On Fri, Mar 20, 2009 at 06:50:58AM -0700, Arjan van de Ven wrote: > On Thu, 19 Mar 2009 21:45:41 -0700 > "Paul E. McKenney" <paulmck@xxxxxxxxxxxxxxxxxx> wrote: > > > > single CPU is soooo last decade ;-) > > > But seriously I no longer have systems that aren't dual core or SMT > > > in some form... > > > > OK, I will ask the stupid question... > > > > Why not delay bringing up the non-boot CPUs until later in boot? > > that'd be throwing out the baby with the bathwater... I'm trying to use > the other cpus to do some of the boot work (so that the total goes > faster); not using the other cpus would be counter productive to that. > (As is just sitting in synchronize_rcu() when the other cpu is > working.. hence this discussion ;-) OK, so you are definitely running multiple CPUs when the offending synchronize_rcu() executes, then? If so, here are some follow-on questions: 1. How many synchronize_rcu() calls are you seeing on the critical boot path and what value of HZ are you running? If each synchronize_rcu() is taking (say) tens of jiffies, then, as Peter Zijlstra notes earlier in this thread, we need to focus on what is taking too long to get through its RCU read-side critical sections. Otherwise, if each synchronize_rcu() is in the 3-5 jiffy range, I may finally be forced to create an expedited version of the synchronize_rcu() API. 2. If expediting is required, then the code calling synchronize_rcu() might or might not have any idea whether or not expediting is appropriate. If it does not, then we would need some sort of way to tell synchronize_rcu() that it should act more aggressively, perhaps /proc flag or kernel global variable indicating that boot is in progress. No, we do not want to make synchronize_rcu() aggressive all the time, as this would harm performance and energy efficiency in the normal runtime situation. So, if it turns out that synchronize_rcu()'s caller does not know whether or not expediting is appropriate, can the boot path manipulate such a flag or variable? 3. Which RCU implementation are you using? CONFIG_CLASSIC_RCU, CONFIG_TREE_RCU, or CONFIG_PREEMPT_RCU? Thanx, Paul -- To unsubscribe from this list: send the line "unsubscribe linux-input" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
