Re: [LSF/MM/BPF TOPIC] Restricting or migrating unmovable kernel allocations from slow tier

Gregory Price
2025-02-07T09:54:17Z

On Fri, Feb 07, 2025 at 06:34:43PM +0900, Honggyu Kim wrote:
> On 2/7/2025 5:57 PM, Gregory Price wrote:
>
> > The default kernel stack size is like 16kb. You'd need like 100,000
> > threads to eat up 1.5GB, and 2048 threads only eats like 32MB.
> >
> > It's not an interesting amount of memory if you have a 20TB system.
>
> The amount might be small, but having those data in slow tier can
> make performance degradation if it is heavily accessed.
>
> The number of accesses isn't linearly corelated to the size of the
> memory region.
>

Right, I started by saying:

[CXL is] "generally not fit for kernel use"

I have the opinion that CXL memory should be defaulted to ZONE_MOVABLE,
but I understand the pressure on ZONE_NORMAL means this may not be
possible for large capacities.

I don't think the solution is to make kernel memory migratable and allow
kernel allocations on CXL.

There's a reason most kernel allocations are not swappable.

> Thanks,
> Honggyu