linux-kernel - Re: [RFC PATCH] rcu: don't ignore preempt

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Message-ID: <20100310022800.GG6203@linux.vnet.ibm.com>
Date:	Tue, 9 Mar 2010 18:28:00 -0800
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	Lai Jiangshan <laijs@...fujitsu.com>
Cc:	Ingo Molnar <mingo@...e.hu>, Peter Zijlstra <peterz@...radead.org>,
	Steven Rostedt <rostedt@...dmis.org>,
	Mathieu Desnoyers <mathieu.desnoyers@...ymtl.ca>,
	josh@...htriplett.org, LKML <linux-kernel@...r.kernel.org>,
	Frederic Weisbecker <fweisbec@...il.com>
Subject: Re: [RFC PATCH] rcu: don't ignore preempt_disable() in the idle loop

On Wed, Mar 10, 2010 at 10:13:56AM +0800, Lai Jiangshan wrote:
> Paul E. McKenney wrote:
> >>
> >> This is another try, it uses the fact that idle loops
> >> are executing with preept_count()=1.
> >> But I didn't look deep into all idle loops.
> > 
> > Hello, Lai!
> > 
> > One question below...
> > 
> > 						Thanx, Paul
> > 
> 
> [...]
> 
> >> +
> >> +	if ((preempt_count() & PREEMPT_MASK) > IDLE_CORE_LOOP_PREEMPT_COUNT)
> >> +		return 0;
> > 
> > How does this work in CONFIG_PREEMPT=n kernels?  I don't see how it
> > does, regardless of what preempt_count() returns in this case.
> 
> You are right, It cannot work in CONFIG_PREEMPT=n kernels.
> ignore this stupid patch.

Don't be too hard on yourself -- it is an interesting idea, just doesn't
seem to quite work out.  Perhaps it will lead you to another idea that
does work.

							Thanx, Paul
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