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

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Date:   Mon, 18 Apr 2022 09:50:33 -0700
From:   Dave Hansen <dave.hansen@...el.com>
To:     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 4/18/22 09:45, Davidlohr Bueso wrote:
> 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.

But how does this help you?  Does it save you a few lines in a shell
script to find the nodes that have memory and CPUs?

>> 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.

It seems like something arbitrary that everyone will just disagree on.
I think we should try to stick to cold, hard facts as must as possible
rather than trying to have the *kernel* dictate as a policy what is fast
versus slow.