Re: [RFC PATCH V1] mm: Disable demotion from proactive reclaim

[Mina Almasry]

On Tue, Nov 29, 2022 at 7:56 PM Huang, Ying <ying.huang@xxxxxxxxx> wrote:
>
> Johannes Weiner <hannes@xxxxxxxxxxx> writes:
>
> > Hello Ying,
> >
> > On Thu, Nov 24, 2022 at 01:51:20PM +0800, Huang, Ying wrote:
> >> Johannes Weiner <hannes@xxxxxxxxxxx> writes:
> >> > The fallback to reclaim actually strikes me as wrong.
> >> >
> >> > Think of reclaim as 'demoting' the pages to the storage tier. If we
> >> > have a RAM -> CXL -> storage hierarchy, we should demote from RAM to
> >> > CXL and from CXL to storage. If we reclaim a page from RAM, it means
> >> > we 'demote' it directly from RAM to storage, bypassing potentially a
> >> > huge amount of pages colder than it in CXL. That doesn't seem right.
> >> >
> >> > If demotion fails, IMO it shouldn't satisfy the reclaim request by
> >> > breaking the layering. Rather it should deflect that pressure to the
> >> > lower layers to make room. This makes sure we maintain an aging
> >> > pipeline that honors the memory tier hierarchy.
> >>
> >> Yes.  I think that we should avoid to fall back to reclaim as much as
> >> possible too.  Now, when we allocate memory for demotion
> >> (alloc_demote_page()), __GFP_KSWAPD_RECLAIM is used.  So, we will trigger
> >> kswapd reclaim on lower tier node to free some memory to avoid fall back
> >> to reclaim on current (higher tier) node.  This may be not good enough,
> >> for example, the following patch from Hasan may help via waking up
> >> kswapd earlier.
> >>
> >> https://lore.kernel.org/linux-mm/b45b9bf7cd3e21bca61d82dcd1eb692cd32c122c.1637778851.git.hasanalmaruf@xxxxxx/
> >>
> >> Do you know what is the next step plan for this patch?
> >>
> >> Should we do even more?
> >>
> >> From another point of view, I still think that we can use falling back
> >> to reclaim as the last resort to avoid OOM in some special situations,
> >> for example, most pages in the lowest tier node are mlock() or too hot
> >> to be reclaimed.
> >
> > If they're hotter than reclaim candidates on the toptier, shouldn't
> > they get promoted instead and make room that way? We may have to tweak
> > the watermark logic a bit to facilitate that (allow promotions where
> > regular allocations already fail?). But this sort of resorting would
> > be preferable to age inversions.
>
> Now it's legal to enable demotion and disable promotion.  Yes, this is
> wrong configuration in general.  But should we trigger OOM for these
> users?
>
> And now promotion only works for default NUMA policy (and MPOL_BIND to
> both promotion source and target nodes with MPOL_F_NUMA_BALANCING).  If
> we use some other NUMA policy, the pages cannot be promoted too.
>
> > The mlock scenario sounds possible. In that case, it wouldn't be an
> > aging inversion, since there is nothing colder on the CXL node.
> >
> > Maybe a bypass check should explicitly consult the demotion target
> > watermarks against its evictable pages (similar to the file_is_tiny
> > check in prepare_scan_count)?
>
> Yes.  This sounds doable.
>
> > Because in any other scenario, if there is a bug in the promo/demo
> > coordination, I think we'd rather have the OOM than deal with age
> > inversions causing intermittent performance issues that are incredibly
> > hard to track down.
>
> Previously, I thought that people will always prefer performance
> regression than OOM.  Apparently, I am wrong.
>
> Anyway, I think that we need to reduce the possibility of OOM or falling
> back to reclaim as much as possible firstly.  Do you agree?
>

I've been discussing this with a few folks here. I think FWIW general
feeling here is that demoting from top tier nodes is preferred, except
in extreme circumstances we would indeed like to run with a
performance issue rather than OOM a customer VM. I wonder if there is
another way to debug mis-tiered pages rather than trigger an oom to
debug.

One thing I think/hope we can trivially agree on is that proactive
reclaim/demotion is _not_ an extreme circumstance. I would like me or
someone from the team to follow up with a patch that disables fallback
to reclaim on proactive reclaim/demotion (sc->proactive).

> One possibility, can we fall back to reclaim only if the sc->priority is
> small enough (even 0)?
>

This makes sense to me.

> Best Regards,
> Huang, Ying
>