linux-kernel - Re: [PATCH tip/core/rcu 13/23] rcu: Control grace-period duration from sysfs

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Message-ID: <20120903093104.GB5574@leaf>
Date:	Mon, 3 Sep 2012 02:31:05 -0700
From:	Josh Triplett <josh@...htriplett.org>
To:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
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, eric.dumazet@...il.com, darren@...art.com,
	fweisbec@...il.com, sbw@....edu, patches@...aro.org,
	"Paul E. McKenney" <paul.mckenney@...aro.org>
Subject: Re: [PATCH tip/core/rcu 13/23] rcu: Control grace-period duration
 from sysfs

On Mon, Sep 03, 2012 at 02:30:16AM -0700, Josh Triplett wrote:
> On Thu, Aug 30, 2012 at 11:18:28AM -0700, Paul E. McKenney wrote:
> > From: "Paul E. McKenney" <paul.mckenney@...aro.org>
> > 
> > Some uses of RCU benefit from shorter grace periods, while others benefit
> > more from the greater efficiency provided by longer grace periods.
> > Therefore, this commit allows the durations to be controlled from sysfs.
> > There are two sysfs parameters, one named "jiffies_till_first_fqs" that
> > specifies the delay in jiffies from the end of grace-period initialization
> > until the first attempt to force quiescent states, and the other named
> > "jiffies_till_next_fqs" that specifies the delay (again in jiffies)
> > between subsequent attempts to force quiescent states.  They both default
> > to three jiffies, which is compatible with the old hard-coded behavior.
> > 
> > Signed-off-by: Paul E. McKenney <paul.mckenney@...aro.org>
> > Signed-off-by: Paul E. McKenney <paulmck@...ux.vnet.ibm.com>
> 
> Signed-off-by: Josh Triplett <josh@...htriplett.org>

Er, sorry, typo:
Reviewed-by: Josh Triplett <josh@...htriplett.org>

> >  Documentation/kernel-parameters.txt |   11 +++++++++++
> >  kernel/rcutree.c                    |   25 ++++++++++++++++++++++---
> >  2 files changed, 33 insertions(+), 3 deletions(-)
> > 
> > diff --git a/Documentation/kernel-parameters.txt b/Documentation/kernel-parameters.txt
> > index ad7e2e5..55ada04 100644
> > --- a/Documentation/kernel-parameters.txt
> > +++ b/Documentation/kernel-parameters.txt
> > @@ -2385,6 +2385,17 @@ bytes respectively. Such letter suffixes can also be entirely omitted.
> >  	rcutree.rcu_cpu_stall_timeout= [KNL,BOOT]
> >  			Set timeout for RCU CPU stall warning messages.
> >  
> > +	rcutree.jiffies_till_first_fqs= [KNL,BOOT]
> > +			Set delay from grace-period initialization to
> > +			first attempt to force quiescent states.
> > +			Units are jiffies, minimum value is zero,
> > +			and maximum value is HZ.
> > +
> > +	rcutree.jiffies_till_next_fqs= [KNL,BOOT]
> > +			Set delay between subsequent attempts to force
> > +			quiescent states.  Units are jiffies, minimum
> > +			value is one, and maximum value is HZ.
> > +
> >  	rcutorture.fqs_duration= [KNL,BOOT]
> >  			Set duration of force_quiescent_state bursts.
> >  
> > diff --git a/kernel/rcutree.c b/kernel/rcutree.c
> > index ed1be62..1d33240 100644
> > --- a/kernel/rcutree.c
> > +++ b/kernel/rcutree.c
> > @@ -226,6 +226,12 @@ int rcu_cpu_stall_timeout __read_mostly = CONFIG_RCU_CPU_STALL_TIMEOUT;
> >  module_param(rcu_cpu_stall_suppress, int, 0644);
> >  module_param(rcu_cpu_stall_timeout, int, 0644);
> >  
> > +static ulong jiffies_till_first_fqs = RCU_JIFFIES_TILL_FORCE_QS;
> > +static ulong jiffies_till_next_fqs = RCU_JIFFIES_TILL_FORCE_QS;
> > +
> > +module_param(jiffies_till_first_fqs, ulong, 0644);
> > +module_param(jiffies_till_next_fqs, ulong, 0644);
> > +
> >  static void force_qs_rnp(struct rcu_state *rsp, int (*f)(struct rcu_data *));
> >  static void force_quiescent_state(struct rcu_state *rsp);
> >  static int rcu_pending(int cpu);
> > @@ -1193,6 +1199,7 @@ static void rcu_gp_cleanup(struct rcu_state *rsp)
> >  static int rcu_gp_kthread(void *arg)
> >  {
> >  	int fqs_state;
> > +	unsigned long j;
> >  	int ret;
> >  	struct rcu_state *rsp = arg;
> >  	struct rcu_node *rnp = rcu_get_root(rsp);
> > @@ -1213,14 +1220,18 @@ static int rcu_gp_kthread(void *arg)
> >  
> >  		/* Handle quiescent-state forcing. */
> >  		fqs_state = RCU_SAVE_DYNTICK;
> > +		j = jiffies_till_first_fqs;
> > +		if (j > HZ) {
> > +			j = HZ;
> > +			jiffies_till_first_fqs = HZ;
> > +		}
> >  		for (;;) {
> > -			rsp->jiffies_force_qs = jiffies +
> > -						RCU_JIFFIES_TILL_FORCE_QS;
> > +			rsp->jiffies_force_qs = jiffies + j;
> >  			ret = wait_event_interruptible_timeout(rsp->gp_wq,
> >  					(rsp->gp_flags & RCU_GP_FLAG_FQS) ||
> >  					(!ACCESS_ONCE(rnp->qsmask) &&
> >  					 !rcu_preempt_blocked_readers_cgp(rnp)),
> > -					RCU_JIFFIES_TILL_FORCE_QS);
> > +					j);
> >  			/* If grace period done, leave loop. */
> >  			if (!ACCESS_ONCE(rnp->qsmask) &&
> >  			    !rcu_preempt_blocked_readers_cgp(rnp))
> > @@ -1234,6 +1245,14 @@ static int rcu_gp_kthread(void *arg)
> >  				cond_resched();
> >  				flush_signals(current);
> >  			}
> > +			j = jiffies_till_next_fqs;
> > +			if (j > HZ) {
> > +				j = HZ;
> > +				jiffies_till_next_fqs = HZ;
> > +			} else if (j < 1) {
> > +				j = 1;
> > +				jiffies_till_next_fqs = 1;
> > +			}
> >  		}
> >  
> >  		/* Handle grace-period end. */
> > -- 
> > 1.7.8
> > 
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