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" <paul...@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 <pau...@au1.ibm.com>
> > Signed-off-by: Paul E. McKenney <paul...@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
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