Re: [PATCH] mm, oom: avoid printk() iteration under RCU

[Shakeel Butt <shakeelb@google.com>]

On Wed, Jul 17, 2019 at 3:55 AM Tetsuo Handa
<penguin-kernel@i-love.sakura.ne.jp> wrote:
>
> Currently dump_tasks() might call printk() for many thousands times under
> RCU, which might take many minutes for slow consoles. Therefore, split
> dump_tasks() into three stages; take a snapshot of possible OOM victim
> candidates under RCU, dump the snapshot from reschedulable context, and
> destroy the snapshot.
>
> In a future patch, the first stage would be moved to select_bad_process()
> and the third stage would be moved to after oom_kill_process(), and will
> simplify refcount handling.
>
> Signed-off-by: Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp>
> Cc: Shakeel Butt <shakeelb@google.com>
> Cc: Michal Hocko <mhocko@suse.com>
> Cc: Roman Gushchin <guro@fb.com>
> ---
>  include/linux/sched.h |  1 +
>  mm/oom_kill.c         | 67 +++++++++++++++++++++++++--------------------------
>  2 files changed, 34 insertions(+), 34 deletions(-)
>
> diff --git a/include/linux/sched.h b/include/linux/sched.h
> index 8dc1811..cb6696b 100644
> --- a/include/linux/sched.h
> +++ b/include/linux/sched.h
> @@ -1246,6 +1246,7 @@ struct task_struct {
>  #ifdef CONFIG_MMU
>         struct task_struct              *oom_reaper_list;
>  #endif
> +       struct list_head                oom_victim_list;

Shouldn't there be INIT_LIST_HEAD(&tsk->oom_victim_list) somewhere?

>  #ifdef CONFIG_VMAP_STACK
>         struct vm_struct                *stack_vm_area;
>  #endif
> diff --git a/mm/oom_kill.c b/mm/oom_kill.c
> index eda2e2a..bd22ca0 100644
> --- a/mm/oom_kill.c
> +++ b/mm/oom_kill.c
> @@ -377,36 +377,13 @@ static void select_bad_process(struct oom_control *oc)
>         }
>  }
>
> -static int dump_task(struct task_struct *p, void *arg)
> -{
> -       struct oom_control *oc = arg;
> -       struct task_struct *task;
> -
> -       if (oom_unkillable_task(p))
> -               return 0;
> -
> -       /* p may not have freeable memory in nodemask */
> -       if (!is_memcg_oom(oc) && !oom_cpuset_eligible(p, oc))
> -               return 0;
>
> -       task = find_lock_task_mm(p);
> -       if (!task) {
> -               /*
> -                * This is a kthread or all of p's threads have already
> -                * detached their mm's.  There's no need to report
> -                * them; they can't be oom killed anyway.
> -                */
> -               return 0;
> +static int add_candidate_task(struct task_struct *p, void *arg)
> +{
> +       if (!oom_unkillable_task(p)) {
> +               get_task_struct(p);
> +               list_add_tail(&p->oom_victim_list, (struct list_head *) arg);
>         }
> -
> -       pr_info("[%7d] %5d %5d %8lu %8lu %8ld %8lu         %5hd %s\n",
> -               task->pid, from_kuid(&init_user_ns, task_uid(task)),
> -               task->tgid, task->mm->total_vm, get_mm_rss(task->mm),
> -               mm_pgtables_bytes(task->mm),
> -               get_mm_counter(task->mm, MM_SWAPENTS),
> -               task->signal->oom_score_adj, task->comm);
> -       task_unlock(task);
> -
>         return 0;
>  }
>
> @@ -422,19 +399,41 @@ static int dump_task(struct task_struct *p, void *arg)
>   */
>  static void dump_tasks(struct oom_control *oc)
>  {
> -       pr_info("Tasks state (memory values in pages):\n");
> -       pr_info("[  pid  ]   uid  tgid total_vm      rss pgtables_bytes swapents oom_score_adj name\n");
> +       static LIST_HEAD(list);
> +       struct task_struct *p;
> +       struct task_struct *t;
>
>         if (is_memcg_oom(oc))
> -               mem_cgroup_scan_tasks(oc->memcg, dump_task, oc);
> +               mem_cgroup_scan_tasks(oc->memcg, add_candidate_task, &list);
>         else {
> -               struct task_struct *p;
> -
>                 rcu_read_lock();
>                 for_each_process(p)
> -                       dump_task(p, oc);
> +                       add_candidate_task(p, &list);
>                 rcu_read_unlock();
>         }
> +       pr_info("Tasks state (memory values in pages):\n");
> +       pr_info("[  pid  ]   uid  tgid total_vm      rss pgtables_bytes swapents oom_score_adj name\n");
> +       list_for_each_entry(p, &list, oom_victim_list) {
> +               cond_resched();
> +               /* p may not have freeable memory in nodemask */
> +               if (!is_memcg_oom(oc) && !oom_cpuset_eligible(p, oc))
> +                       continue;
> +               /* All of p's threads might have already detached their mm's. */
> +               t = find_lock_task_mm(p);
> +               if (!t)
> +                       continue;
> +               pr_info("[%7d] %5d %5d %8lu %8lu %8ld %8lu         %5hd %s\n",
> +                       t->pid, from_kuid(&init_user_ns, task_uid(t)),
> +                       t->tgid, t->mm->total_vm, get_mm_rss(t->mm),
> +                       mm_pgtables_bytes(t->mm),
> +                       get_mm_counter(t->mm, MM_SWAPENTS),
> +                       t->signal->oom_score_adj, t->comm);
> +               task_unlock(t);
> +       }
> +       list_for_each_entry_safe(p, t, &list, oom_victim_list) {
> +               list_del(&p->oom_victim_list);
> +               put_task_struct(p);
> +       }
>  }
>
>  static void dump_oom_summary(struct oom_control *oc, struct task_struct *victim)
> --
> 1.8.3.1
>