linux-kernel - Re: [PATCH 6/6] mm/migrate: export whether or not node is toptier in sysf

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Message-ID: <20220418164503.jfips3aiwhnlfjrq@offworld>
Date:   Mon, 18 Apr 2022 09:45:03 -0700
From:   Davidlohr Bueso <dave@...olabs.net>
To:     Dave Hansen <dave.hansen@...el.com>
Cc:     linux-mm@...ck.org, mhocko@...nel.org, akpm@...ux-foundation.org,
        rientjes@...gle.com, yosryahmed@...gle.com, hannes@...xchg.org,
        shakeelb@...gle.com, dave.hansen@...ux.intel.com,
        tim.c.chen@...ux.intel.com, roman.gushchin@...ux.dev,
        gthelen@...gle.com, a.manzanares@...sung.com,
        heekwon.p@...sung.com, gim.jongmin@...sung.com,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH 6/6] mm/migrate: export whether or not node is toptier in
 sysf

On Mon, 18 Apr 2022, Dave Hansen wrote:

>On 4/16/22 20:49, Davidlohr Bueso wrote:
>> This allows userspace to know if the node is considered fast
>> memory (with CPUs attached to it). While this can be already
>> derived without a new file, this helps further encapsulate the
>> concept.
>
>What is userspace supposed to *do* with this, though?

This came as a scratch to my own itch. I wanted to start testing
more tiering patches overall that I see pop up, and wanted a way
to differentiate the slow vs the fast memories in order to better
configure workload(s) working set sizes beyond what is your typical
grep MemTotal /proc/meminfo. If there is a better way I'm all
for it.

>
>What does "attached" mean?

I'll rephrase.

>Isn't it just asking for trouble to add (known) redundancy to the ABI?
>It seems like a recipe for future inconsistency.

Perhaps. It was mostly about the fact that the notion of top tier
could also change as technology evolves.

Thanks,
Davidlohr