[PATCH v2] powerpc: increase MIN RMA size for CAS negotiation

[Avnish Chouhan]

Hi Michael,
Hope you're doing wonderful!

Thank you so much for your response. I have checked on your queries. 
Please find the findings below:

1. Did you consider just increasing it to 1GB?

We have observed in our recent Out Of Memory issues, a shortage of 
around 50-60 MBs space in RMA in current issues. So we decided to 
increase the RMA by 256 MBs. Please give me couple of days, I'm 
analyzing this 1 GB change and update you on it soon.

2. an LPAR with less than 768MB of RAM

I have analyzed the multiple RAM scenarios. The behavior seems similar 
regardless of RMA size 512 or 768 MBs, as the RMA region is used by PFW 
and GRUB2 for booting. Even if GRUB is able to load the kernel for 
booting, the machines isn't booting and behaving well in low amount of 
RAM. We observe kernel panics, mostly "Out of memory: Killed 
process...." when RAM is less than 3 GBs. The different RAM configs (via 
HMC LPAR properties) and behaviors are given below:

i. RAM (3 GBs)
    System boot fine when RAM is minimum 3 GBs (It does depend on system 
config as well).
ii. RMA (512 MBs)
     With RAM as 512 MB, the system fails to boot with firmware error (eg 
B2006006).
iii. RAM (768 MB and 1 GB)
      With RAM as 768 MB and 1 GB, System boot with kernel panic as 
"Kernel panic - not syncing: System is    deadlocked on memory".
iv. RAM (2 GBs)
     System does boot fine, but abnormal behavior after the boot. I 
observed system panic in one scenario while doing a reboot. "Out of 
memory: Killed process 167....."


Regards,
Avnish Chouhan


On 2024-12-07 07:28, Michael Ellerman wrote:
> Avnish Chouhan <avnish at linux.ibm.com> writes:
>> Change RMA size from 512 MB to 768 MB which will result
>> in more RMA at boot time for PowerPC.
> 
> Did you consider just increasing it to 1GB?
> 
> It's possible there's some folks running LPARs with less than 1GB, but
> they are unlikely to continue doing so by the time this change trickles
> into distros. To be supported modern RHEL requires 2GB minimum RAM
> anyway.
> 
> Can you also describe the behaviour users will see when they configure
> an LPAR with less than 768MB of RAM.
> 
> cheers