Messages in this thread | | | Date | Fri, 12 Apr 2013 16:54:02 -0700 | From | Josh Triplett <> | Subject | Re: [PATCH tip/core/rcu 6/7] rcu: Drive quiescent-state-forcing delay from HZ |
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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?
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)?
- Josh Triplett
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