linux-kernel - Re: [PATCH v5 1/9] mm/demotion: Add support for explicit memory tiers

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Date:   Thu, 9 Jun 2022 09:04:31 -0700
From:   Yang Shi <shy828301@...il.com>
To:     Aneesh Kumar K V <aneesh.kumar@...ux.ibm.com>
Cc:     Linux MM <linux-mm@...ck.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Wei Xu <weixugc@...gle.com>, Huang Ying <ying.huang@...el.com>,
        Greg Thelen <gthelen@...gle.com>,
        Davidlohr Bueso <dave@...olabs.net>,
        Tim C Chen <tim.c.chen@...el.com>,
        Brice Goglin <brice.goglin@...il.com>,
        Michal Hocko <mhocko@...nel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Hesham Almatary <hesham.almatary@...wei.com>,
        Dave Hansen <dave.hansen@...el.com>,
        Jonathan Cameron <Jonathan.Cameron@...wei.com>,
        Alistair Popple <apopple@...dia.com>,
        Dan Williams <dan.j.williams@...el.com>,
        Feng Tang <feng.tang@...el.com>,
        Jagdish Gediya <jvgediya@...ux.ibm.com>,
        Baolin Wang <baolin.wang@...ux.alibaba.com>,
        David Rientjes <rientjes@...gle.com>
Subject: Re: [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@...ux.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@...ux.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