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

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

On Fri, Apr 18, 2025 at 10:07:29PM +0000, Roman Gushchin wrote:
> On Fri, Apr 18, 2025 at 01:30:03PM -0700, Shakeel Butt wrote:
> > 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.
> 
> +1
> 

Any reasons to prefer one over the other? To me having separate
files/interfaces seem more clean and are more script friendly. Also
let's see what others have to say or prefer.