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

[Mina Almasry]

On Tue, Nov 29, 2022 at 9:40 PM Huang, Ying <ying.huang@xxxxxxxxx> wrote:
>
> Mina Almasry <almasrymina@xxxxxxxxxx> writes:
>
> > On Wed, Nov 23, 2022 at 9:52 PM Huang, Ying <ying.huang@xxxxxxxxx> wrote:
> >>
> >> Hi, Johannes,
> >>
> >> 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
> >
> > I may be missing something but as far I can tell reclaim is disabled
> > for allocations from lower tier memory:
> > https://elixir.bootlin.com/linux/v6.1-rc7/source/mm/vmscan.c#L1583
>
> #define GFP_NOWAIT      (__GFP_KSWAPD_RECLAIM)
>
> We have GFP_NOWAIT set in gfp.
>

Ah, thanks. I missed that.

> > I think this is maybe a good thing when doing proactive demotion. In
> > this case we probably don't want to try to reclaim from lower tier
> > nodes and instead fail the proactive demotion.
>
> Do you have some real use cases for this?  If so, we can tweak the
> logic.
>

Nothing real at the moment. I was thinking this may be something
desirable to tune at some point.

> > However I can see this being desirable when the top tier nodes are
> > under real memory pressure to deflect that pressure to the lower tier
> > nodes.
>
> Yes.
>
> Best Regards,
> Huang, Ying
>
> >> 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.
> >>
> >> > So I'm hesitant to design cgroup controls around the current behavior.
> >
> > I sent RFC v2 patch:
> > https://lore.kernel.org/linux-mm/20221130020328.1009347-1-almasrymina@xxxxxxxxxx/T/#u
> >
> > Please take a look when convenient. Thanks!
> >
> >> >
> >>
> >> Best Regards,
> >> Huang, Ying
> >>
>