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

[Peter Zijlstra]

On Thu, Aug 27, 2026 at 04:29:44PM -0700, David Stevens wrote:
> @@ -4295,8 +4296,6 @@ int try_to_wake_up(struct task_struct *p, unsigned int state, int wake_flags)
>  		if (!ttwu_state_match(p, state, &success))
>  			break;
>  
> -		trace_sched_waking(p);
> -
>  		/*
>  		 * Ensure we load p->on_rq _after_ p->state, otherwise it would
>  		 * be possible to, falsely, observe p->on_rq == 0 and get stuck
> @@ -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);
>  
>  		/*
>  		 * Ensure we load p->on_cpu _after_ p->on_rq, otherwise it would be

Don't move that tracepoint. That's simply lying about how long it takes
to wake up the task.