linux-kernel - Re: [PATCH] Revert arm64: drop ranges in definition of ARCH_FORCE_MAX

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Date:   Tue, 2 May 2023 15:07:41 +0100
From:   Catalin Marinas <catalin.marinas@....com>
To:     Justin Forbes <jforbes@...oraproject.org>
Cc:     Mike Rapoport <rppt@...nel.org>, Will Deacon <will@...nel.org>,
        linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
        jmforbes@...uxtx.org, Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [PATCH] Revert arm64: drop ranges in definition of
 ARCH_FORCE_MAX_ORDER

On Mon, May 01, 2023 at 04:24:38PM -0500, Justin Forbes wrote:
> On Sat, Apr 29, 2023 at 11:02 PM Mike Rapoport <rppt@...nel.org> wrote:
> > Why the default MAX_ORDER was not acceptable on arm64 server machines but
> > it is fine on, say, x86 and s390?
> > I'm not asking how you made it possible in Fedora and RHEL, I'm asking why
> > did you switch from the default order at all.
> 
> Because the MAX_ORDER on aarch64 with 4K pages is more tuned to the
> needs of the average edge client, not so much those of a server class
> machine.  And I get it, I would say well over 90% of the Fedora users
> running aarch64 are indeed running on a rPi or similar with a small
> memory footprint, and workloads which match that.  But we do support
> and run a 4K page size aarch64 kernel on proper server class hardware,
> running typical server workloads, and RHEL has a lot more users in the
> server class than edge clients.   RHEL could probably default to 64K
> pages, and most users would be happy with that. Fedora certainly could
> not. 

I was talking to Marc Zyngier earlier and he reckons the need for a
higher MAX_ORDER is the GIC driver ITS allocation for Thunder-X. I'm
happy to make ARCH_MAX_ORDER higher in defconfig (12, 13?) if
CONFIG_ARCH_THUNDER. Mobile vendors won't enable this platform.

Regarding EXPERT, we could drop it and do like the other architectures
but we'll have randconfig occasionally hitting weird values that won't
build (like -1). Not sure EXPERT helps here.

-- 
Catalin