Re: [PATCH] oom: break after selecting process to kill

[Niv Yehezkel <executerx@gmail.com>]

[-- Attachment #1: Type: text/plain, Size: 3012 bytes --]

On Fri, Sep 12, 2014 at 10:08:53AM +0200, Michal Hocko wrote:
> On Thu 11-09-14 17:33:39, Niv Yehezkel wrote:
> > There is no need to fallback and continue computing
> > badness for each running process after we have found a
> > process currently performing the swapoff syscall. We ought to
> > immediately select this process for killing.
> 
> a) this is not only about swapoff. KSM (run_store) is currently
>    considered oom origin as well.
> b) you forgot to tell us what led you to this change. It sounds like a
>    minor optimization to me. We can potentially skip scanning through
>    many tasks but this is not guaranteed at all because our task might
>    be at the very end of the tasks list as well.
> c) finally this might select thread != thread_group_leader which is a
>    minor issue affecting oom report
> 
> I am not saying the change is wrong but please make sure you first
> describe your motivation. Does it fix any issue you are seeing?  Is this
> just something that struck you while reading the code? Maybe it was 
> /* always select this thread first */ comment for OOM_SCAN_SELECT.
> Besides that your process_selected is not really needed. You could test
> for chosen_points == ULONG_MAX as well. This would be even more
> straightforward because any score like that is ultimate candidate.
> 
> > Signed-off-by: Niv Yehezkel <executerx@gmail.com>
> > ---
> >  mm/oom_kill.c |    6 +++++-
> >  1 file changed, 5 insertions(+), 1 deletion(-)
> > 
> > diff --git a/mm/oom_kill.c b/mm/oom_kill.c
> > index 1e11df8..68ac30e 100644
> > --- a/mm/oom_kill.c
> > +++ b/mm/oom_kill.c
> > @@ -305,6 +305,7 @@ static struct task_struct *select_bad_process(unsigned int *ppoints,
> >  	struct task_struct *g, *p;
> >  	struct task_struct *chosen = NULL;
> >  	unsigned long chosen_points = 0;
> > +	bool process_selected = false;
> >  
> >  	rcu_read_lock();
> >  	for_each_process_thread(g, p) {
> > @@ -315,7 +316,8 @@ static struct task_struct *select_bad_process(unsigned int *ppoints,
> >  		case OOM_SCAN_SELECT:
> >  			chosen = p;
> >  			chosen_points = ULONG_MAX;
> > -			/* fall through */
> > +			process_selected = true;
> > +			break;
> >  		case OOM_SCAN_CONTINUE:
> >  			continue;
> >  		case OOM_SCAN_ABORT:
> > @@ -324,6 +326,8 @@ static struct task_struct *select_bad_process(unsigned int *ppoints,
> >  		case OOM_SCAN_OK:
> >  			break;
> >  		};
> > +		if (process_selected)
> > +			break;
> >  		points = oom_badness(p, NULL, nodemask, totalpages);
> >  		if (!points || points < chosen_points)
> >  			continue;
> > -- 
> > 1.7.10.4
> > 
> 
> -- 
> Michal Hocko
> SUSE Labs

Been reviewing kernel code lately and looking for implementations not fulfilling their actual intention. That's about most of the patches I tend to send.
Motivation is pretty much derived from the Eudyptula challenge so there is not concrete reason for this patch.

To the point: I have not witnessed any major affects to performance due to this.
I fixed the patch and attached it to this mail.

[-- Attachment #2: 0001-break-after-selecting-process-to-kill.patch --]
[-- Type: text/x-diff, Size: 0 bytes --]