Re: [PATCH] oom: skip frozen tasks

[David Rientjes <rientjes@google.com>]

On Fri, 26 Aug 2011, Michal Hocko wrote:

> Let's give all frozen tasks a bonus (OOM_SCORE_ADJ_MAX/2) so that we do
> not consider them unless really necessary and if we really pick up one
> then thaw its threads before we try to kill it.
> 

I don't like arbitrary heuristics like this because they polluted the old 
oom killer before it was rewritten and made it much more unpredictable.  
The only heuristic it includes right now is a bonus for root tasks so that 
when two processes have nearly the same amount of memory usage (within 3% 
of available memory), the non-root task is chosen instead.

This bonus is actually saying that a single frozen task can use up to 50% 
more of the machine's capacity in a system-wide oom condition than the 
task that will now be killed instead.  That seems excessive.

I do like the idea of automatically thawing the task though and if that's 
possible then I don't think we need to manipulate the badness heuristic at 
all.  I know that wouldn't be feasible when we've frozen _all_ threads and 
that's why we have oom_killer_disable(), but we'll have to check with 
Rafael to see if something like this could work.  Rafael?

> TODO
> - given bonus might be too big?
> - aren't we racing with try_to_freeze_tasks?
> ---
>  mm/oom_kill.c |   13 +++++++++++++
>  1 files changed, 13 insertions(+), 0 deletions(-)
> 
> diff --git a/mm/oom_kill.c b/mm/oom_kill.c
> index 626303b..fd194bc 100644
> --- a/mm/oom_kill.c
> +++ b/mm/oom_kill.c
> @@ -32,6 +32,7 @@
>  #include <linux/mempolicy.h>
>  #include <linux/security.h>
>  #include <linux/ptrace.h>
> +#include <linux/freezer.h>
>  
>  int sysctl_panic_on_oom;
>  int sysctl_oom_kill_allocating_task;
> @@ -214,6 +215,14 @@ unsigned int oom_badness(struct task_struct *p, struct mem_cgroup *mem,
>  	points += p->signal->oom_score_adj;
>  
>  	/*
> +	 * Do not try to kill frozen tasks unless there is nothing else to kill.
> +	 * We do not want to give it 1 point because we still want to select a good
> +	 * candidate among all frozen tasks. Let's give it a reasonable bonus.
> +	 */
> +	if (frozen(p))
> +		points -= OOM_SCORE_ADJ_MAX/2;
> +
> +	/*
>  	 * Never return 0 for an eligible task that may be killed since it's
>  	 * possible that no single user task uses more than 0.1% of memory and
>  	 * no single admin tasks uses more than 3.0%.
> @@ -450,6 +459,10 @@ static int oom_kill_task(struct task_struct *p, struct mem_cgroup *mem)
>  			pr_err("Kill process %d (%s) sharing same memory\n",
>  				task_pid_nr(q), q->comm);
>  			task_unlock(q);
> +
> +			if (frozen(q))
> +				thaw_process(q);
> +
>  			force_sig(SIGKILL, q);
>  		}
>  

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