread is unkillable
> - */
> - child_points = oom_badness(child,
> - oc->memcg, oc->nodemask, oc->totalpages);
> - if (child_points > victim_points) {
> - put_task_struct(victim);
> - victim = child;
> - victim_points = child_points;
> - get_task_struct(victim);
> - }
> - }
> - }
> - put_task_struct(p);
> - read_unlock(&tasklist_lock);
> + dump_header(oc, victim);
>
> /*
> * Do we need to kill the entire memory cgroup?
> @@ -1017,14 +974,15 @@ static void oom_kill_process(struct oom_control *oc, const char *message)
> */
> oom_group = mem_cgroup_get_oom_group(victim, oc->memcg);
>
> - __oom_kill_process(victim);
> + __oom_kill_process(victim, message);
>
> /*
> * If necessary, kill all tasks in the selected memory cgroup.
> */
> if (oom_group) {
> mem_cgroup_print_oom_group(oom_group);
> - mem_cgroup_scan_tasks(oom_group, oom_kill_memcg_member, NULL);
> + mem_cgroup_scan_tasks(oom_group, oom_kill_memcg_member,
> + (void*) message);
> mem_cgroup_put(oom_group);
> }
> }
> --
> 2.20.1.321.g9e740568ce-goog
>

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