linux-kernel - Re: [PATCH RFC tip/core/rcu] accelerate grace period if last non-dynticked CPU

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Message-ID: <1264432097.31321.409.camel@gandalf.stny.rr.com>
Date:	Mon, 25 Jan 2010 10:08:17 -0500
From:	Steven Rostedt <rostedt@...dmis.org>
To:	Lai Jiangshan <laijs@...fujitsu.com>
Cc:	paulmck@...ux.vnet.ibm.com, linux-kernel@...r.kernel.org,
	mingo@...e.hu, dipankar@...ibm.com, akpm@...ux-foundation.org,
	mathieu.desnoyers@...ymtl.ca, josh@...htriplett.org,
	dvhltc@...ibm.com, niv@...ibm.com, tglx@...utronix.de,
	peterz@...radead.org, Valdis.Kletnieks@...edu, dhowells@...hat.com
Subject: Re: [PATCH RFC tip/core/rcu] accelerate grace period if last
 non-dynticked CPU

On Mon, 2010-01-25 at 20:28 +0800, Lai Jiangshan wrote:
> Paul E. McKenney wrote:

> > +	/* Don't bother unless we are the last non-dyntick-idle CPU. */
> > +	for_each_cpu(thatcpu, nohz_cpu_mask)
> > +		if (thatcpu != cpu)
> > +			return rcu_needs_cpu_quick_check(cpu);
> 
> The comment and the code are not the same, I think.
> 

I once heard this quote, but I don't know who said it:

"If the comment and the code do not match, they probably are both wrong"

Anyway, you are correct, the comment does not match, but I think the
code is wrong. The code returns if any cpu is in non-dyntick-idle state.
Reading the change log, that looks wrong. Perhaps this is what was
needed:


	for_each_online_cpu(thatcpu) {
		if (thatcpu != cpu && !cpumask_test_cpu(thatcpu, nohz_cpu_mask)
			return rcu_needs_cpu_quick_check(cpu);
	}

-- Steve


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