On Thu, Mar 19, 2009 at 08:20:32PM -0700, Arjan van de Ven wrote: > On Thu, 19 Mar 2009 19:07:50 -0700 > "Paul E. McKenney" <paulmck@xxxxxxxxxxxxxxxxxx> wrote: > > > On Thu, Mar 19, 2009 at 07:18:41AM -0700, Arjan van de Ven wrote: > > > On Thu, 19 Mar 2009 14:26:28 +0530 > > > Dipankar Sarma <dipankar@xxxxxxxxxx> wrote: > > > > > > > On Wed, Mar 18, 2009 at 09:58:12PM -0700, Arjan van de Ven wrote: > > > > > Hi, > > > > > > > > > > the input layer does a "synchronize_rcu()" after a > > > > > list_add_tail_rcu(), which is costing me 1 second of boot > > > > > time..... And based on my understanding of the RCU concept, you > > > > > only need to synchronize on delete, not on addition... so I > > > > > think the synchronize is entirely redundant here... > > > > > > > > The more appropriate question is - why is synchronize_rcu() taking > > > > 1 second ? Any idea what the other CPUs are doing at the time > > > > of calling synchronize_rcu() ? > > > > > > one cpu is doing a lot of i2c traffic which is a bunch of udelay()s > > > in loops.. then it does quite a bit of uncached memory access, and > > > the lot takes quite while. > > > > > > > What driver is this ? How early > > > > in the boot is this happening ? > > > > > > during kernel boot. > > > > > > I suppose my question is also more generic.. why synchronize when > > > it's not needed? At least based on my understanding of RCU (but > > > you're the expert), you don't need to synchronize for an add, only > > > between a delete and a (k)free..... > > > > I don't claim to understand the code in question, so it is entirely > > possible that the following is irrelevant. But one other reason for > > synchronize_rcu() is: > > > > 1. Make change. > > > > 2. synchronize_rcu() > > > > 3. Now you are guaranteed that all CPUs/tasks/whatever > > currently running either are not messing with you on the one hand, or > > have seen the change on the other. > > ok so this is for the case where someone is already iterating the list. > > I don't see anything in the code that assumes this.. I must let the networking guys sort this out. > > It sounds like you are seeing these delays later in boot, however, > > yeah it's during driver init/ > > > Alternatively, again assuming a single-CPU system > > 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? The first patch in my earlier email (which is in mainline) will shortcut synchronize_rcu() whenever there is only one CPU is online, at least for Classic RCU and Hierarchical 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
