Re: [PATCH tip/core/rcu 6/7] rcu: Drive quiescent-state-forcing delay from HZ

["Paul E. McKenney" <paulmck@linux.vnet.ibm.com>]

On Fri, Apr 12, 2013 at 04:54:02PM -0700, Josh Triplett wrote:
> On Fri, Apr 12, 2013 at 04:19:13PM -0700, Paul E. McKenney wrote:
> > From: "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>
> > 
> > Systems with HZ=100 can have slow bootup times due to the default
> > three-jiffy delays between quiescent-state forcing attempts.  This
> > commit therefore auto-tunes the RCU_JIFFIES_TILL_FORCE_QS value based
> > on the value of HZ.  However, this would break very large systems that
> > require more time between quiescent-state forcing attempts.  This
> > commit therefore also ups the default delay by one jiffy for each
> > 256 CPUs that might be on the system (based off of nr_cpu_ids at
> > runtime, -not- NR_CPUS at build time).
> > 
> > Reported-by: Paul Mackerras <paulus@au1.ibm.com>
> > Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
> 
> Something seems very wrong if RCU regularly hits the fqs code during
> boot; feels like there's some more straightforward solution we're
> missing.  What causes these CPUs to fall under RCU's scrutiny during
> boot yet not actually hit the RCU codepaths naturally?

The problem is that they are running HZ=100, so that RCU will often
take 30-60 milliseconds per grace period.  At that point, you only
need 16-30 grace periods to chew up a full second, so it is not all
that hard to eat up the additional 8-12 seconds of boot time that
they were seeing.  IIRC, UP boot was costing them 4 seconds.

For HZ=1000, this would translate to 800ms to 1.2s, which is nowhere
near as annoying.

> Also, a comment below.
> 
> > --- a/kernel/rcutree.h
> > +++ b/kernel/rcutree.h
> > @@ -342,7 +342,17 @@ struct rcu_data {
> >  #define RCU_FORCE_QS		3	/* Need to force quiescent state. */
> >  #define RCU_SIGNAL_INIT		RCU_SAVE_DYNTICK
> >  
> > -#define RCU_JIFFIES_TILL_FORCE_QS	 3	/* for rsp->jiffies_force_qs */
> > +#if HZ > 500
> > +#define RCU_JIFFIES_TILL_FORCE_QS	 3	/* for jiffies_till_first_fqs */
> > +#elif HZ > 250
> > +#define RCU_JIFFIES_TILL_FORCE_QS	 2
> > +#else
> > +#define RCU_JIFFIES_TILL_FORCE_QS	 1
> > +#endif
> 
> This seems like it really wants to use a duration calculated directly
> from HZ; perhaps (HZ/100)?

Very possibly to the direct calculation, but HZ/100 would get 10 ticks
delay at HZ=1000, which is too high -- the value of 3 ticks for HZ=1000
works well.  But I could do something like this:

#define RCU_JIFFIES_TILL_FORCE_QS (((HZ + 199) / 300) + ((HZ + 199) / 300 ? 0 : 1))

Or maybe a bit better:

#define RCU_JTFQS_SE ((HZ + 199) / 300)
#define RCU_JIFFIES_TILL_FORCE_QS (RCU_JTFQS_SE + (RCU_JTFQS_SE ? 0 : 1))

This would come reasonably close to the values shown above.  Would
this work for you?

							Thanx, Paul