linux-kernel - Re: [RFC] Make need_resched() return true when rcu_urgent

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Message-ID: <20180706162905.GZ2476@hirez.programming.kicks-ass.net>
Date:   Fri, 6 Jul 2018 18:29:05 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     David Woodhouse <dwmw2@...radead.org>
Cc:     Paul McKenney <paulmck@...ux.vnet.ibm.com>, mhillenb@...zon.de,
        linux-kernel <linux-kernel@...r.kernel.org>
Subject: Re: [RFC] Make need_resched() return true when rcu_urgent_qs
 requested

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.