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

[Josh Triplett <josh@joshtriplett.org>]

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