From mboxrd@z Thu Jan 1 00:00:00 1970 From: Eric Dumazet Subject: Re: Question about usage of RCU in the input layer Date: Fri, 20 Mar 2009 06:28:11 +0100 Message-ID: <49C3296B.1020209@cosmosbay.com> References: <20090318215812.15496a86@infradead.org> <20090319085628.GA6167@in.ibm.com> <20090319071841.63334eff@infradead.org> <20090320020750.GA6807@linux.vnet.ibm.com> <20090319202032.4c971d92@infradead.org> <20090320044541.GE6807@linux.vnet.ibm.com> Mime-Version: 1.0 Content-Type: text/plain; charset=ISO-8859-1 Content-Transfer-Encoding: QUOTED-PRINTABLE Return-path: In-Reply-To: <20090320044541.GE6807@linux.vnet.ibm.com> Sender: linux-kernel-owner@vger.kernel.org To: paulmck@linux.vnet.ibm.com Cc: Arjan van de Ven , dipankar@in.ibm.com, linux-input@vger.kernel.org, dmitry.torokhov@gmail.com, linux-kernel@vger.kernel.org List-Id: linux-input@vger.kernel.org Paul E. McKenney a =E9crit : > 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" 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 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() takin= g >>>>> 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 ?=20 >>>> 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 fo= r >>> 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 li= st. >> >> I don't see anything in the code that assumes this.. >=20 > I must let the networking guys sort this out. >=20 >>> 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...=20 >=20 > OK, I will ask the stupid question... >=20 > 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 short= cut > synchronize_rcu() whenever there is only one CPU is online, at least > for Classic RCU and Hierarchical RCU. >=20 Hmm... point is to make linux boot as fast as possible, so ... Use a special variant of udelay() in offending drivers that make approp= riate RCU call to increment quiescent state ?