linux-kernel - Re: [PATCH 22/32] rcu: Restart tick if we enqueue a callback in a nohz/cpuset CPU

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Message-ID: <20110817021824.GC32132@somewhere.redhat.com>
Date:	Wed, 17 Aug 2011 04:18:26 +0200
From:	Frederic Weisbecker <fweisbec@...il.com>
To:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
Cc:	LKML <linux-kernel@...r.kernel.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Anton Blanchard <anton@....ibm.com>,
	Avi Kivity <avi@...hat.com>, Ingo Molnar <mingo@...e.hu>,
	Lai Jiangshan <laijs@...fujitsu.com>,
	Paul Menage <menage@...gle.com>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Stephen Hemminger <shemminger@...tta.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Tim Pepper <lnxninja@...ux.vnet.ibm.com>
Subject: Re: [PATCH 22/32] rcu: Restart tick if we enqueue a callback in a
 nohz/cpuset CPU

On Tue, Aug 16, 2011 at 01:20:05PM -0700, Paul E. McKenney wrote:
> > @@ -1546,6 +1547,13 @@ __call_rcu(struct rcu_head *head, void (*func)(struct rcu_head *rcu),
> >  	rdp->nxttail[RCU_NEXT_TAIL] = &head->next;
> >  	rdp->qlen++;
> > 
> > +	/* Restart the timer if needed to handle the callbacks */
> > +	if (tick_nohz_adaptive_mode()) {
> > +		/* Make updates on nxtlist visible to self IPI */
> > +		barrier();
> > +		smp_cpuset_update_nohz(smp_processor_id());
> > +	}
> > +
> 
> But this must be happening in a system call or interrupt handler, right?
> If so, won't we get a chance to check things on exit from the system call
> or interrupt?  Or are you hooking only into syscall entry?

Sure in theory when we call call_rcu() we are in the kernel and thus not
very far from a syscall/exception/irq exit into userspace or a scheduling
out.

We could rely on that assumption and I bet it won't ever be a problem
in practice. I just wanted to ensure we don't spend too much time
before handling that callback.
 
> >  	/* If interrupts were disabled, don't dive into RCU core. */
> >  	if (irqs_disabled_flags(flags)) {
> >  		local_irq_restore(flags);
> > -- 
> > 1.7.5.4
> > 
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