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?
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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