Re: [RFC] memory reserve for userspace oom-killer

[Michal Hocko]

On Wed 21-04-21 06:57:43, Shakeel Butt wrote:
> On Wed, Apr 21, 2021 at 12:16 AM Michal Hocko <mhocko@xxxxxxxx> wrote:
> >
> [...]
> > > To decide when to kill, the oom-killer has to read a lot of metrics.
> > > It has to open a lot of files to read them and there will definitely
> > > be new allocations involved in those operations. For example reading
> > > memory.stat does a page size allocation. Similarly, to perform action
> > > the oom-killer may have to read cgroup.procs file which again has
> > > allocation inside it.
> >
> > True but many of those can be avoided by opening the file early. At
> > least seq_file based ones will not allocate later if the output size
> > doesn't increase. Which should be the case for many. I think it is a
> > general improvement to push those who allocate during read to an open
> > time allocation.
> >
> 
> I agree that this would be a general improvement but it is not always
> possible (see below).

It would be still great to invest into those improvements. And I would
be really grateful to learn about bottlenecks from the existing kernel
interfaces you have found on the way.

> > > Regarding sophisticated oom policy, I can give one example of our
> > > cluster level policy. For robustness, many user facing jobs run a lot
> > > of instances in a cluster to handle failures. Such jobs are tolerant
> > > to some amount of failures but they still have requirements to not let
> > > the number of running instances below some threshold. Normally killing
> > > such jobs is fine but we do want to make sure that we do not violate
> > > their cluster level agreement. So, the userspace oom-killer may
> > > dynamically need to confirm if such a job can be killed.
> >
> > What kind of data do you need to examine to make those decisions?
> >
> 
> Most of the time the cluster level scheduler pushes the information to
> the node controller which transfers that information to the
> oom-killer. However based on the freshness of the information the
> oom-killer might request to pull the latest information (IPC and RPC).

I cannot imagine any OOM handler to be reliable if it has to depend on
other userspace component with a lower resource priority. OOM handlers
are fundamentally complex components which has to reduce their
dependencies to the bare minimum.
 
> [...]
> > >
> > > I was thinking of simply prctl(SET_MEMPOOL, bytes) to assign mempool
> > > to a thread (not shared between threads) and prctl(RESET_MEMPOOL) to
> > > free the mempool.
> >
> > I am not a great fan of prctl. It has become a dumping ground for all
> > mix of unrelated functionality. But let's say this is a minor detail at
> > this stage.
> 
> I agree this does not have to be prctl().
> 
> > So you are proposing to have a per mm mem pool that would be
> 
> I was thinking of per-task_struct instead of per-mm_struct just for simplicity.
> 
> > used as a fallback for an allocation which cannot make a forward
> > progress, right?
> 
> Correct
> 
> > Would that pool be preallocated and sitting idle?
> 
> Correct
> 
> > What kind of allocations would be allowed to use the pool?
> 
> I was thinking of any type of allocation from the oom-killer (or
> specific threads). Please note that the mempool is the backup and only
> used in the slowpath.
> 
> > What if the pool is depleted?
> 
> This would mean that either the estimate of mempool size is bad or
> oom-killer is buggy and leaking memory.
> 
> I am open to any design directions for mempool or some other way where
> we can provide a notion of memory guarantee to oom-killer.

OK, thanks for clarification. There will certainly be hard problems to
sort out[1] but the overall idea makes sense to me and it sounds like a
much better approach than a OOM specific solution.


[1] - how the pool is going to be replenished without hitting all
potential reclaim problems (thus dependencies on other all tasks
directly/indirectly) yet to not rely on any background workers to do
that on the task behalf without a proper accounting etc...
-- 
Michal Hocko
SUSE Labs