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

["Huang, Ying" <ying.huang@intel.com>]

Yang Shi <shy828301@gmail.com> writes:

> On Wed, Nov 23, 2022 at 9:52 PM Huang, Ying <ying.huang@intel.com> wrote:
>>
>> Hi, Johannes,
>>
>> Johannes Weiner <hannes@cmpxchg.org> 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@fb.com/
>>
>> 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?

> 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