linux-kernel - Re: [PATCH tip/core/rcu 13/13] rcu: Shrink TINY

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Message-ID: <20130521210957.GV3578@linux.vnet.ibm.com>
Date:	Tue, 21 May 2013 14:09:57 -0700
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	Josh Triplett <josh@...htriplett.org>
Cc:	linux-kernel@...r.kernel.org, mingo@...e.hu, laijs@...fujitsu.com,
	dipankar@...ibm.com, akpm@...ux-foundation.org,
	mathieu.desnoyers@...ymtl.ca, niv@...ibm.com, tglx@...utronix.de,
	peterz@...radead.org, rostedt@...dmis.org, Valdis.Kletnieks@...edu,
	dhowells@...hat.com, edumazet@...gle.com, darren@...art.com,
	fweisbec@...il.com, sbw@....edu
Subject: Re: [PATCH tip/core/rcu 13/13] rcu: Shrink TINY_RCU by reworking
 CPU-stall ifdefs

On Tue, May 21, 2013 at 10:31:41AM -0700, Josh Triplett wrote:
> On Mon, May 20, 2013 at 07:58:20AM -0700, Paul E. McKenney wrote:
> > From: "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
> > 
> > TINY_RCU's reset_cpu_stall_ticks() and check_cpu_stalls() functions
> > are defined unconditionally, and are empty functions if CONFIG_RCU_TRACE
> > is disabled (which in turns disables detection of RCU CPU stalls).
> > These empty functions can add a bit of bloat to TINY_RCU, so this
> > commit reworks the ifdefs so that these functions are defined only
> > if they actually do something.
> > 
> > Signed-off-by: Paul E. McKenney <paulmck@...ux.vnet.ibm.com>
> 
> Strange.  Both of these functions are static, and called in only one
> place, so shouldn't the compiler inline them (and thus throw them away
> completely when empty)?  Why does it not do so?

Good point for most architectures.  But given Tiny RCU's purpose, it
should allow for stupid compilers.  And I do need to fix the commit
message to call out the other benefit, namely saving a couple of
lines of source code.

							Thanx, Paul

> - Josh Triplett
> 
> > ---
> >  kernel/rcutiny.c        | 4 ++--
> >  kernel/rcutiny_plugin.h | 6 ++----
> >  2 files changed, 4 insertions(+), 6 deletions(-)
> > 
> > diff --git a/kernel/rcutiny.c b/kernel/rcutiny.c
> > index 4adc9e2..aa34411 100644
> > --- a/kernel/rcutiny.c
> > +++ b/kernel/rcutiny.c
> > @@ -204,7 +204,7 @@ static int rcu_is_cpu_rrupt_from_idle(void)
> >   */
> >  static int rcu_qsctr_help(struct rcu_ctrlblk *rcp)
> >  {
> > -	reset_cpu_stall_ticks(rcp);
> > +	RCU_TRACE(reset_cpu_stall_ticks(rcp));
> >  	if (rcp->rcucblist != NULL &&
> >  	    rcp->donetail != rcp->curtail) {
> >  		rcp->donetail = rcp->curtail;
> > @@ -251,7 +251,7 @@ void rcu_bh_qs(int cpu)
> >   */
> >  void rcu_check_callbacks(int cpu, int user)
> >  {
> > -	check_cpu_stalls();
> > +	RCU_TRACE(check_cpu_stalls());
> >  	if (user || rcu_is_cpu_rrupt_from_idle())
> >  		rcu_sched_qs(cpu);
> >  	else if (!in_softirq())
> > diff --git a/kernel/rcutiny_plugin.h b/kernel/rcutiny_plugin.h
> > index 65ef180..0cd385a 100644
> > --- a/kernel/rcutiny_plugin.h
> > +++ b/kernel/rcutiny_plugin.h
> > @@ -158,15 +158,11 @@ static void check_cpu_stall(struct rcu_ctrlblk *rcp)
> >  		rcp->jiffies_stall = jiffies + rcu_jiffies_till_stall_check();
> >  }
> >  
> > -#endif /* #ifdef CONFIG_RCU_TRACE */
> > -
> >  static void reset_cpu_stall_ticks(struct rcu_ctrlblk *rcp)
> >  {
> > -#ifdef CONFIG_RCU_TRACE
> >  	rcp->ticks_this_gp = 0;
> >  	rcp->gp_start = jiffies;
> >  	rcp->jiffies_stall = jiffies + rcu_jiffies_till_stall_check();
> > -#endif /* #ifdef CONFIG_RCU_TRACE */
> >  }
> >  
> >  static void check_cpu_stalls(void)
> > @@ -174,3 +170,5 @@ static void check_cpu_stalls(void)
> >  	RCU_TRACE(check_cpu_stall(&rcu_bh_ctrlblk));
> >  	RCU_TRACE(check_cpu_stall(&rcu_sched_ctrlblk));
> >  }
> > +
> > +#endif /* #ifdef CONFIG_RCU_TRACE */
> > -- 
> > 1.8.1.5
> > 
> --
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