Re: [PATCH v13 2/3] mm: Fix OOM killer inaccuracy on large many-core systems

[Michal Hocko <mhocko@suse.com>]

On Mon 12-01-26 19:47:54, Mathieu Desnoyers wrote:
> On 2026-01-12 14:48, Michal Hocko wrote:
> > On Mon 12-01-26 14:37:49, Mathieu Desnoyers wrote:
> > > On 2026-01-12 03:42, Michal Hocko wrote:
> > > > Hi,
> > > > sorry to jump in this late but the timing of previous versions didn't
> > > > really work well for me.
> > > > 
> > > > On Sun 11-01-26 14:49:57, Mathieu Desnoyers wrote:
> > > > [...]
> > > > > Here is a (possibly incomplete) list of the prior approaches that were
> > > > > used or proposed, along with their downside:
> > > > > 
> > > > > 1) Per-thread rss tracking: large error on many-thread processes.
> > > > > 
> > > > > 2) Per-CPU counters: up to 12% slower for short-lived processes and 9%
> > > > >      increased system time in make test workloads [1]. Moreover, the
> > > > >      inaccuracy increases with O(n^2) with the number of CPUs.
> > > > > 
> > > > > 3) Per-NUMA-node counters: requires atomics on fast-path (overhead),
> > > > >      error is high with systems that have lots of NUMA nodes (32 times
> > > > >      the number of NUMA nodes).
> > > > > 
> > > > > The approach proposed here is to replace this by the hierarchical
> > > > > per-cpu counters, which bounds the inaccuracy based on the system
> > > > > topology with O(N*logN).
> > > > 
> > > > The concept of hierarchical pcp counter is interesting and I am
> > > > definitely not opposed if there are more users that would benefit.
> > > > 
> > > >   From the OOM POV, IIUC the primary problem is that get_mm_counter
> > > > (percpu_counter_read_positive) is too imprecise on systems when the task
> > > > is moving around a large number of cpus. In the list of alternative
> > > > solutions I do not see percpu_counter_sum_positive to be mentioned.
> > > > oom_badness() is a really slow path and taking the slow path to
> > > > calculate a much more precise value seems acceptable. Have you
> > > > considered that option?
> > > I must admit I assumed that since there was already a mechanism in place
> > > to ensure it's not necessary to sum per-cpu counters when the oom killer
> > > is trying to select tasks, it must be because this
> > > 
> > >    O(nr_possible_cpus * nr_processes)
> > > 
> > > operation must be too slow for the oom killer requirements.
> > > 
> > > AFAIU, the oom killer is executed when the memory allocator fails to
> > > allocate memory, which can be within code paths which need to progress
> > > eventually. So even though it's a slow path compared to the allocator
> > > fast path, there must be at least _some_ expectations about it
> > > completing within a decent amount of time. What would that ballpark be ?
> > 
> > I do not think we have ever promissed more than the oom killer will try
> > to unlock the system blocked on memory shortage.
> > 
> > > To give an order of magnitude, I've tried modifying the upstream
> > > oom killer to use percpu_counter_sum_positive and compared it to
> > > the hierarchical approach:
> > > 
> > > AMD EPYC 9654 96-Core (2 sockets)
> > > Within a KVM, configured with 256 logical cpus.
> > > 
> > >                     nr_processes=40    nr_processes=10000
> > > Counter sum:            0.4 ms             81.0 ms
> > > HPCC with 2-pass:       0.3 ms              9.3 ms
> > 
> > These are peanuts for the global oom situations. We have had situations
> > when soft lockup detector triggered because of the process tree
> > traversal so adding 100ms is not really critical.
> > 
> > > So as we scale up the number of processes on large SMP systems,
> > > the latency caused by the oom killer task selection greatly
> > > increases with the counter sums compared with the hierarchical
> > > approach.
> > 
> > Yes, I am not really questioning the hierarchical approach will perform
> > much better but I am thinking of a good enough solution and calculating
> > the number might be just that stop gap solution (that would be also
> > suitable for stable tree backports). I am not ruling out improving on
> > top of that by a more clever solution like your hierarchical counters
> > approach. Especially if there are more benefits from that elsewhere.
> > 
> 
> Would you be OK with introducing changes in the following order ?
> 
> 1) Fix the OOM killer inaccuracy by using counter sum (iteration on all
>    cpu counters) in task selection. This may slow down the oom killer,
>    but would at least fix its current inaccuracy issues. This could be
>    backported to stable kernels.
> 
> 2) Introduce the hierarchical percpu counters on top, as a oom killer
>    task selection performance optimization (reduce latency of oom kill).
> 
> This way, (2) becomes purely a performance optimization, so it's easy
> to bissect and revert if it causes issues.

Yes, this makes more sense.

> I agree that bringing a fix along with a performance optimization within
> a single commit makes it hard to backport to stable, and tricky to
> revert if it causes problems.
> 
> As for finding other users of the hpcc, I have ideas, but not so much
> time available to try them out, as I'm pretty much doing this in my
> spare time.

I do understand this constrain and motivation to have OOM situation
addressed with a priority. I am pretty sure that if you see issues in
OOM path then other consumers of get_mm_counter would be affected as
well. Namely /proc/<pid>/stat. There might be others but I can imagine
that some of them are more performance than precision sensitive.
All that being said it seems that we need slow-and-precise and
fast-approximate interfaces to have incremental path for other users as
well. Looking at patch 1 it seems there are interfaces available for
that. I think it would be great to call those out explicitly in the
highlevel doc to give some guidance what to use when with what kind of
expectations.

Thanks!
-- 
Michal Hocko
SUSE Labs