Re: [PATCH v16 3/3] mm: Reduce latency of OOM killer task selection with 2-pass algorithm

[Michal Hocko <mhocko@suse.com>]

On Mon 26-01-26 11:39:33, Mathieu Desnoyers wrote:
> On 2026-01-16 16:55, Michal Hocko wrote:
> > On Wed 14-01-26 14:36:44, Mathieu Desnoyers wrote:
> > > On 2026-01-14 12:06, Michal Hocko wrote:
> > > > On Wed 14-01-26 09:59:15, Mathieu Desnoyers wrote:
> > [...]
> > Thanks to those clarifications
> > > > My overall impression is that the implementation is really involved and
> > > > at this moment I do not really see a big benefit of all the complexity.
> > > 
> > > Note that we can get the proc ABI RSS accuracy improvements with the
> > > previous 2 patches without this 2-pass algo. Do you see more value in
> > > the RSS accuracy improvements than in the oom killer latency reduction ?
> > 
> > Yes, TBH I do not see oom latency as a big problem. As already mention
> > this is a slow path and we are not talking about a huge latency anyway.
> > proc numbers are much more sensitive to latency as they are regularly
> > read by user space tools and accuracy for those matters as well (being
> > off by 100s MB or GBs is simply making those numbers completely bogus).
> 
> It makes sense.
> 
> > > > It would help to explicitly mention what is the the overall imprecision
> > > > of the oom victim selection with the new data structure (maybe this is
> > > > good enough[*]). What if we go with exact precision with the new data
> > > > structure comparing to the original pcp counters.
> > > 
> > > Do you mean comparing using approximate sums with the new data
> > > structure (which has a bounded accuracy of O(nr_cpus*log(nr_cpus)))
> > > compared to the old data structure which had an inaccuracy of
> > > O(nr_cpus^2) ? So if the inaccuracy provided by the new data structure
> > > is good enough for OOM task selection, we could go from precise sum
> > > back to an approximation and just use that with the new data
> > > structure.
> > 
> > Exactly!
> OK, so based on your feedback, I plan to remove this 2-pass algo
> from the series, and simply keep using the precise sum for the OOM
> killer. If people complain about its latency, then we can eventually
> use the approximation provided by the hierarchical counters. But let's
> wait until someone asks for it rather than add this complexity when
> there is no need.
> 
> The hierarchical counters are still useful as they increase the
> accuracy of approximations exported through /proc.
> 
> How does that sound ?

Works for me.

Thanks!
-- 
Michal Hocko
SUSE Labs