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

[Yang Shi]

On Tue, Nov 29, 2022 at 9:33 PM Huang, Ying <ying.huang@xxxxxxxxx> wrote:
>
> Yang Shi <shy828301@xxxxxxxxx> writes:
>
> > On Mon, Nov 28, 2022 at 4:54 PM Huang, Ying <ying.huang@xxxxxxxxx> wrote:
> >>
> >> Yang Shi <shy828301@xxxxxxxxx> 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
> >> >> 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.
> >> >
> >> > For the ideal case, I do agree with Johannes to demote the page tier
> >> > by tier rather than reclaiming them from the higher tiers. But I also
> >> > agree with your premature OOM concern.
> >> >
> >> >>
> >> >> 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?
> >> >
> >> > In my initial implementation I implemented a simple throttle logic
> >> > when the demotion is not going to succeed if the demotion target has
> >> > not enough free memory (just check the watermark) to make migration
> >> > succeed without doing any reclamation. Shall we resurrect that?
> >>
> >> Can you share the link to your throttle patch?  Or paste it here?
> >
> > I just found this on the mailing list.
> > https://lore.kernel.org/linux-mm/1560468577-101178-8-git-send-email-yang.shi@xxxxxxxxxxxxxxxxx/
>
> Per my understanding, this patch will avoid demoting if there's no free
> space on demotion target?  If so, I think that we should trigger kswapd
> reclaiming on demotion target before that.  And we can simply avoid to
> fall back to reclaim firstly, then avoid to scan as an improvement as
> that in your patch above.

Yes, it should. The rough idea looks like:

if (the demote target is contended)
    wake up kswapd
    reclaim_throttle(VMSCAN_THROTTLE_DEMOTION)
    retry demotion

The kswapd is responsible for clearing the contention flag.

>
> Best Regards,
> Huang, Ying
>
> > But it didn't have the throttling logic, I may not submit that version
> > to the mailing list since we decided to drop this and merge mine and
> > Dave's.
> >
> > Anyway it is not hard to add the throttling logic, we already have a
> > few throttling cases in vmscan, for example, "mm/vmscan: throttle
> > reclaim until some writeback completes if congested".
> >>
> >> > Waking kswapd sooner is fine to me, but it may be not enough, for
> >> > example, the kswapd may not keep up so remature OOM may happen on
> >> > higher tiers or reclaim may still happen. I think throttling the
> >> > reclaimer/demoter until kswapd makes progress could avoid both. And
> >> > since the lower tiers memory typically is quite larger than the higher
> >> > tiers, so the throttle should happen very rarely IMHO.
> >> >
> >> >>
> >> >> 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.
> >>
> >> Best Regards,
> >> Huang, Ying