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SubjectRe: [PATCH] copy over oom_adj value at fork time
On Fri, 17 Jul 2009, Paul Menage wrote:

> > The only way to workaround that is by using the highest oom_adj user for
> > the mm_struct from the array in reporting /proc/pid/oom_score, as well.
>
> That sounds fine to me.
>

It's better than what we did before my patchset, which would be report an
erroneous oom_score if another thread attached to the same mm_struct had a
less immune (more positive) oom_adj score.

With my patches in 2.6.31-rc3, there is no ambiguity and /proc/pid/oom_adj
and /proc/pid/oom_score are consistent for all threads. Because of that,
they now accurately reflect in all circumstances how the oom killer
chooses a victim, which should be very important to users who are tuning
oom_adj values in the first place.

> > But that would lead to /proc/pid/oom_adj not affecting oom_score at all,
> > which isn't consistent.
>
> Isn't consistent with what? It's perfectly consistent with saying "the
> oom_score of a task is based on the highest oom_adj value of any task
> sharing the same mm". Admittedly it's not 100% consistent with the old
> semantics, but I'm having trouble imagining a scenario where someone
> was relying on the changed semantics.
>

I'm sure that most users who tune /proc/pid/oom_adj for a group of tasks,
especially when tuning them in terms of killing priority in oom conditions
as opposed to simply using OOM_DISABLE, rely on /proc/pid/oom_score to
determine when a task should be killed because it is using an egregious
amount of memory. That's why oom_score is exported to userspace;
otherwise, tuning oom_adj for anything other than OOM_DISABLE would be
difficult.

> But taking a completely different approach, is there a reason that we
> couldn't have just moved the do_each_thread() check for OOM_DISABLED
> out of oom_kill_task() and into select_bad_process() at the point
> where we've decided that the thread in question is a better victim
> than the current victim? That would fix the OOM livelock while
> allowing us to keep exactly the same oom_adj/oom_score semantics as in
> previous kernels.
>

My patches don't merely address threads that have an oom_adj value of
OOM_DISABLE while others sharing the same memory do not, they address a
consistency issue whereas those threads may all share memory but have
radically different oom_adj values: that means that only the highest
oom_adj value amongst them is actually used by the oom killer and all
other oom_adj values set by the user for those threads are wrong.

> > The inheritance issue should be fixed with Rik's patch with the exception
> > of vfork -> change /proc/pid-of-child/oom_adj -> execve.  If scripts were
> > written to do that with the old behavior, they'll have to adjust to change
> > oom_adj _after_ the execve
>
> Think about what you're suggesting here - execve() replaces your code
> with whatever you're execing, so unless that code is also written to
> handle oom_adj (which for something like a generic job scheduler, the
> exec'd code is unlikely to do) you're stuck.
>

The burden to adjust /proc/pid/oom_adj has always been on the admin, the
only thing the kernel needs to provide is an inheritance property (as
added with Rik's patch) to have any consistency with a priority based
killing approach, i.e. forked children should have the same oom_adj value
to begin with to not be more immune or a higher priority to kill than the
parent unless explicitly changed by the admin.
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