From mboxrd@z Thu Jan 1 00:00:00 1970 From: "Paul E. McKenney" Subject: Re: [PATCH 2/2] ipvs: Use cond_resched_rcu_lock() helper when dumping connections Date: Mon, 29 Apr 2013 14:30:02 -0700 Message-ID: <20130429213002.GP3780@linux.vnet.ibm.com> References: <1366940708-10180-1-git-send-email-horms@verge.net.au> <1366940708-10180-3-git-send-email-horms@verge.net.au> <20130426080313.GC8669@dyad.programming.kicks-ass.net> <20130426154547.GC3860@linux.vnet.ibm.com> <20130426171948.GA31467@dyad.programming.kicks-ass.net> <20130426174815.GI3860@linux.vnet.ibm.com> <1367000815.8964.243.camel@edumazet-glaptop> <20130427162049.GB3780@linux.vnet.ibm.com> Reply-To: paulmck@linux.vnet.ibm.com Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Cc: Eric Dumazet , Peter Zijlstra , Simon Horman , Ingo Molnar , lvs-devel@vger.kernel.org, netdev@vger.kernel.org, netfilter-devel@vger.kernel.org, linux-kernel@vger.kernel.org, Pablo Neira Ayuso , Dipankar Sarma , dhaval.giani@gmail.com To: Julian Anastasov Return-path: Content-Disposition: inline In-Reply-To: Sender: linux-kernel-owner@vger.kernel.org List-Id: netfilter-devel.vger.kernel.org On Tue, Apr 30, 2013 at 12:08:18AM +0300, Julian Anastasov wrote: > > Hello, > > On Sat, 27 Apr 2013, Paul E. McKenney wrote: > > > On Sat, Apr 27, 2013 at 02:32:48PM +0300, Julian Anastasov wrote: > > > > > > So, I assume, to help realtime kernels and rcu_barrier > > > it is not a good idea to guard rcu_read_unlock with checks. > > > I see that rcu_read_unlock will try to reschedule in the > > > !CONFIG_PREEMPT_RCU case (via preempt_enable), can we > > > use ifdefs to avoid double TIF_NEED_RESCHED check?: > > > > > > rcu_read_unlock(); > > > #if !defined(CONFIG_PREEMPT) || defined(CONFIG_PREEMPT_RCU) > > > > I would instead suggest something like: > > > > #ifndef CONFIG_PREEMPT_RCU > > > > But yes, in the CONFIG_PREEMPT_RCU case, the cond_resched() is not > > needed. > > Hm, is this correct? If I follow the ifdefs > preempt_schedule is called when CONFIG_PREEMPT is > defined _and_ CONFIG_PREEMPT_RCU is not defined. > Your example for CONFIG_PREEMPT_RCU is the opposite to this? Yep, I really did intend to say "#ifndef CONFIG_PREEMPT_RCU". A couple of things to keep in mind: 1. Although rcu_read_unlock() does map to preempt_enable() for CONFIG_TINY_RCU and CONFIG_TREE_RCU, the current Kconfig refuses to allow either CONFIG_TINY_RCU or CONFIG_TREE_RCU to be selected if CONFIG_PREEMPT=y. 2. In the CONFIG_PREEMPT_RCU case, __rcu_read_unlock() will check to see if the RCU core needs to be informed, so there is no need to invoke cond_resched() in that case. 3. If we drop your "|| defined(CONFIG_PREEMPT_RCU)", we get an almost-synonym for my "#ifndef CONFIG_PREEMPT_RCU". The "almost" applies to older kernels due to the possibility of having a CONFIG_TINY_PREEMPT_RCU kernel -- but this possibility is going away soon. Make sense? Thanx, Paul > > > cond_resched(); > > > #endif > > > rcu_read_lock(); > > Regards > > -- > Julian Anastasov >