Messages in this thread | | | Date | Mon, 11 Aug 2008 20:16:42 +0200 | From | Ingo Molnar <> | Subject | Re: [PATCH] Resurect proper handling of maxcpus= kernel option |
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* 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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