On Thu, Nov 27, 2025 at 03:56:59PM +0000, Ryan Roberts wrote: > On 27/11/2025 15:03, Ard Biesheuvel wrote: > > So the question is really whether we want to dedicate 16 bytes per > > task for this. I wouldn't mind personally, but it is something our > > internal QA engineers tend to obsess over. > > Yeah that's a good point.
I think it's a fair point that some people will obsesses over this, but I think the concern is misplaced. I know that people were very happy for the kernel FPSIMD context to disappear from task_struct, but 16 bytes is a fair amount smaller, and I'm pretty sure we can offset that with a small/moderate amount of work. AFAICT there are extant holes in task_struct that could easily account for 16 bytes. I can also see a few ways to rework arm64's thread_info and thread_struct (which are both embedded within task_struct) to save some space. > Is this something we could potentially keep at the start of the > kstack? Is there any precident for keeping state there at the moment? > For arm64, I know there is a general feeling that 16K for the stack > more than enough (but we are stuck with it because 8K isn't quite > enough). So it would be "for free". I guess it would be tricky to do > this in an arch-agnostic way though... We went out of our way to stop playing silly games like that when we moved thread_info into task_struct; please let's not bring that back. Mark.
