linux-kernel - Re: [PATCH RFC tip/core/rcu 2/5] rcu: Short-circuit normal GPs via expedited GPs

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Message-ID: <20150701100352.GO19282@twins.programming.kicks-ass.net>
Date:	Wed, 1 Jul 2015 12:03:52 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
Cc:	linux-kernel@...r.kernel.org, mingo@...nel.org,
	laijs@...fujitsu.com, dipankar@...ibm.com,
	akpm@...ux-foundation.org, mathieu.desnoyers@...icios.com,
	josh@...htriplett.org, tglx@...utronix.de, rostedt@...dmis.org,
	dhowells@...hat.com, edumazet@...gle.com, dvhart@...ux.intel.com,
	fweisbec@...il.com, oleg@...hat.com, bobby.prani@...il.com
Subject: Re: [PATCH RFC tip/core/rcu 2/5] rcu: Short-circuit normal GPs via
 expedited GPs

On Tue, Jun 30, 2015 at 02:48:27PM -0700, Paul E. McKenney wrote:
> @@ -2121,17 +2137,24 @@ static int __noreturn rcu_gp_kthread(void *arg)
>  					       TPS("fqswait"));
>  			rsp->gp_state = RCU_GP_WAIT_FQS;
>  			ret = wait_event_interruptible_timeout(rsp->gp_wq,
> -					((gf = READ_ONCE(rsp->gp_flags)) &
> -					 RCU_GP_FLAG_FQS) ||
> -					(!READ_ONCE(rnp->qsmask) &&
> -					 !rcu_preempt_blocked_readers_cgp(rnp)),
> -					j);
> +				((gf = READ_ONCE(rsp->gp_flags)) &
> +				 RCU_GP_FLAG_FQS) ||
> +				(!READ_ONCE(rnp->qsmask) &&
> +				 !rcu_preempt_blocked_readers_cgp(rnp)) ||
> +				rcu_exp_gp_seq_done(rsp->exp_rsp,
> +						    rsp->gp_exp_snap),
> +				j);

How about using a helper function there?

static inline bool rcu_gp_done(rsp, rnp)
{
	/* Forced Quiescent State complete */
	if (READ_ONCE(rsp->gp_flags) & RCU_GP_FLAG_FQS)
		return true;

	/* QS not masked and not blocked by preempted readers */
	if (!READ_ONCE(rnp->qsmask) &&
	    !rcu_preempt_blocked_readers_cgp(rnp))
		return true;

	/* Expedited Grace Period completed */
	if (rcu_exp_gp_seq_done(rsp))
		return true;

	return false;
}

	ret = wait_event_interruptible_timeout(rsp->gp_wq,
		rcu_gp_done(rsp, rnp), j);


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