On Wed 17-06-15 21:31:21, Tetsuo Handa wrote:
> Michal Hocko wrote:
[...]
> > I think we can rely on timers. A downside would be that we cannot dump
> > the full OOM report from the IRQ context because we rely on task_lock
> > which is not IRQ safe. But I do not think we really need it. An OOM
> > report will be in the log already most of the time and show_mem will
> > tell us the current memory situation.
> >
> > What do you think?
>
> We can rely on timers, but we can't rely on global timer.
Why not?
>
> > + if (sysctl_panic_on_oom_timeout) {
> > + if (sysctl_panic_on_oom > 1) {
> > + pr_warn("panic_on_oom_timeout is ignored for panic_on_oom=2\n");
> > + } else {
> > + /*
> > + * Only schedule the delayed panic_on_oom when this is
> > + * the first OOM triggered. oom_lock will protect us
> > + * from races
> > + */
> > + if (atomic_read(&oom_victims))
> > + return;
> > +
> > + mod_timer(&panic_on_oom_timer,
> > + jiffies + (sysctl_panic_on_oom_timeout * HZ));
> > + return;
> > + }
> > + }
>
> Since this version uses global panic_on_oom_timer, you cannot handle
> OOM race like below.
>
> (1) p1 in memcg1 calls out_of_memory().
> (2) 5 seconds of timeout is started by p1.
> (3) p1 takes 3 seconds for some reason.
> (4) p2 in memcg2 calls out_of_memory().
> (5) p1 calls unmark_oom_victim() but timer continues.
> (6) p2 takes 2 seconds for some reason.
> (7) 5 seconds of timeout expires despite individual delay was less than
> 5 seconds.
Yes it is not intended to handle such a race. Timeout is completely
ignored for panic_on_oom=2 and contrained oom context doesn't trigger
this path for panic_on_oom=1.
But you have a point that we could have
- constrained OOM which elevates oom_victims
- global OOM killer strikes but wouldn't start the timer
This is certainly possible and timer_pending(&panic_on_oom) replacing
oom_victims check should help here. I will think about this some more.
But this sounds like a minor detail.
The important thing is to decide what is the reasonable way forward. We
have two two implementations of panic based timeout. So we should decide
- Should be the timeout bound to panic_on_oom?
- Should we care about constrained OOM contexts?
- If yes should they use the same timeout?
- If yes should each memcg be able to define its own timeout?
My thinking is that it should be bound to panic_on_oom=1 only until we
hear from somebody actually asking for a constrained oom and even then
do not allow for too large configuration space (e.g. no per-memcg
timeout) or have separate mempolicy vs. memcg timeouts.
Let's start simple and make things more complicated later!
--
Michal Hocko
SUSE Labs
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