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

Guenter Roeck
2024-03-19T18:26:18Z

On 3/19/24 09:21, Thomas Gleixner wrote:
> 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...
>

That makes me wonder if the number is affected by the total memory size.
How about a system with 1GB of memory or less ?

Guenter