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SubjectRe: [patch 4/5] sched: Delay task stack freeing on RT
On Tue, Sep 28, 2021 at 02:24:30PM +0200, Thomas Gleixner wrote:

> --- a/kernel/exit.c
> +++ b/kernel/exit.c
> @@ -172,6 +172,11 @@ static void delayed_put_task_struct(stru
> kprobe_flush_task(tsk);
> perf_event_delayed_put(tsk);
> trace_sched_process_free(tsk);
> +
> + /* RT enabled kernels delay freeing the VMAP'ed task stack */
> + if (IS_ENABLED(CONFIG_PREEMPT_RT))
> + put_task_stack(tsk);
> +
> put_task_struct(tsk);
> }

> --- a/kernel/sched/core.c
> +++ b/kernel/sched/core.c
> @@ -4846,8 +4846,12 @@ static struct rq *finish_task_switch(str
> if (prev->sched_class->task_dead)
> prev->sched_class->task_dead(prev);
>
> - /* Task is done with its stack. */
> - put_task_stack(prev);
> + /*
> + * Release VMAP'ed task stack immediate for reuse. On RT
> + * enabled kernels this is delayed for latency reasons.
> + */
> + if (!IS_ENABLED(CONFIG_PREEMPT_RT))
> + put_task_stack(prev);
>
> put_task_struct_rcu_user(prev);
> }


Having this logic split across two files seems unfortunate and prone to
'accidents'. Is there a real down-side to unconditionally doing it in
delayed_put_task_struct() ?

/me goes out for lunch... meanwhile tglx points at: 68f24b08ee89.

Bah.. Andy?

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