linux-kernel - Re: [PATCH v3 13/13] mm: introduce arch_wants_lazy_mmu

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Message-ID: <35df96ba-a004-4eb4-8d26-5935892a852c@arm.com>
Date: Fri, 24 Oct 2025 14:17:20 +0200
From: Kevin Brodsky <kevin.brodsky@....com>
To: David Hildenbrand <david@...hat.com>, linux-mm@...ck.org
Cc: linux-kernel@...r.kernel.org, Alexander Gordeev <agordeev@...ux.ibm.com>,
 Andreas Larsson <andreas@...sler.com>,
 Andrew Morton <akpm@...ux-foundation.org>,
 Boris Ostrovsky <boris.ostrovsky@...cle.com>, Borislav Petkov
 <bp@...en8.de>, Catalin Marinas <catalin.marinas@....com>,
 Christophe Leroy <christophe.leroy@...roup.eu>,
 Dave Hansen <dave.hansen@...ux.intel.com>,
 "David S. Miller" <davem@...emloft.net>, "H. Peter Anvin" <hpa@...or.com>,
 Ingo Molnar <mingo@...hat.com>, Jann Horn <jannh@...gle.com>,
 Juergen Gross <jgross@...e.com>, "Liam R. Howlett"
 <Liam.Howlett@...cle.com>, Lorenzo Stoakes <lorenzo.stoakes@...cle.com>,
 Madhavan Srinivasan <maddy@...ux.ibm.com>,
 Michael Ellerman <mpe@...erman.id.au>, Michal Hocko <mhocko@...e.com>,
 Mike Rapoport <rppt@...nel.org>, Nicholas Piggin <npiggin@...il.com>,
 Peter Zijlstra <peterz@...radead.org>, Ryan Roberts <ryan.roberts@....com>,
 Suren Baghdasaryan <surenb@...gle.com>, Thomas Gleixner
 <tglx@...utronix.de>, Vlastimil Babka <vbabka@...e.cz>,
 Will Deacon <will@...nel.org>, Yeoreum Yun <yeoreum.yun@....com>,
 linux-arm-kernel@...ts.infradead.org, linuxppc-dev@...ts.ozlabs.org,
 sparclinux@...r.kernel.org, xen-devel@...ts.xenproject.org, x86@...nel.org
Subject: Re: [PATCH v3 13/13] mm: introduce arch_wants_lazy_mmu_mode()

On 23/10/2025 22:10, David Hildenbrand wrote:
> On 15.10.25 10:27, Kevin Brodsky wrote:
>> powerpc decides at runtime whether the lazy MMU mode should be used.
>>
>> To avoid the overhead associated with managing
>> task_struct::lazy_mmu_state if the mode isn't used, introduce
>> arch_wants_lazy_mmu_mode() and bail out of lazy_mmu_mode_* if it
>> returns false. Add a default definition returning true, and an
>> appropriate implementation for powerpc.
>>
>> Signed-off-by: Kevin Brodsky <kevin.brodsky@....com>
>> ---
>> This patch seemed like a good idea to start with, but now I'm not so
>> sure that the churn added to the generic layer is worth it.
>
> Exactly my thoughts :)
>
> I think we need evidence that this is really worth it for optimizing
> out basically a counter update on powerpc.

Ack, I'll drop that patch in v4 unless someone sees a better
justification for it.

- Kevin