On Mon 04-05-20 06:54:40, Shakeel Butt wrote:

On Sun, May 3, 2020 at 11:56 PM Michal Hocko [off-list ref] wrote:

quoted

On Thu 30-04-20 11:27:12, Shakeel Butt wrote:

quoted

Lowering memory.max can trigger an oom-kill if the reclaim does not
succeed. However if oom-killer does not find a process for killing, it
dumps a lot of warnings.

It shouldn't dump much more than the regular OOM report AFAICS. Sure
there is "Out of memory and no killable processes..." message printed as
well but is that a real problem?

quoted

Deleting a memcg does not reclaim memory from it and the memory can
linger till there is a memory pressure. One normal way to proactively
reclaim such memory is to set memory.max to 0 just before deleting the
memcg. However if some of the memcg's memory is pinned by others, this
operation can trigger an oom-kill without any process and thus can log a
lot un-needed warnings. So, ignore all such warnings from memory.max.

OK, I can see why you might want to use memory.max for that purpose but
I do not really understand why the oom report is a problem here.

It may not be a problem for an individual or small scale deployment
but when "sweep before tear down" is the part of the workflow for
thousands of machines cycling through hundreds of thousands of cgroups
then we can potentially flood the logs with not useful dumps and may
hide (or overflow) any useful information in the logs.

If you are doing this in a large scale and the oom report is really a
problem then you shouldn't be resetting hard limit to 0 in the first
place.

quoted

memory.max can trigger the oom kill and user should be expecting the oom
report under that condition. Why is "no eligible task" so special? Is it
because you know that there won't be any tasks for your particular case?
What about other use cases where memory.max is not used as a "sweep
before tear down"?

What other such use-cases would be? The only use-case I can envision
of adjusting limits dynamically of a live cgroup are resource
managers. However for cgroup v2, memory.high is the recommended way to
limit the usage, so, why would resource managers be changing
memory.max instead of memory.high? I am not sure. What do you think?

There are different reasons to use the hard limit. Mostly to contain
potential runaways. While high limit might be a sufficient measure to
achieve that as well the hard limit is the last resort. And it clearly
has the oom killer semantic so I am not really sure why you are
comparing the two.

FB is moving away from limits setting, so, not sure if they have
thought of these cases.

BTW for such use-cases, shouldn't we be taking the memcg's oom_lock?

This is a good question. I would have to go and double check the code
but I suspect that this is an omission.
-- 
Michal Hocko
SUSE Labs