Paul E. McKenney a écrit : > 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. > Hmm... point is to make linux boot as fast as possible, so ... Use a special variant of udelay() in offending drivers that make appropriate RCU call to increment quiescent state ? -- 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
