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SubjectRe: [patch -mm v2] mm: introduce oom_adj_child
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
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

- /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.

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).

> 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.

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