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SubjectRe: [PATCH] OOM killer: wait for tasks with pending SIGKILL to exit
On Wed, 25 Sep 2013 13:31:32 -0700 (PDT)
David Rientjes <rientjes@google.com> wrote:

> On Wed, 11 Sep 2013, Sergey Dyasly wrote:
>
> > > > /*
> > > > * If this task is not being ptraced on exit, then wait for it
> > > > * to finish before killing some other task unnecessarily.
> > > > */
> > > > - if (!(task->group_leader->ptrace & PT_TRACE_EXIT))
> > > > + if (!(task->group_leader->ptrace & PT_TRACE_EXIT)) {
> > > > + set_tsk_thread_flag(task, TIF_MEMDIE);
> > >
> > > This does not, we do not give access to memory reserves unless the process
> > > needs it to allocate memory. The task here, which is not current, can
> > > call into the oom killer and be granted memory reserves if necessary.
> >
> > True. However, why TIF_MEMDIE is set for PF_EXITING task in oom_kill_process()
> > then?
>
> If current needs access to memory reserves while PF_EXITING, it should
> call the page allocator, find that it is out of memory, and call the oom
> killer to silently be granted memory reserves.

I understand this and you are repeating yourself :)
What you are saying contradicts current OOMk code the way I read it. Comment in
oom_kill_process() says:

"If the task is already exiting ... set TIF_MEMDIE so it can die quickly"

I just want to know the right solution.

> > > > @@ -412,16 +415,6 @@ void oom_kill_process(struct task_struct *p, gfp_t gfp_mask, int order,
> > > > static DEFINE_RATELIMIT_STATE(oom_rs, DEFAULT_RATELIMIT_INTERVAL,
> > > > DEFAULT_RATELIMIT_BURST);
> > > >
> > > > - /*
> > > > - * If the task is already exiting, don't alarm the sysadmin or kill
> > > > - * its children or threads, just set TIF_MEMDIE so it can die quickly
> > > > - */
> > > > - if (p->flags & PF_EXITING) {
> > > > - set_tsk_thread_flag(p, TIF_MEMDIE);
> > > > - put_task_struct(p);
> > > > - return;
> > > > - }
> > >
> > > I think you misunderstood the point of this; if a selected process is
> > > already in the exit path then this is simply avoiding dumping oom kill
> > > lines to the kernel log. We want to keep doing that.
> >
> > This happens in oom_kill_process() after victim has been selected by
> > select_bad_process(). But there is already PF_EXITING check in
> > oom_scan_process_thread() and in this case OOM code won't call oom_kill_process.
>
> select_bad_process() is one of three callers to oom_kill_process().

You are mistaken, oom_kill_process() is only called from out_of_memory()
and mem_cgroup_out_of_memory().

> > The only difference is in force_kill flag, and the only case where it's set
> > is SysRq. And I think in this case OOM killer messages are a good thing to have
> > even when victim is already exiting, instead of just silence.
> >
>
> Read the comment about why we don't emit anything to the kernel log in
> this case; the process is already exiting, there's no need to kill it or
> make anyone believe that it was killed.

Yes, but there is already the PF_EXITING check in oom_scan_process_thread(),
and in this case oom_kill_process() won't be even called. That's why it's
redundant.

--
Sergey Dyasly <dserrg@gmail.com>


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