Re: [PATCH] memcg: introduce non-blocking limit setting interfaces

[Shakeel Butt <shakeel.butt@linux.dev>]

On Fri, Apr 18, 2025 at 01:18:53PM -0700, Greg Thelen wrote:
> On Fri, Apr 18, 2025 at 1:00 PM Shakeel Butt <shakeel.butt@linux.dev> wrote:
> >
> > Setting the max and high limits can trigger synchronous reclaim and/or
> > oom-kill if the usage is higher than the given limit. This behavior is
> > fine for newly created cgroups but it can cause issues for the node
> > controller while setting limits for existing cgroups.
> >
> > In our production multi-tenant and overcommitted environment, we are
> > seeing priority inversion when the node controller dynamically adjusts
> > the limits of running jobs of different priorities. Based on the system
> > situation, the node controller may reduce the limits of lower priority
> > jobs and increase the limits of higher priority jobs. However we are
> > seeing node controller getting stuck for long period of time while
> > reclaiming from lower priority jobs while setting their limits and also
> > spends a lot of its own CPU.
> >
> > One of the workaround we are trying is to fork a new process which sets
> > the limit of the lower priority job along with setting an alarm to get
> > itself killed if it get stuck in the reclaim for lower priority job.
> > However we are finding it very unreliable and costly. Either we need a
> > good enough time buffer for the alarm to be delivered after setting
> > limit and potentialy spend a lot of CPU in the reclaim or be unreliable
> > in setting the limit for much shorter but cheaper (less reclaim) alarms.
> >
> > Let's introduce new limit setting interfaces which does not trigger
> > reclaim and/or oom-kill and let the processes in the target cgroup to
> > trigger reclaim and/or throttling and/or oom-kill in their next charge
> > request. This will make the node controller on multi-tenant
> > overcommitted environment much more reliable.
> 
> Would opening the typical synchronous files (e.g. memory.max) with
> O_NONBLOCK be a more general way to tell the kernel that the user
> space controller doesn't want to wait? It's not quite consistent with
> traditional use of O_NONBLOCK, which would make operations to
> fully succeed or fail, rather than altering the operation being requested.
> But O_NONBLOCK would allow for a semantics of non-blocking
> reclaim, if that's fast enough for your controller.
> 

We actually thought about O_NONBLOCK but the challenge with that is how
would the node controller knows if the underlying kernel has O_NONBLOCK
implying no-reclaim/no-oom-kill feature. I don't think opening
memory.max with O_NONBLOCK will fail today, so the node controller would
still need to implement the complicated fork+set-limit+alarm logic
until the whole fleet has moved away from older kernel. Also I have
checked with systemd folks and they are not happy to implement that
complicated fork+set-limit+alarm logic.