On Fri, 20 Mar 2009 07:31:04 -0700 "Paul E. McKenney" <paulmck@xxxxxxxxxxxxxxxxxx> wrote: > > > > 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? absolutely. (and I'm using bootgraph.pl in scripts to track who's stalling etc) > > If so, here are some follow-on questions: > > 1. How many synchronize_rcu() calls are you seeing on the > critical boot path I've seen only this (input) one to take a long time > and what value of HZ are you running? 1000 > > 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 I know that "the other guy" is not optimal and takes waaay too long. > 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. I think a simplified API for the "add to a list" case might make sense. Because the request isn't for a full sync for sure... (independent of that .. the open question is if this specific case is even needed; I think the code confused "send to others" with "wait until everyone sees"; afaik synchronize_rcu() has no pushing behavior at all, nor should it) > > 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? CLASSIC -- Arjan van de Ven Intel Open Source Technology Centre For development, discussion and tips for power savings, visit http://www.lesswatts.org -- 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
