linux-kernel - Re: [PATCH tip/core/rcu 04/18] rcu: Use single-stage IPI algorithm for RCU expedited grace period

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Message-ID: <20151007133545.GB3604@twins.programming.kicks-ass.net>
Date:	Wed, 7 Oct 2015 15:35:45 +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,
	jiangshanlai@...il.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 tip/core/rcu 04/18] rcu: Use single-stage IPI algorithm
 for RCU expedited grace period

On Tue, Oct 06, 2015 at 09:29:23AM -0700, Paul E. McKenney wrote:

> +/* Flags for rcu_preempt_ctxt_queue() decision table. */
> +#define RCU_GP_TASKS	0x8
> +#define RCU_EXP_TASKS	0x4
> +#define RCU_GP_BLKD	0x2
> +#define RCU_EXP_BLKD	0x1

Purely cosmetic, but that's backwards ;-) Most of our flags etc.. are in
increasing order.

> +static void rcu_preempt_ctxt_queue(struct rcu_node *rnp, struct rcu_data *rdp,
> +				   unsigned long flags) __releases(rnp->lock)
> +{
> +	int blkd_state = (rnp->gp_tasks ? RCU_GP_TASKS : 0) +
> +			 (rnp->exp_tasks ? RCU_EXP_TASKS : 0) +
> +			 (rnp->qsmask & rdp->grpmask ? RCU_GP_BLKD : 0) +
> +			 (rnp->expmask & rdp->grpmask ? RCU_EXP_BLKD : 0);

An alternative way is:

	int blkd_state = RCU_GP_TASKS  * !!rnp->gp_tasks +
			 RCU_EXP_TASKS * !!rnp->exp_tasks +
			 RCU_GP_BLKD   * !!(rnp->qsmask  & rdp->grpmask) +
			 RCU_EXP_BLKD  * !!(rnp->expmask & rdp->grpmask);

I suppose it depends on how your brain is wired which version reads
easier :-)
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