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SubjectRe: [PATCH] Resurect proper handling of maxcpus= kernel option

* Max.Krasnyansky@qualcomm.com <Max.Krasnyansky@qualcomm.com> wrote:

> From: Max Krasnyansky <maxk@qualcomm.com>
>
> For some reason we had redundant parsers registered for maxcpus=. One
> in init/main.c and another in arch/x86/smpboot.c So I nuked the one in
> arch/x86.
>
> Also 64-bit kernels used to handle maxcpus= as documented in
> Documentation/cpu-hotplug.txt. CPUs with 'id > maxcpus' are
> initialized but not booted. 32-bit version for some reason ignored
> them even though all the infrastructure for booting them later is
> there.
>
> In the current mainline both 64 and 32 bit versions are broken. I'm
> too lazy to look through git history but I'm guessing it happened as
> part of the i386 and x86_64 unification.

yes in essence. 32-bit always had maxcpus as a hard restriction in the
number of CPUs. This got extended to 64-bit as well, via commit
89b08200ad:

x86: make x86_64 accept the max_cpus parameter

in v2.6.25. Two major kernel releases and nobody noticed - it's a rarely
used option.

> This patch restores the correct behaviour. I've tested x86_64 version
> on 4- and 8- way Core2 and 2-way Opteron based machines. Various
> config combinations SMP, !SMP, CPU_HOTPLUG, !CPU_HOTPLUG. Booted with
> maxcpus=1 and maxcpus=4, etc. Everything is working as expected.
>
> I cannot test 32-bit version (no 32-bit machines here).

This will need some test time on 32-bit as that is where this represents
a material change. ( albeit what matters most is the maxcpus=1
distinction - and for that nosmp can be used as well to turn off
multi-cpu support altogether. So we could do this in v2.6.27 as well. )

Also, a quick grep shows that your patch will very likely break the
visws build:

arch/x86/kernel/visws_quirks.c:extern unsigned int __cpuinitdata maxcpus;
arch/x86/kernel/visws_quirks.c: if (ncpus > maxcpus)
arch/x86/kernel/visws_quirks.c: ncpus = maxcpus;

could you please fix that?

Ingo


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