Re: [patch] mm, oom: avoid checking set of allowed nodes twice when selecting a victim

[Michal Hocko <mhocko@suse.cz>]

On Tue 03-04-12 16:34:36, David Rientjes wrote:
> For systems with high CONFIG_NODES_SHIFT, checking nodes_intersect() for
> each thread's set of allowed nodes is very expensive.  It's unnecessary
> to do this twice for each thread, once in select_bad_process() and once
> in oom_badness().  We've already filtered unkillable threads at the point
> where oom_badness() is called.
> 
> oom_badness() must still check if a thread is a kthread, however, to
> ensure /proc/pid/oom_score doesn't report one as killable.
> 
> This significantly speeds up the tasklist iteration when there are a
> large number of threads on the system and CONFIG_NODES_SHIFT is high.
> 
> Signed-off-by: David Rientjes <rientjes@google.com>

Reviewed-by: Michal Hocko <mhocko@suse.cz>

> ---
>  mm/oom_kill.c |   16 +++++++++-------
>  1 file changed, 9 insertions(+), 7 deletions(-)
> 
> diff --git a/mm/oom_kill.c b/mm/oom_kill.c
> --- a/mm/oom_kill.c
> +++ b/mm/oom_kill.c
> @@ -151,13 +151,16 @@ struct task_struct *find_lock_task_mm(struct task_struct *p)
>  	return NULL;
>  }
>  
> +static bool is_unkillable_kthread(struct task_struct *p)
> +{
> +	return is_global_init(p) || (p->flags & PF_KTHREAD);
> +}
> +
>  /* return true if the task is not adequate as candidate victim task. */
>  static bool oom_unkillable_task(struct task_struct *p,
>  		const struct mem_cgroup *memcg, const nodemask_t *nodemask)
>  {
> -	if (is_global_init(p))
> -		return true;
> -	if (p->flags & PF_KTHREAD)
> +	if (is_unkillable_kthread(p))
>  		return true;
>  
>  	/* When mem_cgroup_out_of_memory() and p is not member of the group */
> @@ -185,7 +188,7 @@ unsigned int oom_badness(struct task_struct *p, struct mem_cgroup *memcg,
>  {
>  	long points;
>  
> -	if (oom_unkillable_task(p, memcg, nodemask))
> +	if (is_unkillable_kthread(p))
>  		return 0;
>  
>  	p = find_lock_task_mm(p);
> @@ -478,9 +481,8 @@ static void oom_kill_process(struct task_struct *p, gfp_t gfp_mask, int order,
>  
>  			if (child->mm == p->mm)
>  				continue;
> -			/*
> -			 * oom_badness() returns 0 if the thread is unkillable
> -			 */
> +			if (oom_unkillable_task(child, memcg, nodemask))
> +				continue;
>  			child_points = oom_badness(child, memcg, nodemask,
>  								totalpages);
>  			if (child_points > victim_points) {
> 
> --
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-- 
Michal Hocko
SUSE Labs
SUSE LINUX s.r.o.
Lihovarska 1060/12
190 00 Praha 9    
Czech Republic

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