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

[David Stevens]

On Fri, Aug 28, 2026 at 5:04 AM Peter Zijlstra <peterz at infradead.org> wrote:
>
> On Thu, Aug 27, 2026 at 04:29:44PM -0700, David Stevens wrote:
> > @@ -4320,8 +4319,18 @@ int try_to_wake_up(struct task_struct *p, unsigned int state, int wake_flags)
> >                * A similar smp_rmb() lives in __task_needs_rq_lock().
> >                */
> >               smp_rmb();
> > -             if (READ_ONCE(p->on_rq) && ttwu_runnable(p, wake_flags))
> > +             if (READ_ONCE(p->on_rq) && ttwu_runnable(p, wake_flags)) {
> > +                     trace_sched_waking(p);
> > +                     break;
> > +             }
> > +
> > +             if (!ensure_stack_is_present(p, &need_deferred_repopulate)) {
> > +                     WRITE_ONCE(p->__state, TASK_STACK_RECLAIM);
> > +                     do_deferred_repopulate_wake = need_deferred_repopulate;
> >                       break;
> > +             }
> > +
> > +             trace_sched_waking(p);
>
> Absolutely not; ensure_stack_is_present() must not call
> repopulate_stack() while holding ->pi_lock. Not happening.

The optimistic fast path for repopulate_stack() could be modified to
try pulling from a pre-allocated pool of zero'ed pages. That would
reduce the function to a couple of memcg_kmem_charge_page() calls and
then vmap_pages_range() to repopulate the stack's page tables. That
wouldn't require touching any locks except a raw_spinlock protecting
the pre-allocated pool (or just make it per_cpu). In terms of cost,
this would involve a couple of atomic operations for the page pool
lock and the memcg charging plus non-atomic operations on 5-10 other
cache lines.

Is that within the scope of what can be done under the pi_lock? If
that's still not happening, I can see how things look if we always
defer wakeup to a workqueue.

-David