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

Thomas Gleixner
2024-03-19T16:21:05Z

On Mon, Mar 18 2024 at 20:13, Arnd Bergmann wrote:
> 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.

With a SMP=y, NR_CPUS=1 build on x86 64bit I get:

total used free shared buff/cache available
Mem: 32882056 498068 32590580 4884 128884 32383988
Swap: 998396 0 998396

Same config just SMP=n:

total used free shared buff/cache available
Mem: 32885804 461704 32635284 4876 119480 32424100
Swap: 998396 0 998396

So the delta for available is ~40 MiB.

But if I look at it with init=/bin/sh on the command line then the delta
is significantly different:

With a SMP=y, NR_CPUS=1 build on x86 64bit I get:

total used free shared buff/cache available
Mem: 32883680 324120 32822728 216 10864 32559560
Swap: 0 0 0

Same config just SMP=n:

total used free shared buff/cache available
Mem: 32885804 326876 32821972 216 11100 32558928
Swap: 0 0 0

Delta available = 632 KiB

I haven't had the time to stare at that in detail, but comparing
/proc/meminfo for the full boot case above does not immediately give me
a hint. It's confusing at best...

Thanks,

tglx