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

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Date:   Tue, 2 May 2023 19:12:14 +0300
From:   Mike Rapoport <rppt@...nel.org>
To:     Marc Zyngier <maz@...nel.org>
Cc:     Justin Forbes <jforbes@...oraproject.org>,
        Catalin Marinas <catalin.marinas@....com>,
        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 Tue, May 02, 2023 at 03:21:17PM +0100, Marc Zyngier wrote:
> On Tue, 02 May 2023 15:07:41 +0100,
> Catalin Marinas <catalin.marinas@....com> wrote:
> > 
> > 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. 

The memory size of the machine or how heavy the workloads it runs have
nothing to do with MAX_ORDER. Again, x86 and s390 are perfectly fine with
MAX_ORDER == 10 ...

> > 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.

... but this indeed could be the reason to increase MAX_ORDER.

> > I'm happy to make ARCH_MAX_ORDER higher in defconfig (12, 13?) if
> > CONFIG_ARCH_THUNDER. Mobile vendors won't enable this platform.
> 
> In any case, I'd like to know exactly *what* requires it. The only
> platform I know would benefit from this is the old TX1, but this
> machine is more a boat anchor than a real server.

Yeah, if we'd knew what exactly requires such huge contiguous allocation,
we probably could fix that and leave Kconfig alone.
 
-- 
Sincerely yours,
Mike.