linux-kernel - Re: [PATCH tip/core/rcu 14/47] rcu: Limit lazy-callback duration

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Date:	Sat, 4 Feb 2012 15:32:25 +0100
From:	Frederic Weisbecker <fweisbec@...il.com>
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, josh@...htriplett.org,
	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, patches@...aro.org,
	"Paul E. McKenney" <paul.mckenney@...aro.org>
Subject: Re: [PATCH tip/core/rcu 14/47] rcu: Limit lazy-callback duration

On Sat, Feb 04, 2012 at 06:30:22AM -0800, Paul E. McKenney wrote:
> On Sat, Feb 04, 2012 at 02:54:58PM +0100, Frederic Weisbecker wrote:
> > On Fri, Feb 03, 2012 at 05:44:49PM -0800, Paul E. McKenney wrote:
> > > From: "Paul E. McKenney" <paul.mckenney@...aro.org>
> > > 
> > > Currently, a given CPU is permitted to remain in dyntick-idle mode
> > > indefinitely if it has only lazy RCU callbacks queued.  This is vulnerable
> > > to corner cases in NUMA systems, so limit the time to six seconds by
> > > default.  (Currently controlled by a cpp macro.)
> > > 
> > > Signed-off-by: Paul E. McKenney <paul.mckenney@...aro.org>
> > > Signed-off-by: Paul E. McKenney <paulmck@...ux.vnet.ibm.com>
> > > ---
> > >  kernel/rcutree_plugin.h |   12 +++++++++++-
> > >  1 files changed, 11 insertions(+), 1 deletions(-)
> > > 
> > > diff --git a/kernel/rcutree_plugin.h b/kernel/rcutree_plugin.h
> > > index eeb2cc6..8041310 100644
> > > --- a/kernel/rcutree_plugin.h
> > > +++ b/kernel/rcutree_plugin.h
> > > @@ -2050,6 +2050,9 @@ static void rcu_prepare_for_idle(int cpu)
> > >   *	number, be warned: Setting RCU_IDLE_GP_DELAY too high can hang your
> > >   *	system.  And if you are -that- concerned about energy efficiency,
> > >   *	just power the system down and be done with it!
> > > + * RCU_IDLE_LAZY_GP_DELAY gives the number of jiffies that a CPU is
> > > + *	permitted to sleep in dyntick-idle mode with only lazy RCU
> > > + *	callbacks pending.  Setting this too high can OOM your system.
> > >   *
> > >   * The values below work well in practice.  If future workloads require
> > >   * adjustment, they can be converted into kernel config parameters, though
> > > @@ -2058,11 +2061,13 @@ static void rcu_prepare_for_idle(int cpu)
> > >  #define RCU_IDLE_FLUSHES 5		/* Number of dyntick-idle tries. */
> > >  #define RCU_IDLE_OPT_FLUSHES 3		/* Optional dyntick-idle tries. */
> > >  #define RCU_IDLE_GP_DELAY 6		/* Roughly one grace period. */
> > > +#define RCU_IDLE_LAZY_GP_DELAY (6 * HZ)	/* Roughly six seconds. */
> > >  
> > >  static DEFINE_PER_CPU(int, rcu_dyntick_drain);
> > >  static DEFINE_PER_CPU(unsigned long, rcu_dyntick_holdoff);
> > >  static DEFINE_PER_CPU(struct hrtimer, rcu_idle_gp_timer);
> > > -static ktime_t rcu_idle_gp_wait;
> > > +static ktime_t rcu_idle_gp_wait;	/* If some non-lazy callbacks. */
> > > +static ktime_t rcu_idle_lazy_gp_wait;	/* If only lazy callbacks. */
> > >  
> > >  /*
> > >   * Allow the CPU to enter dyntick-idle mode if either: (1) There are no
> > > @@ -2151,6 +2156,8 @@ static void rcu_prepare_for_idle_init(int cpu)
> > >  		unsigned int upj = jiffies_to_usecs(RCU_IDLE_GP_DELAY);
> > >  
> > >  		rcu_idle_gp_wait = ns_to_ktime(upj * (u64)1000);
> > > +		upj = jiffies_to_usecs(6 * HZ);
> > 
> > I think you meant to use RCU_IDLE_LAZY_GP_DELAY here?
> 
> Good catch!!!  Indeed I do, fixed.
> 
> > 
> > > +		rcu_idle_lazy_gp_wait = ns_to_ktime(upj * (u64)1000);
> > >  		firsttime = 0;
> > >  	}
> > >  }
> > > @@ -2225,6 +2232,9 @@ static void rcu_prepare_for_idle(int cpu)
> > >  		if (rcu_cpu_has_nonlazy_callbacks(cpu))
> > >  			hrtimer_start(&per_cpu(rcu_idle_gp_timer, cpu),
> > >  				      rcu_idle_gp_wait, HRTIMER_MODE_REL);
> > > +		else
> > > +			hrtimer_start(&per_cpu(rcu_idle_gp_timer, cpu),
> > > +				      rcu_idle_lazy_gp_wait, HRTIMER_MODE_REL);
> > 
> > Might be a good idea to cancel it from rcu_cleanup_after_idle() to avoid
> > needless hrtimer interrupt (if something else woke up from idle before it
> > triggered).
> 
> Very much so!  And rcu_cleanup_after_idle() is in fact defined as:
> 
> static void rcu_cleanup_after_idle(int cpu)
> {
> 	hrtimer_cancel(&per_cpu(rcu_idle_gp_timer, cpu));
> }
> 
> The reason this does not appear in this patch is that it is already in
> mainline.

Oh I saw that in your branch but didn't realize this is the same timer that
can carry different functions. I see.

Thanks.
 
> > >  		return; /* Nothing more to do immediately. */
> > >  	} else if (--per_cpu(rcu_dyntick_drain, cpu) <= 0) {
> > >  		/* We have hit the limit, so time to give up. */
> > > -- 
> > > 1.7.8
> > > 
> > 
> 
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