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Date: Tue, 21 Dec 2010 00:49:38 +0100 From: Frederic Weisbecker <fweisbec@...il.com> To: Peter Zijlstra <a.p.zijlstra@...llo.nl> Cc: LKML <linux-kernel@...r.kernel.org>, Thomas Gleixner <tglx@...utronix.de>, "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>, Ingo Molnar <mingo@...e.hu>, Steven Rostedt <rostedt@...dmis.org>, Lai Jiangshan <laijs@...fujitsu.com>, Andrew Morton <akpm@...ux-foundation.org>, Anton Blanchard <anton@....ibm.com>, Tim Pepper <lnxninja@...ux.vnet.ibm.com> Subject: Re: [RFC PATCH 06/15] nohz_task: Keep the tick if rcu needs it On Mon, Dec 20, 2010 at 04:58:20PM +0100, Peter Zijlstra wrote: > On Mon, 2010-12-20 at 16:24 +0100, Frederic Weisbecker wrote: > > > @@ -1634,7 +1633,7 @@ static int __rcu_pending(struct rcu_state *rsp, struct rcu_data *rdp) > > * by the current CPU, returning 1 if so. This function is part of the > > * RCU implementation; it is -not- an exported member of the RCU API. > > */ > > -static int rcu_pending(int cpu) > > +int rcu_pending(int cpu) > > /me wonders about that comment. Yeah I'll need to update that. > > { > > return __rcu_pending(&rcu_sched_state, &per_cpu(rcu_sched_data, cpu)) || > > __rcu_pending(&rcu_bh_state, &per_cpu(rcu_bh_data, cpu)) || > > diff --git a/kernel/sched.c b/kernel/sched.c > > index 6dbae46..45bd6e2 100644 > > --- a/kernel/sched.c > > +++ b/kernel/sched.c > > @@ -2470,10 +2470,16 @@ static void nohz_task_cpu_update(void *unused) > > int nohz_task_can_stop_tick(void) > > { > > struct rq *rq = this_rq(); > > + int cpu; > > > > if (rq->nr_running > 1) > > return 0; > > > > + cpu = smp_processor_id(); > > + > > + if (rcu_pending(cpu) || rcu_needs_cpu(cpu)) > > + return 0; > > Arguable, rcu_needs_cpu() should imply rcu_pending(), because if there's > work still to be done, it needs the cpu, hmm? We certainly need to change the naming there. rcu_needs_cpu() checks if we need to do something with the local callbacks. rcu_pending() checks if we the current CPU needs to notify quiescent states because a new grace period has started. So now that rcu_pending() is exported we probably need to refine the naming. rcu_callbacks_pending() and rcu_grace_period_pending(), or something like this. > > return 1; > > } > > > > This patch also implies you broke stuff with #4 because it would put the > machine to sleep while RCU still had bits to do, not very nice. Nope, the new config can only be built after [RFC PATCH 11/15] x86: Nohz task support I know I split up the patches in some unusual way but I did that on purpose: I wanted to have a finegrained patchset so that it's more reviewable than a big "core support" - "arch support" dual patch based style. But I ensured the new config can not be enabled before it's entirely buildable and has no known bugs. -- 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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