Re: cgroup/test_zswap failed with "zswpout does not increase after test program"

On Fri, Jul 10, 2026 at 10:30 AM Nhat Pham <[email protected]> wrote:
>
> On Wed, Jul 8, 2026 at 12:08 PM Yosry Ahmed <[email protected]> wrote:
> >
> > On Wed, Jul 8, 2026 at 12:00 PM Michal Koutný <[email protected]> wrote:
> > >
> > > On Tue, Jul 07, 2026 at 02:49:56PM -0700, Yosry Ahmed <[email protected]> wrote:
> > > > I would honestly rather use more memory. I think there might be cases
> > > > where the flusher is delayed. The flush being slightly delayed is not
> > > > technically a bug that we want to see a failure for, but if a large
> > > > stats change is not visible that's a user-noticeable behavior that we
> > > > want a failure for.
> > > >
> > > > WDYT?
> > >
> > > There's already the (recent) page size-based scaling, so the idea with
> > > nr_cpus scaling could make the selftest useful on wider range of setups
> > > (even page size can be considered as a slight implementation detail
> > > leak, thus the justification of nr_cpus dependency).
> > >
> > > Also, I still think that internally the threshold should be changed to
> > > the "harmonic" formula [1] but the selftest can go with the linear
> > > dependency for more pronounced effects.
> >
> > Yeah I agree the threshold formula can be improved, but we need to
> > make sure performance doesn't regress.
>
> Yeah that's a separate discussion indeed. For now I'm fine with fixing
> the selftests by using more memory.
>
> Is there a good generic formula for this? I understand there will be
> hardcode values, but at least making it scalable (by how much memory
> we have, page size, # of cpus) if possible? :)

The rstat formula depends on the number of CPUs and a constant factor
(see memcg_vmstats_needs_flush()), but this formula is for the number
of updates (e.g. number of pages reclaimed), and that value will
decrease as the page size increases. So I think we want to scale with
both the page size and the number of CPUs. The amount of memory in the
system is not a factor for now.