Re: [patch v2] mm, oom: normalize oom scores to oom_score_adj scale only for userspace

[Andrew Morton <akpm@linux-foundation.org>]

On Wed, 23 May 2012 00:15:03 -0700 (PDT)
David Rientjes <rientjes@google.com> wrote:

> The oom_score_adj scale ranges from -1000 to 1000 and represents the
> proportion of memory available to the process at allocation time.  This
> means an oom_score_adj value of 300, for example, will bias a process as
> though it was using an extra 30.0% of available memory and a value of
> -350 will discount 35.0% of available memory from its usage.
> 
> The oom killer badness heuristic also uses this scale to report the oom
> score for each eligible process in determining the "best" process to
> kill.  Thus, it can only differentiate each process's memory usage by
> 0.1% of system RAM.
> 
> On large systems, this can end up being a large amount of memory: 256MB
> on 256GB systems, for example.
> 
> This can be fixed by having the badness heuristic to use the actual
> memory usage in scoring threads and then normalizing it to the
> oom_score_adj scale for userspace.  This results in better comparison
> between eligible threads for kill and no change from the userspace
> perspective.
> 
> ...
>
> @@ -367,12 +354,13 @@ static struct task_struct *select_bad_process(unsigned int *ppoints,
>  		}
>  
>  		points = oom_badness(p, memcg, nodemask, totalpages);
> -		if (points > *ppoints) {
> +		if (points > chosen_points) {
>  			chosen = p;
> -			*ppoints = points;
> +			chosen_points = points;
>  		}
>  	} while_each_thread(g, p);
>  
> +	*ppoints = chosen_points * 1000 / totalpages;
>  	return chosen;
>  }
>  

It's still not obvious that we always avoid the divide-by-zero here. 
If there's some weird way of convincing constrained_alloc() to look at
an empty nodemask, or a nodemask which covers only empty nodes then
blam.

Now, it's probably the case that this is a can't-happen but that
guarantee would be pretty convoluted and fragile?