From mboxrd@z Thu Jan 1 00:00:00 1970 From: Ingo Molnar Subject: Re: [PATCH RFC] v2 expedited "big hammer" RCU grace periods Date: Sun, 26 Apr 2009 22:22:55 +0200 Message-ID: <20090426202255.GA5365@elte.hu> References: <20090426052340.GA24931@linux.vnet.ibm.com> <20090426112717.GE10391@elte.hu> <20090426191343.GC29238@Krystal> Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Cc: "Paul E. McKenney" , linux-kernel@vger.kernel.org, netdev@vger.kernel.org, netfilter-devel@vger.kernel.org, akpm@linux-foundation.org, torvalds@linux-foundation.org, davem@davemloft.net, dada1@cosmosbay.com, zbr@ioremap.net, jeff.chua.linux@gmail.com, paulus@samba.org, laijs@cn.fujitsu.com, jengelh@medozas.de, r000n@r000n.net, benh@kernel.crashing.org To: Mathieu Desnoyers Return-path: Content-Disposition: inline In-Reply-To: <20090426191343.GC29238@Krystal> Sender: linux-kernel-owner@vger.kernel.org List-Id: netfilter-devel.vger.kernel.org * Mathieu Desnoyers wrote: > * Ingo Molnar (mingo@elte.hu) wrote: > > > > * Paul E. McKenney wrote: > > > > > Second cut of "big hammer" expedited RCU grace periods, but only > > > for rcu_bh. This creates another softirq vector, so that entering > > > this softirq vector will have forced an rcu_bh quiescent state (as > > > noted by Dave Miller). Use smp_call_function() to invoke > > > raise_softirq() on all CPUs in order to cause this to happen. > > > Track the CPUs that have passed through a quiescent state (or gone > > > offline) with a cpumask. > > > > hm, i'm still asking whether doing this would be simpler via a > > reschedule vector - which not only is an existing facility but also > > forces all RCU domains through a quiescent state - not just bh-RCU > > participants. > > > > Triggering a new softirq is in no way simpler that doing an SMP > > cross-call - in fact softirqs are a finite resource so using some > > other facility would be preferred. > > > > Am i missing something? > > > > I think the reason for this whole thread is that waiting for rcu > quiescent state, when called many times e.g. in multiple iptables > invokations, takes too longs (5 seconds to load the netfilter > rules at boot). [...] I'm aware of the problem space. I was suggesting that to trigger the quiescent state and to wait for it to propagate it would be enough to reuse the reschedule mechanism. It would be relatively straightforward: first a send-reschedule then do a wait_task_context_switch() on rq->curr - both are existing primitives. (a task reference has to be taken but that's pretty much all) By the time wait_task_context_switch() returns from the last CPU we know that the quiescent state has passed. Ingo