Re: [patch 5/9] x86: Cure per CPU madness on UP

Arnd Bergmann
2024-03-18T19:13:28Z

On Mon, Mar 18, 2024, at 18:27, Thomas Gleixner wrote:
> On Sat, Mar 16 2024 at 02:11, Thomas Gleixner wrote:
>> On Fri, Mar 15 2024 at 16:23, Linus Torvalds wrote:
>> The amount of subtle SMP=n fallout has been kinda exponentially
>> increasing over the years and it's just putting burden on the wrong
>> people. TBH, I'm tired of this nonsense.
>
> And for the fun of it I hacked Kconfig to allow a SMP=y NR_CPUS=1 build
> and checked the size of vmlinux:
>
> 64-bit 32-bit
> SMP, NCPUS=1 38438400 22110177
> UP 38393703 21682041
> Delta 44697 428076
> 0.1% 2%
>
> The UP savings are not really impressive...
>
> Let me look what it actually takes to do that.

FWIW, I did some experiments a few weeks ago on 32-bit ARM,
using a fairly minimal kernel in a virtual machine, and
checking the runtime memory consumption rather than compile-time.
In a kvm guest with 32MiB RAM, I saw a difference of multiple
megabytes in memory usage:

Linux testvm 6.8.0-rc4-00410-gc02197fc9076-dirty #1 SMP PREEMPT armv7l
root@testvm:~# free
total used free shared buff/cache available
Mem: 26932 14956 1732 52 12800 11976
Swap: 16360 3632 12728

Linux testvm 6.8.0-rc4-00410-gc02197fc9076-dirty #2 PREEMPT armv7l
root@testvm:~# free
total used free shared buff/cache available
Mem: 26932 13744 5648 32 10092 13188
Swap: 16360 3880 12480

There is a little difference between runs, but this does seem
significant enough to keep it. The SMP build was with
CONFIG_NR_CPUS=2 (the smallest supported compile-time number),
but running on a single-CPU qemu instance.

Arnd