On Mon 19-04-21 18:44:02, 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.

Can you be more specific please?

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.

There is nothing like that. Even memory reserves are a finite resource
which can be consumed as it is sharing those reserves with other users
who are not necessarily coordinated. So before we start discussing
making this even more muddy by handing over memory reserves to the
userspace we should really examine whether pre-allocation is something
that will not work.

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.

I do not think that handing over an unlimited ticket to the memory
reserves to userspace is a good idea. Even the in kernel oom killer is
bound to a partial access to reserves. So if we really want this then
it should be in sync with and bound by the ALLOC_OOM.

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.

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.

Could you elaborate some more on how this would be controlled from the
userspace? A dedicated syscall? A driver?

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.

I would tend to agree.

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.

A kernel module or eBPF to implement oom decisions has already been
discussed few years back. But I am afraid this would be hard to wire in
for anything except for the victim selection. I am not sure it is
maintainable to also control when the OOM handling should trigger.

-- 
Michal Hocko
SUSE Labs