linux-kernel - Re: [PATCH RFC tip/core/rcu 11/11] rcu: move TREE

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Message-ID: <20110223151118.GF2163@linux.vnet.ibm.com>
Date:	Wed, 23 Feb 2011 07:11:18 -0800
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	Frederic Weisbecker <fweisbec@...il.com>
Cc:	linux-kernel@...r.kernel.org, mingo@...e.hu, laijs@...fujitsu.com,
	dipankar@...ibm.com, akpm@...ux-foundation.org,
	mathieu.desnoyers@...ymtl.ca, josh@...htriplett.org,
	niv@...ibm.com, tglx@...utronix.de, peterz@...radead.org,
	rostedt@...dmis.org, Valdis.Kletnieks@...edu, dhowells@...hat.com,
	eric.dumazet@...il.com, darren@...art.com,
	"Paul E. McKenney" <paul.mckenney@...aro.org>
Subject: Re: [PATCH RFC tip/core/rcu 11/11] rcu: move TREE_RCU from softirq
 to kthread

On Wed, Feb 23, 2011 at 03:44:24AM +0100, Frederic Weisbecker wrote:
> On Tue, Feb 22, 2011 at 05:39:40PM -0800, Paul E. McKenney wrote:
> > +static int rcu_node_kthread(void *arg)
> > +{
> > +	int cpu;
> > +	unsigned long flags;
> > +	unsigned long mask;
> > +	struct rcu_node *rnp = (struct rcu_node *)arg;
> > +	struct sched_param sp;
> > +	struct task_struct *t;
> > +
> > +	for (;;) {
> > +		wait_event_interruptible(rnp->node_wq, rnp->wakemask != 0 ||
> > +						       kthread_should_stop());
> > +		if (kthread_should_stop())
> > +			break;
> > +		raw_spin_lock_irqsave(&rnp->lock, flags);
> > +		mask = rnp->wakemask;
> > +		rnp->wakemask = 0;
> > +		raw_spin_unlock_irqrestore(&rnp->lock, flags);
> > +		for (cpu = rnp->grplo; cpu <= rnp->grphi; cpu++, mask <<= 1) {
> 
> I may be confused, but shouldn't it be mask >>= 1 instead?

You are not confused, but I sure was!  ;-)

> rnp->wakemask is the unioned rdp->grpmask of the cpu(s) for which we woke that
> node thread up. Those mask start from 0, so what you want with the below
> check is to watch if the next CPU in group range is in the wakeup mask by shifting
> to the right.
> 
> No?

Not only are you are quite correct, but this bug might well explain the
slowdown in grace-period latency that I was seeing in tests.

Thank you very much!!!

							Thanx, Paul

> > +			if ((mask & 0x1) == 0)
> > +				continue;
> > +			preempt_disable();
> > +			per_cpu(rcu_cpu_has_work, cpu) = 1;
> > +			t = per_cpu(rcu_cpu_kthread_task, cpu);
> > +			if (t == NULL) {
> > +				preempt_enable();
> > +				continue;
> > +			}
> > +			sp.sched_priority = RCU_KTHREAD_PRIO;
> > +			sched_setscheduler_nocheck(t, cpu, &sp);
> > +			wake_up_process(t);
> > +			preempt_enable();
> > +		}
> > +	}
> > +	return 0;
> > +}
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