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Date: Wed, 24 Nov 2010 03:33:21 +0100 From: Frederic Weisbecker <fweisbec@...il.com> To: paulmck@...ux.vnet.ibm.com Cc: LKML <linux-kernel@...r.kernel.org>, Lai Jiangshan <laijs@...fujitsu.com>, Ingo Molnar <mingo@...e.hu>, Thomas Gleixner <tglx@...utronix.de>, Peter Zijlstra <a.p.zijlstra@...llo.nl>, Steven Rostedt <rostedt@...dmis.org> Subject: Re: [PATCH 1/2] rcu: Don't chase unnecessary quiescent states after extended grace periods 2010/11/24 Frederic Weisbecker <fweisbec@...il.com>: > On Tue, Nov 23, 2010 at 04:58:20PM -0800, Paul E. McKenney wrote: >> On Wed, Nov 24, 2010 at 01:31:12AM +0100, Frederic Weisbecker wrote: >> > When a cpu is in an extended quiescent state, which includes idle >> > nohz or CPU offline, others CPUs will take care of the grace periods >> > on its behalf. >> > >> > When this CPU exits its extended quiescent state, it will catch up >> > with the last started grace period and start chasing its own >> > quiescent states to end the current grace period. >> > >> > However in this case we always start to track quiescent states if the >> > grace period number has changed since we started our extended >> > quiescent state. And we do this because we always assume that the last >> > grace period is not finished and needs us to complete it, which is >> > sometimes wrong. >> > >> > This patch verifies if the last grace period has been completed and >> > if so, start hunting local quiescent states like we always did. >> > Otherwise don't do anything, this economizes us some work and >> > an unnecessary softirq. >> >> Interesting approach! I can see how this helps in the case where the >> CPU just came online, but I don't see it in the nohz case, because the >> nohz case does not update the rdp->completed variable. In contrast, >> the online path calls rcu_init_percpu_data() which sets up this variable. >> >> So, what am I missing here? >> >> Thanx, Paul >> >> PS. It might well be worthwhile for the online case alone, but >> the commit message does need to be accurate. > > > So, let's take this scenario (inspired from a freshly dumped trace to > clarify my ideas): > > CPU 1 was idle, it has missed several grace periods, but CPU 0 took care > of that. > > Hence, CPU 0's rdp->gpnum = rdp->completed = 4294967000 > > But the last grace period was 4294967002 and it's completed > (rnp->pgnum = rnp->completed = rsp->pgnum = 4294967002). > > Now CPU 0 gets a tick for a random reason, it calls rcu_check_callbacks() > and then rcu_pending() which raises the softirq because of this: > > /* Has another RCU grace period completed? */ > if (ACCESS_ONCE(rnp->completed) != rdp->completed) { /* outside lock */ > rdp->n_rp_gp_completed++; > return 1; > } > > The softirq fires, we call rcu_process_gp_end() which will > update rdp->completed into the global state: > (rsp->completed = rnp->pgnum = rnp->completed = rsp->pgnum = 4294967002). > > But rsp->pgnum is still 2 offsets backwards. > > Now we call rcu_check_quiescent_state() -> check_for_new_grace_period() > -> note_new_gpnum() and then we end up a requested quiescent state while > every grace periods are completed. Sorry I should have described that in the changelogs but my ideas weren't as clear as they are now (at least I feel they are, doesn't mean they actually are ;) Chasing these RCU bugs for too much hours has toasted my brain.. -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@...r.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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