Re: [PATCH] oom: skip frozen tasks

[Michal Hocko <mhocko@suse.cz>]

On Fri 26-08-11 02:21:42, David Rientjes wrote:
> 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.

Yes, the number is probably too high. I just wanted to start up with
something. Maybe we can give it another root bonus. But I agree whatever
we use it will be just a random value...

> 
> 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 

Why it wouldn't be feasible for all threads? If you have all tasks
frozen (suspend going on, whole cgroup or all tasks in a cpuset/nodemask
are frozen) then the selection is more natural because all of them are
equal (with or without a bonus). The bonus tries to reduce thawing if
not all of them are frozen.
I am not saying the bonus is necessary, though. It depends on what
the freezer is used for (e.g. freeze a process which went wild and
debug what went wrong wouldn't welcome that somebody killed it or other
(mis)use which relies on D state).

> 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);
> >  		}
> >  

-- 
Michal Hocko
SUSE Labs
SUSE LINUX s.r.o.
Lihovarska 1060/12
190 00 Praha 9    
Czech Republic

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