Re: [RFC 06/10] Reclaim memory from blocked kernel stacks

[Sebastian Andrzej Siewior <bigeasy@linutronix.de>]

On 2026-08-28 15:59:47 [+0200], Peter Zijlstra wrote:
> > > +
> > > +	  The wakeup latency of tasks with reclaimed stacks may increase,
> > > +	  especially while the system is under memory pressure.
> > 
> > It says *may* increase and on RT it _definitely_ will increase since
> > there is a kworker involved not to mention the memory allocation itself.
> > Anyway. This either needs to stay away from PREEMPT_RT or find a way to
> > exclude at the very least mlock()ed tasks.
> > Did lockdep see this?
> 
> It should have. They're taking spinlock inside raw_spinlock and lockdep
> should very much warn about that by default.

Yes. My point was that this was hidden from lockdep.

> > If I understood the whole exercise correct then you have a kernel stack
> > of two pages and in best case you can unmap and release the second page
> > while the task is napping.
> 
> THREAD_SIZE_ORDER	2
> THREAD_SIZE		(PAGE_SIZE << THREAD_SIZE_ORDER)
> 
> that makes for 4 pages.

Oh. I wasn't aware that we have 16kib stacks these days. But looking
at it we have it now for over 10 years… Judging from 6538b8ea886e4
("x86_64: expand kernel stack to 16K") it might be temporary and things
are better now? Arm64 has a different story according to 845ad05ec31e0
("arm64: Change kernel stack size to 16K"). Risc-V also mentions "for
now" in 0cac21b02ba5f ("riscv: use 16KB kernel stack on 64-bit").

I just booted my XFS kvm box and did things and 8KiB works so far.

> > What might be a tad simpler is to memset(,0,) the remaining part of the
> > stack. Since the stack is vmap-ed it should be swapped out on its own
> > without additional tricks. That memset() would help zram to compress
> > better so it uses less memory. ta-da.
> 
> That would still be a 12k memset with IRQs-disabled and rq->lock held.

Right, because the stack grew a bit. Probably still cheaper compared to
the other things done here ;)

> > What also should be simpler (and I am not saying just to move you away
> > from the scheduler) is to have a shrinker which iterates over all tasks
> > which are marked for reclaim and then similar to swap just unmap both
> > stack pages and release the second page which is not used.
> > Upon wake up the task should create a page_fault which would be used to
> > allocate the second stack page and map the whole stack again.
> 
> Right, so you can FREEZE the task, unmap its stack and then thaw it or
> something. But there should be a definite opt-out on all this, because
> taking faults on your stack will be horrible.

Definitely. Not something for the currently visible app.

> Not to mention you'll suffer wakeup latencies while frozen.

Right but you would use it under memory pressure and steal the stack
from the most idle tasks rather from everyone. 

> This all really sounds like what should be addressed is this insane
> number of tasks rather than trying to cope with the consequences of
> that.

Sebastian