Re: [RFC 00/14] Dynamic Kernel Stacks

[H. Peter Anvin]

On 3/12/24 12:45, Pasha Tatashin wrote:
> > Ok, first of all, talking about "kernel memory" here is misleading.
>
> Hi Peter,
>
> I re-read my cover letter, and I do not see where "kernel memory" is
> mentioned. We are talking about kernel stacks overhead that is
> proportional to the user workload, as every active thread has an
> associated kernel stack. The idea is to save memory by not
> pre-allocating all pages of kernel-stacks, but instead use it as a
> safeguard when a stack actually becomes deep. Come-up with a solution
> that can handle rare deeper stacks only when needed. This could be
> done through faulting on the supported hardware (as proposed in this
> series), or via pre-map on every schedule event, and checking the
> access when thread goes off cpu (as proposed by Andy Lutomirski to
> avoid double faults on x86) .
>
> In other words, this feature is only about one very specific type of
> kernel memory that is not even directly mapped (the feature required
> vmapped stacks).
>
> > Unless your threads are spending nearly all their time sleeping, the
> > threads will occupy stack and TLS memory in user space as well.
>
> Can you please elaborate, what data is contained in the kernel stack
> when thread is in user space? My series requires thread_info not to be
> in the stack by depending on THREAD_INFO_IN_TASK.

My point is that what matters is total memory use, not just memory used 
in the kernel. Amdahl's law.

	-hpa