On Mon, Apr 19, 2021 at 06:44:02PM -0700, Shakeel Butt wrote:

Proposal: Provide memory guarantees to userspace oom-killer.

Background:

Issues with kernel oom-killer:
1. Very conservative and prefer to reclaim. Applications can suffer
for a long time.
2. Borrows the context of the allocator which can be resource limited
(low sched priority or limited CPU quota).
3. Serialized by global lock.
4. Very simplistic oom victim selection policy.

These issues are resolved through userspace oom-killer by:
1. Ability to monitor arbitrary metrics (PSI, vmstat, memcg stats) to
early detect suffering.
2. Independent process context which can be given dedicated CPU quota
and high scheduling priority.
3. Can be more aggressive as required.
4. Can implement sophisticated business logic/policies.

Android's LMKD and Facebook's oomd are the prime examples of userspace
oom-killers. One of the biggest challenges for userspace oom-killers
is to potentially function under intense memory pressure and are prone
to getting stuck in memory reclaim themselves. Current userspace
oom-killers aim to avoid this situation by preallocating user memory
and protecting themselves from global reclaim by either mlocking or
memory.min. However a new allocation from userspace oom-killer can
still get stuck in the reclaim and policy rich oom-killer do trigger
new allocations through syscalls or even heap.

Our attempt of userspace oom-killer faces similar challenges.
Particularly at the tail on the very highly utilized machines we have
observed userspace oom-killer spectacularly failing in many possible
ways in the direct reclaim. We have seen oom-killer stuck in direct
reclaim throttling, stuck in reclaim and allocations from interrupts
keep stealing reclaimed memory. We have even observed systems where
all the processes were stuck in throttle_direct_reclaim() and only
kswapd was running and the interrupts kept stealing the memory
reclaimed by kswapd.

To reliably solve this problem, we need to give guaranteed memory to
the userspace oom-killer. At the moment we are contemplating between
the following options and I would like to get some feedback.

1. prctl(PF_MEMALLOC)

The idea is to give userspace oom-killer (just one thread which is
finding the appropriate victims and will be sending SIGKILLs) access
to MEMALLOC reserves. Most of the time the preallocation, mlock and
memory.min will be good enough but for rare occasions, when the
userspace oom-killer needs to allocate, the PF_MEMALLOC flag will
protect it from reclaim and let the allocation dip into the memory
reserves.

The misuse of this feature would be risky but it can be limited to
privileged applications. Userspace oom-killer is the only appropriate
user of this feature. This option is simple to implement.

Hello Shakeel!

If ordinary PAGE_SIZE and smaller kernel allocations start to fail,
the system is already in a relatively bad shape. Arguably the userspace
OOM killer should kick in earlier, it's already a bit too late.
Allowing to use reserves just pushes this even further, so we're risking
the kernel stability for no good reason.

But I agree that throttling the oom daemon in direct reclaim makes no sense.
I wonder if we can introduce a per-task flag which will exclude the task from
throttling, but instead all (large) allocations will just fail under a
significant memory pressure more easily. In this case if there is a significant
memory shortage the oom daemon will not be fully functional (will get -ENOMEM
for an attempt to read some stats, for example), but still will be able to kill
some processes and make the forward progress.
But maybe it can be done in userspace too: by splitting the daemon into
a core- and extended part and avoid doing anything behind bare minimum
in the core part.

2. Mempool

The idea is to preallocate mempool with a given amount of memory for
userspace oom-killer. Preferably this will be per-thread and
oom-killer can preallocate mempool for its specific threads. The core
page allocator can check before going to the reclaim path if the task
has private access to the mempool and return page from it if yes.

This option would be more complicated than the previous option as the
lifecycle of the page from the mempool would be more sophisticated.
Additionally the current mempool does not handle higher order pages
and we might need to extend it to allow such allocations. Though this
feature might have more use-cases and it would be less risky than the
previous option.

It looks like an over-kill for the oom daemon protection, but if there
are other good use cases, maybe it's a good feature to have.

Another idea I had was to use kthread based oom-killer and provide the
policies through eBPF program. Though I am not sure how to make it
monitor arbitrary metrics and if that can be done without any
allocations.

To start this effort it would be nice to understand what metrics various
oom daemons use and how easy is to gather them from the bpf side. I like
this idea long-term, but not sure if it has been settled down enough.
I imagine it will require a fair amount of work on the bpf side, so we
need a good understanding of features we need.

Thanks!