linux-kernel - Re: [PATCHv4 1/8] mm: Add support for unaccepted memory

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Date:   Wed, 13 Apr 2022 18:07:46 +0200
From:   David Hildenbrand <david@...hat.com>
To:     Dave Hansen <dave.hansen@...el.com>,
        "Kirill A. Shutemov" <kirill@...temov.name>
Cc:     "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
        Borislav Petkov <bp@...en8.de>,
        Andy Lutomirski <luto@...nel.org>,
        Sean Christopherson <seanjc@...gle.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Joerg Roedel <jroedel@...e.de>,
        Ard Biesheuvel <ardb@...nel.org>,
        Andi Kleen <ak@...ux.intel.com>,
        Kuppuswamy Sathyanarayanan 
        <sathyanarayanan.kuppuswamy@...ux.intel.com>,
        David Rientjes <rientjes@...gle.com>,
        Vlastimil Babka <vbabka@...e.cz>,
        Tom Lendacky <thomas.lendacky@....com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Peter Zijlstra <peterz@...radead.org>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Ingo Molnar <mingo@...hat.com>,
        Varad Gautam <varad.gautam@...e.com>,
        Dario Faggioli <dfaggioli@...e.com>,
        Brijesh Singh <brijesh.singh@....com>,
        Mike Rapoport <rppt@...nel.org>, x86@...nel.org,
        linux-mm@...ck.org, linux-coco@...ts.linux.dev,
        linux-efi@...r.kernel.org, linux-kernel@...r.kernel.org,
        Mike Rapoport <rppt@...ux.ibm.com>
Subject: Re: [PATCHv4 1/8] mm: Add support for unaccepted memory

On 13.04.22 17:36, Dave Hansen wrote:
> On 4/13/22 04:30, Kirill A. Shutemov wrote:
>>> 2) Fast boot; after boot, all memory will slowly but steadily get
>>>    accepted in the background. After a while, all memory is accepted and
>>>    can be signaled to user space.
> ...
>> Frankly, I think option 2 is the worst one. You still CPU cycles from the
>> workload after boot to do the job that may or may not be needed. It is an
>> half-measure that helps nobody.
> 
> Let's not be too hyperbolic here.  "Worst" is entirely subjective and it
> totally depends on your perspective and what you care about.

Right. Some people might want to start their workload as soon as the
pain is really over. Some might want to have a functional system before
that, others might not care.

> 
> There are basically four options:
> 
>  * Accept everything in early boot
>  * Accept with deferred page free
>  * Accept with kthread after boot
>  * Accept on demand
> 
> and four things that matter:
> 
>  * Code complexity
>  * Time to a shell prompt
>  * CPU/Memory waste
>  * Deterministic overhead
> 
> Did I miss any?

Nothing that comes to mind.

> 
> News flash: none of the options wins on all the things that matter.
> We're going to have to pick one (or maybe two).  I'm also not horribly
> convinced that there's a problem here worth solving, especially one that
> requires surgery in the core of the buddy allocator.
> 
> This is essentially making a performance argument: it takes too long to
> boot if we go with a simpler solution.  Yet, I haven't seen any data.  I
> think we need to go with the simplest approach(es) until there's some
> actual data to guide us here.

Simplest meaning: accept everything during early boot and don't touch
core-mm/buddy code, correct?

> 
> Here's another way to look at it:
> 
>> https://docs.google.com/spreadsheets/d/1Fpv0Yp0CTF5_JXHR2pywvNtImTwUVGTxDMlJ5t8qiis/edit?usp=sharing
> 


-- 
Thanks,

David / dhildenb