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SubjectRe: [PATCH v5 1/9] mm/demotion: Add support for explicit memory tiers
On Thu, Jun 9, 2022 at 1:18 AM Aneesh Kumar K V
<aneesh.kumar@linux.ibm.com> wrote:
>
> On 6/8/22 10:12 PM, Yang Shi wrote:
> > On Tue, Jun 7, 2022 at 9:58 PM Aneesh Kumar K V
> > <aneesh.kumar@linux.ibm.com> wrote:
>
> ....
>
> >> config TIERED_MEMORY
> >> bool "Support for explicit memory tiers"
> >> - def_bool n
> >> - depends on MIGRATION && NUMA
> >> - help
> >> - Support to split nodes into memory tiers explicitly and
> >> - to demote pages on reclaim to lower tiers. This option
> >> - also exposes sysfs interface to read nodes available in
> >> - specific tier and to move specific node among different
> >> - possible tiers.
> >> + def_bool MIGRATION && NUMA
> >
> > CONFIG_NUMA should be good enough. Memory tiering doesn't have to mean
> > demotion/promotion has to be supported IMHO.
> >
> >>
> >> config HUGETLB_PAGE_SIZE_VARIABLE
> >> def_bool n
> >>
> >> ie, we just make it a Kconfig variable without exposing it to the user?
> >>
>
> We can do that but that would also mean in order to avoid building the
> demotion targets etc we will now have to have multiple #ifdef
> CONFIG_MIGRATION in mm/memory-tiers.c . It builds without those #ifdef
> So these are not really build errors, but rather we will be building all
> the demotion targets for no real use with them.

Can we have default demotion targets for !MIGRATION? For example, all
demotion targets are -1.

>
> What usecase do you have to expose memory tiers on a system with
> CONFIG_MIGRATION disabled? CONFIG_MIGRATION gets enabled in almost all
> configs these days due to its dependency against COMPACTION and
> TRANSPARENT_HUGEPAGE.

Johannes's interleave series is an example,
https://lore.kernel.org/lkml/20220607171949.85796-1-hannes@cmpxchg.org/

It doesn't do any demotion/promotion, just make allocations interleave
on different tiers.

>
> Unless there is a real need, I am wondering if we can avoid sprinkling
> #ifdef CONFIG_MIGRATION in mm/memory-tiers.c
>
> -aneesh

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