On Mon, Nov 18, 2013 at 05:51:10PM +0100, Michal Hocko wrote:

On Mon 18-11-13 10:41:15, Johannes Weiner wrote:

quoted

On Thu, Nov 14, 2013 at 03:26:51PM -0800, David Rientjes wrote:

quoted

When current has a pending SIGKILL or is already in the exit path, it
only needs access to memory reserves to fully exit.  In that sense, the
memcg is not actually oom for current, it simply needs to bypass memory
charges to exit and free its memory, which is guarantee itself that
memory will be freed.

We only want to notify userspace for actionable oom conditions where
something needs to be done (and all oom handling can already be deferred
to userspace through this method by disabling the memcg oom killer with
memory.oom_control), not simply when a memcg has reached its limit, which
would actually have to happen before memcg reclaim actually frees memory
for charges.

Even though the situation may not require a kill, the user still wants
to know that the memory hard limit was breached and the isolation
broken in order to prevent a kill.  We just came really close and the

You can observe that you are getting into troubles from fail counter
already. The usability without more reclaim statistics is a bit
questionable but you get a rough impression that something is wrong at
least.

quoted

fact that current is exiting is coincidental.  Not everybody is having
OOM situations on a frequent basis and they might want to know when
they are redlining the system and that the same workload might blow up
the next time it's run.

I am just concerned that signaling temporal OOM conditions which do not
require any OOM killer action (user or kernel space) might be confusing.
Userspace would have harder times to tell whether any action is required
or not.

But userspace in all likeliness DOES need to take action.

Reclaim is a really long process.  If 5 times doing 12 priority cycles
and scanning thousands of pages is not enough to reclaim a single
page, what does that say about the health of the memcg?

But more importantly, OOM handling is just inherently racy.  A task
might receive the kill signal a split second *after* userspace was
notified.  Or a task may exit voluntarily a split second after a
victim was chosen and killed.

We have to draw a line somewhere, right now this is "reclaim failed".
This patch doesn't fix a problem, it just blurs that line and makes
OOM notifications less predictable.