linux-kernel - Re: CONFIG_PREEMPT

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Date:	Sun, 9 Aug 2009 04:32:28 +0100 (BST)
From:	Hugh Dickins <hugh.dickins@...cali.co.uk>
To:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
cc:	Hugh Dickins <hugh.dickins@...cali.co.uk>,
	Ingo Molnar <mingo@...e.hu>,
	Ben Herrenschmidt <benh@...nel.crashing.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	linux-kernel@...r.kernel.org
Subject: Re: CONFIG_PREEMPT_RCU in next/mmotm

On Sat, 8 Aug 2009, Paul E. McKenney wrote:
> On Sat, Aug 08, 2009 at 08:34:03PM +0100, Hugh Dickins wrote:
> 
> > CONFIG_PREEMPT_TREE=y seems okay on PowerPC; and when I briefly tried
> 
> CONFIG_TREE_RCU=y, you mean, right?

Whoops, yes indeed.

> > My guess is that there's some other issue which is triggering
> > the RCU disgrace, but that is just a guess.
> 
> The one other issue I know of is one that apparently happens only on
> one of Ingo's machines, which suddenly decides that it need not inform
> RCU when onlining a CPU (at boot time).
> 
> But you seem to be making it past boot, and I would guess that you are
> not doing any hotplug CPU operations, right?

That's right, I'm well past boot, and don't explicitly get into CPU
hotplug, nor using suspend/hibernate on that machine; and looking at
the config, see I don't even have CONFIG_HOTPLUG_CPU set.

In the morning I'll check that /proc/cpuinfo really shows all four
cpus before I start, and is still showing four cpus when it goes bad,
in case there's nevertheless an issue with that.

Is there some particular attribute of a cpu, most relevant to its RCU
grace status, that I can check on with a printk?

> 
> > Config attached.  Any suggestions?

[ Of course I forgot the config, but sent it just to Paul later. ]

> 
> /me scratches head...  Will try some more CONFIG_PREEMPT_RCU testing
> locally.

Thanks: I'm kind of glad that it's interesting,
though it may sink some of our time.

Hugh
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