Re: [RFC] Make need_resched() return true when rcu_urgent_qs requested

Peter Zijlstra
2018-07-06T16:29:14Z

On Fri, Jul 06, 2018 at 03:53:30PM +0100, David Woodhouse wrote:
> diff --git a/include/linux/sched.h b/include/linux/sched.h
> index e4d4e60..89f5814 100644
> --- a/include/linux/sched.h
> +++ b/include/linux/sched.h
> @@ -1616,7 +1616,8 @@ static inline int spin_needbreak(spinlock_t *lock)
>
> static __always_inline bool need_resched(void)
> {
> - return unlikely(tif_need_resched());
> + return unlikely(tif_need_resched()) ||
> + rcu_urgent_qs_requested();
> }

Instead of making need_resched() touch two cachelines, I think I would
prefer adding resched_cpu() to rcu_request_urgent_qs_task().

The preempt state is alread a bit complicated and shadowed in the
preempt_count (on some architectures) adding additional bits to it like
this is just asking for trouble.