Re: [PATCH tip/core/rcu 11/26] rcu: Exit RCU extended QS on user preemption

[Peter Zijlstra <peterz@infradead.org>]

On Thu, 2012-08-30 at 14:05 -0700, Paul E. McKenney wrote:
> From: Frederic Weisbecker <fweisbec@gmail.com>
> 
> When exceptions or irq are about to resume userspace, if
> the task needs to be rescheduled, the arch low level code
> calls schedule() directly.
> 
> At that time we may be in extended quiescent state from RCU
> POV: the exception is not anymore protected inside
> rcu_user_exit() - rcu_user_enter() and the irq has called
> rcu_irq_exit() already.
> 
> Create a new API schedule_user() that calls schedule() inside
> rcu_user_exit()-rcu_user_enter() in order to protect it. Archs
> will need to rely on it now to implement user preemption safely.

> ---
>  kernel/sched/core.c |    7 +++++++
>  1 files changed, 7 insertions(+), 0 deletions(-)
> 
> diff --git a/kernel/sched/core.c b/kernel/sched/core.c
> index 0bd599b..e841dfc 100644
> --- a/kernel/sched/core.c
> +++ b/kernel/sched/core.c
> @@ -3463,6 +3463,13 @@ asmlinkage void __sched schedule(void)
>  }
>  EXPORT_SYMBOL(schedule);
>  
> +asmlinkage void __sched schedule_user(void)
> +{
> +	rcu_user_exit();
> +	schedule();
> +	rcu_user_enter();
> +}


OK, so colour me unconvinced.. why are we doing this?

Typically when we call schedule nr_running != 1 (we need current to be
running and a possible target to switch to).

So I'd prefer to simply have schedule() disable all this adaptive tick
nonsense and leave it at that.