Re: [RFC 00/14] Dynamic Kernel Stacks

[Pasha Tatashin <pasha.tatashin@soleen.com>]

On Sun, Mar 17, 2024 at 2:58 PM David Laight <David.Laight@aculab.com> wrote:
>
> From: Pasha Tatashin
> > Sent: 16 March 2024 19:18
> ...
> > Expanding on Mathew's idea of an interface for dynamic kernel stack
> > sizes, here's what I'm thinking:
> >
> > - Kernel Threads: Create all kernel threads with a fully populated
> > THREAD_SIZE stack.  (i.e. 16K)
> > - User Threads: Create all user threads with THREAD_SIZE kernel stack
> > but only the top page mapped. (i.e. 4K)
> > - In enter_from_user_mode(): Expand the thread stack to 16K by mapping
> > three additional pages from the per-CPU stack cache. This function is
> > called early in kernel entry points.
> > - exit_to_user_mode(): Unmap the extra three pages and return them to
> > the per-CPU cache. This function is called late in the kernel exit
> > path.
>
> Isn't that entirely horrid for TLB use and so will require a lot of IPI?

The TLB load is going to be exactly the same as today, we already use
small pages for VMA mapped stacks. We won't need to have extra
flushing either, the mappings are in the kernel space, and once pages
are removed from the page table, no one is going to access that VA
space until that thread enters the kernel again. We will need to
invalidate the VA range only when the pages are mapped, and only on
the local cpu.

> Remember, if a thread sleeps in 'extra stack' and is then resheduled
> on a different cpu the extra pages get 'pumped' from one cpu to
> another.

Yes, the per-cpu cache can get unbalanced this way, we can remember
the original CPU where we acquired the pages to return to the same
place.

> I also suspect a stack_probe() is likely to end up being a cache miss
> and also slow???

Can you please elaborate on this point. I am not aware of
stack_probe() and how it is used.

> So you wouldn't want one on all calls.
> I'm not sure you'd want a conditional branch either.
>
> The explicit request for 'more stack' can be required to be allowed
> to sleep - removing a lot of issues.
> It would also be portable to all architectures.
> I'd also suspect that any thread that needs extra stack is likely
> to need to again.
> So while the memory could be recovered, I'd bet is isn't worth
> doing except under memory pressure.
> The call could also return 'no' - perhaps useful for (broken) code
> that insists on being recursive.

The current approach discussed is somewhat different from explicit
more stack requests API. I am investigating how feasible it is to use
kernel stack multiplexing, so the same pages can be re-used by many
threads when they are actually used. If the multiplexing approach
won't work, I will come back to the explicit more stack API.

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