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SubjectRe: [PATCH] OOM killer: wait for tasks with pending SIGKILL to exit
On Mon, 9 Sep 2013 13:07:08 -0700 (PDT)
David Rientjes <rientjes@google.com> 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?
Also, setting TIF_MEMDIE will avoid direct reclaim and memory allocation should
be fast if exiting task needs it.

> > @@ -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.
There is only a slight chance that victim will become PF_EXITING between
scan and kill.

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.

--
Sergey Dyasly <dserrg@gmail.com>


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