Re: [patch 2/7 -mm] oom: sacrifice child with highest badness score for parent

[KOSAKI Motohiro]

> 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.
> 
> Signed-off-by: David Rientjes <rientjes@xxxxxxxxxx>

Probably, kamezawa-san talked about right thing. but this patch is
enough small and it have no regression risk. So, we can choice step-by-step
development.
	Reviewed-by: KOSAKI Motohiro <kosaki.motohiro@xxxxxxxxxxxxxx>



> ---
>  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
> @@ -432,7 +432,10 @@ 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;
> +	unsigned long victim_points = 0;
> +	struct timespec uptime;
>  
>  	if (printk_ratelimit())
>  		dump_header(p, gfp_mask, order, mem);
> @@ -446,17 +449,25 @@ 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) with score %lu or sacrifice child\n",
> +		message, task_pid_nr(p), p->comm, points);
>  
> -	/* Try to kill a child first */
> +	/* Try to sacrifice the worst child first */
> +	do_posix_clock_monotonic_gettime(&uptime);
>  	list_for_each_entry(c, &p->children, sibling) {
> +		unsigned long cpoints;
> +
>  		if (c->mm == p->mm)
>  			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;
> +		}
>  	}
> -	return oom_kill_task(p);
> +	return oom_kill_task(victim);
>  }
>  
>  #ifdef CONFIG_CGROUP_MEM_RES_CTLR
> 
> --
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