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SubjectRe: [patch -mm v2] mm: introduce oom_adj_child
On Sat, 1 Aug 2009 13:26:52 -0700 (PDT)
David Rientjes <rientjes@google.com> wrote:

> On Sat, 1 Aug 2009, KAMEZAWA Hiroyuki wrote:
>
> > Summarizing I think now .....
> > - rename mm->oom_adj as mm->effective_oom_adj
> > - re-add per-thread oom_adj
> > - update mm->effective_oom_adj based on per-thread oom_adj
> > - if necessary, plz add read-only /proc/pid/effective_oom_adj file.
> > or show 2 values in /proc/pid/oom_adj
> > - rewrite documentation about oom_score.
> > " it's calclulated from _process's_ memory usage and oom_adj of
> > all threads which shares a memor context".
> > This behavior is not changed from old implemtation, anyway.
> > - If necessary, rewrite oom_kill itself to scan only thread group
> > leader. It's a way to go regardless of vfork problem.
> >
>
> Ok, so you've abandoned the signal_struct proposal and now want to add it
per-signal is also ok, just I didn't write.

> back to task_struct with an effective member in mm_struct by changing the
> documentation. Hmm.
>
> This solves the livelock problem by adding additional tunables, but
> doesn't match how the documentation describes the use case for
> /proc/pid/oom_adj. Your argument is that the behavior of that value can't
> change: that it must be per-thread. And that allowance leads to one of
> two inconsistent scenarios:
>
> - /proc/pid/oom_score is inconsistent when tuning /proc/pid/oom_adj if it
> relies on the per-thread oom_adj; it now really represents nothing but
> an incorrect value if other threads share that memory and misleads the
> user on how the oom killer chooses victims, or

What's why I said to show effective_oom_adj if necessary..

>
> - /proc/pid/oom_score is inconsistent when the thread that set the
> effective per-mm oom_adj exits and it is now obsolete since you have
> no way to determine what the next effective oom_adj value shall be.
>
plz re-caluculate it. it's not a big job if done in lazy way.


> Determining the next effective per-mm oom_adj isn't possible when the only
> threads sharing the mm remaining have different per-thread oom_adj values.
> That's a horribly inconsistent state to be getting into because it allows
> oom_score to change when a thread exits, which is completely unknown to
> userspace, OR is allows the effective per-mm oom_adj to be different from
> all threads sharing the same memory (and, thus, /proc/pid/oom_score not
> being representative of any thread's /proc/pid/oom_adj).
>
A _sane_ user will just set oom_adj to thread-group-leader.
Do you think users are too fool to set per-thread oom_adj independently ?
No problems in real world.


> > I think documentation is wrong. It should say "you should think of
> > multi-thread effect to oom_adj/oom_score".
> >
>
> It's more likely than not that applications were probably written to the
> way the documentation described the two files: that is, adjust
> /proc/pid/oom_score by tuning /proc/pid/oom_adj instead of relying on an
> undocumented implementation detail concerning the tuning of oom_adj for a
> vfork'd child prior to exec(). The user is probably unaware of the oom
> killer's implementation and simply interprets a higher oom_score as a more
> likely candidate for oom kill. My patches preserve that in all scenarios
> without altering the documentation or adding additional files that would
> be required to leave the oom_adj value itself in an inconsistent state as
> you propose.
>
No. My understanding is this.

- oom_adj is designed considering vfork(), of course. then. per-thread.
- oom_score has been incorrect in multi-threaded system. The user will not
be affected.
- you fixed livelock but breaks the feature.


Thanks,
-Kame







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