linux-kernel - Re: [PATCH tip/core/rcu 2/2] rcu: Make expedited GPs handle CPU 0 being offline

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Message-ID: <20180626104447.GG9125@tardis>
Date:   Tue, 26 Jun 2018 18:44:47 +0800
From:   Boqun Feng <boqun.feng@...il.com>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
        linux-kernel@...r.kernel.org, mingo@...nel.org,
        jiangshanlai@...il.com, dipankar@...ibm.com,
        akpm@...ux-foundation.org, mathieu.desnoyers@...icios.com,
        josh@...htriplett.org, tglx@...utronix.de, rostedt@...dmis.org,
        dhowells@...hat.com, edumazet@...gle.com, fweisbec@...il.com,
        oleg@...hat.com, joel@...lfernandes.org
Subject: Re: [PATCH tip/core/rcu 2/2] rcu: Make expedited GPs handle CPU 0
 being offline

On Tue, Jun 26, 2018 at 11:38:20AM +0200, Peter Zijlstra wrote:
> On Mon, Jun 25, 2018 at 03:43:32PM -0700, Paul E. McKenney wrote:
> > +		preempt_disable();
> > +		for_each_leaf_node_possible_cpu(rnp, cpu) {
> > +			if (cpu_is_offline(cpu)) /* Preemption disabled. */
> > +				continue;
> 
> Create for_each_node_online_cpu() instead? Seems a bit pointless to
> iterate possible mask only to then check it against the online mask.
> Just iterate the online mask directly.
> 
> Or better yet, write this as:
> 
> 	preempt_disable();
> 	cpu = cpumask_next(rnp->grplo - 1, cpu_online_mask);
> 	if (cpu > rnp->grphi)
> 		cpu = WORK_CPU_UNBOUND;
> 	queue_work_on(cpu, rcu_par_gp_wq, &rnp->rew.rew_work);
> 	preempt_enable();
> 
> Which is what it appears to be doing.
> 

Make sense! Thanks ;-)

Applied this and running a TREE03 rcutorture. If all go well, I will
send the updated patch.

Regards,
Boqun

> > +			queue_work_on(cpu, rcu_par_gp_wq, &rnp->rew.rew_work);
> > +			rnp->exp_need_flush = true;
> > +			break;
> > +		}
> > +		preempt_enable();
> > +		if (!rnp->exp_need_flush) { /* All offline, report QSes. */
> > +			queue_work(rcu_par_gp_wq, &rnp->rew.rew_work);
> > +			rnp->exp_need_flush = true;
> > +		}
> >  	}
> >  
> >  	/* Wait for workqueue jobs (if any) to complete. */
> > -- 
> > 2.17.1
> > 

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