linux-kernel - Re: [PATCH tip/core/rcu 1/3] rcu: Optimize rcu_is_nocb_cpu() for RCU_NOCB_CPU

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Message-ID: <20140217233331.GR4250@linux.vnet.ibm.com>
Date:	Mon, 17 Feb 2014 15:33:31 -0800
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	Josh Triplett <josh@...htriplett.org>
Cc:	linux-kernel@...r.kernel.org, mingo@...nel.org,
	laijs@...fujitsu.com, dipankar@...ibm.com,
	akpm@...ux-foundation.org, mathieu.desnoyers@...icios.com,
	niv@...ibm.com, tglx@...utronix.de, peterz@...radead.org,
	rostedt@...dmis.org, dhowells@...hat.com, edumazet@...gle.com,
	darren@...art.com, fweisbec@...il.com, oleg@...hat.com, sbw@....edu
Subject: Re: [PATCH tip/core/rcu 1/3] rcu: Optimize rcu_is_nocb_cpu() for
 RCU_NOCB_CPU_ALL

On Mon, Feb 17, 2014 at 01:50:53PM -0800, Josh Triplett wrote:
> On Mon, Feb 17, 2014 at 01:40:25PM -0800, Paul E. McKenney wrote:
> > From: "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
> > 
> > If CONFIG_RCU_NOCB_CPU_ALL=y, then rcu_is_nocb_cpu() will always
> > return true, however, the current version nevertheless checks
> > rcu_nocb_mask.  This commit therefore creates a static inline
> > implementation of rcu_is_nocb_cpu() that unconditionally returns
> > true when CONFIG_RCU_NOCB_CPU_ALL=y.
> > 
> > Signed-off-by: Paul E. McKenney <paulmck@...ux.vnet.ibm.com>
> 
> One suggestion below.  With or without that change:
> Reviewed-by: Josh Triplett <josh@...htriplett.org>

I like the change -- applied both it and your Reviewed-by.

							Thanx, Paul

> >  include/linux/rcupdate.h | 4 ++++
> >  kernel/rcu/tree_plugin.h | 2 ++
> >  2 files changed, 6 insertions(+)
> > 
> > diff --git a/include/linux/rcupdate.h b/include/linux/rcupdate.h
> > index 72bf3a01a4ee..92365b1fede4 100644
> > --- a/include/linux/rcupdate.h
> > +++ b/include/linux/rcupdate.h
> > @@ -1016,7 +1016,11 @@ static inline notrace void rcu_read_unlock_sched_notrace(void)
> >  	__kfree_rcu(&((ptr)->rcu_head), offsetof(typeof(*(ptr)), rcu_head))
> >  
> >  #ifdef CONFIG_RCU_NOCB_CPU
> > +#ifdef CONFIG_RCU_NOCB_CPU_ALL
> > +static inline bool rcu_is_nocb_cpu(int cpu) { return true; }
> > +#else /* #ifdef CONFIG_RCU_NOCB_CPU_ALL */
> >  bool rcu_is_nocb_cpu(int cpu);
> > +#endif /* #else #ifdef CONFIG_RCU_NOCB_CPU_ALL */
> >  #else
> >  static inline bool rcu_is_nocb_cpu(int cpu) { return false; }
> >  #endif /* #else #ifdef CONFIG_RCU_NOCB_CPU */
> 
> This seem clearer if written without the nesting:
> 
> #if defined(CONFIG_RCU_NOCB_CPU_ALL)
> static inline bool rcu_is_nocb_cpu(int cpu) { return true; }
> #elif defined(CONFIG_RCU_NOCB_CPU)
> bool rcu_is_nocb_cpu(int cpu);
> #else
> static inline bool rcu_is_nocb_cpu(int cpu) { return false; }
> #endif
> 
> That clearly distinguishes the three cases: all, some, none.
> 
> > diff --git a/kernel/rcu/tree_plugin.h b/kernel/rcu/tree_plugin.h
> > index 6e2ef4b2b920..39a50b918bff 100644
> > --- a/kernel/rcu/tree_plugin.h
> > +++ b/kernel/rcu/tree_plugin.h
> > @@ -2101,6 +2101,7 @@ static void rcu_init_one_nocb(struct rcu_node *rnp)
> >  	init_waitqueue_head(&rnp->nocb_gp_wq[1]);
> >  }
> >  
> > +#ifndef CONFIG_RCU_NOCB_CPU_ALL
> >  /* Is the specified CPU a no-CPUs CPU? */
> >  bool rcu_is_nocb_cpu(int cpu)
> >  {
> > @@ -2108,6 +2109,7 @@ bool rcu_is_nocb_cpu(int cpu)
> >  		return cpumask_test_cpu(cpu, rcu_nocb_mask);
> >  	return false;
> >  }
> > +#endif /* #ifndef CONFIG_RCU_NOCB_CPU_ALL */
> >  
> >  /*
> >   * Enqueue the specified string of rcu_head structures onto the specified
> > -- 
> > 1.8.1.5
> > 
> 

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