linux-kernel - Re: [PATCHv7 02/14] mm: Add support for unaccepted memory

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Date:   Fri, 5 Aug 2022 07:53:49 -0700
From:   Dave Hansen <dave.hansen@...el.com>
To:     David Hildenbrand <david@...hat.com>,
        Vlastimil Babka <vbabka@...e.cz>,
        "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>
Cc:     Andi Kleen <ak@...ux.intel.com>,
        Kuppuswamy Sathyanarayanan 
        <sathyanarayanan.kuppuswamy@...ux.intel.com>,
        David Rientjes <rientjes@...gle.com>,
        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>,
        Dario Faggioli <dfaggioli@...e.com>,
        Mike Rapoport <rppt@...nel.org>, marcelo.cerri@...onical.com,
        tim.gardner@...onical.com, khalid.elmously@...onical.com,
        philip.cox@...onical.com, 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>,
        Mel Gorman <mgorman@...hsingularity.net>
Subject: Re: [PATCHv7 02/14] mm: Add support for unaccepted memory

On 8/5/22 07:22, David Hildenbrand wrote:
>> A list that sits besides the existing free_area, contains only >=pageblock
>> order sizes of unaccepted pages (no migratetype distinguished) and we tap
>> into it approximately before __rmqueue_fallback()? There would be some
>> trickery around releasing zone-lock for doing accept_memory(), but should be
>> manageable.
>>
> Just curious, do we have a microbenchmark that is able to reveal the
> impact of such code changes before we start worrying?

Nope.  I went looking to see if I could find any impact.  I think Kirill
did too.  Too bad that effort didn't make it into the changelog yet.

The merging check at least is just checking a field in a cache-hot
'struct page'.  The common case is probably three instructions:

	load to a register
	check the bit
	jump if not set

It adds a wee bit of icache pressure, but it's also the kind of thing
that should be a piece of cake for the branch predictors.

That dynamic check could easily be wrapped by a static branch.  But,
that first requires more code to go dig in the nooks and crannies of the
page allocator to make sure *ALL* pages are accepted.