Re: + mm-oom-avoid-printk-iteration-under-rcu.patch added to -mm tree

[Michal Hocko]

Andrew,
I've had some concerns wrt. this patch - especially the additional
complexity - and I have to say I am not convinced that this is really
needed. Our past experience in this area suggests that more tricky code
leads to different corner cases. So I am really reluctant to add more
complexity without any real world reports.

On Tue 23-07-19 16:14:29, Andrew Morton wrote:
> From: Tetsuo Handa <penguin-kernel@xxxxxxxxxxxxxxxxxxx>
> Subject: mm, oom: avoid printk() iteration under RCU
> 
> 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.
> 
> Link: http://lkml.kernel.org/r/1563360901-8277-1-git-send-email-penguin-kernel@xxxxxxxxxxxxxxxxxxx
> Signed-off-by: Tetsuo Handa <penguin-kernel@xxxxxxxxxxxxxxxxxxx>
> Cc: Shakeel Butt <shakeelb@xxxxxxxxxx>
> Cc: Michal Hocko <mhocko@xxxxxxxx>
> Cc: Roman Gushchin <guro@xxxxxx>
> Cc: David Rientjes <rientjes@xxxxxxxxxx>
> Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
> ---
> 
>  include/linux/sched.h |    1 
>  mm/oom_kill.c         |   67 +++++++++++++++++++---------------------
>  2 files changed, 34 insertions(+), 34 deletions(-)
> 
> --- a/include/linux/sched.h~mm-oom-avoid-printk-iteration-under-rcu
> +++ a/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;
>  #ifdef CONFIG_VMAP_STACK
>  	struct vm_struct		*stack_vm_area;
>  #endif
> --- a/mm/oom_kill.c~mm-oom-avoid-printk-iteration-under-rcu
> +++ a/mm/oom_kill.c
> @@ -377,36 +377,13 @@ static void select_bad_process(struct oo
>  	}
>  }
>  
> -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
>   */
>  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)
> _
> 
> Patches currently in -mm which might be from penguin-kernel@xxxxxxxxxxxxxxxxxxx are
> 
> mm-oom-avoid-printk-iteration-under-rcu.patch
> info-task-hung-in-generic_file_write_iter.patch
> info-task-hung-in-generic_file_write-fix.patch
> kexec-bail-out-upon-sigkill-when-allocating-memory.patch

-- 
Michal Hocko
SUSE Labs