linux-kernel - Re: [RFC PATCH 06/15] nohz_task: Keep the tick if rcu needs it

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Message-ID: <20101220234934.GD1715@nowhere>
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.
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