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

[Max Krasnyansky <maxk@qualcomm.com>]

Ingo Molnar wrote:
> * 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.

btw I think it's rarely used because many people do not realize it's there.
There are at least a couple of use cases that came up recently.
- Busted cpu. You can boot the machine with maxcpus=1 and then bring up cpus
one by one to see which one is busted.
- Recently reported regression that 16cpu box booted fine with NRCPUS=8 but
failed with NRCPUS=16. Again we can boot with maxcpus=8 and bring other cpus
later to see when/where we fail.

Things like that.

Max