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: <20151007154457.GL3910@linux.vnet.ibm.com>
Date:	Wed, 7 Oct 2015 08:44:57 -0700
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	Peter Zijlstra <peterz@...radead.org>
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 Wed, Oct 07, 2015 at 03:35:45PM +0200, Peter Zijlstra wrote:
> 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 :-)

Indeed!  ;-)

I will stick with the ?: for the moment, but your multiplied bang-up
approach might well grow on me.

							Thanx, Paul

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