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Date: Tue, 03 Mar 2009 16:42:18 +0800
From: Li Zefan <lizf@cn.fujitsu.com>
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To: Paul Menage <menage@google.com>
CC: Ingo Molnar <mingo@elte.hu>, Peter Zijlstra <a.p.zijlstra@chello.nl>,
       Andrew Morton <akpm@linux-foundation.org>,
       LKML <linux-kernel@vger.kernel.org>,
       Linux Containers <containers@lists.linux-foundation.org>
Subject: Re: [PATCH] Allow cpusets to be configured/built on non-SMP systems
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Paul Menage wrote:
> On Mon, Mar 2, 2009 at 10:01 PM, Paul Menage <menage@google.com> wrote:
>> On Mon, Mar 2, 2009 at 7:17 PM, Li Zefan <lizf@cn.fujitsu.com> wrote:
>>>> +static int generate_sched_domains(struct cpumask **domains,
>>>> +                     struct sched_domain_attr **attributes)
>>>> +{
>>> Except here should "return 0;", otherwise emit a compile warining.
>>>
>> Good catch - the weird thing is that (in my UML build) it doesn't
>> actually generate that warning. Mysterious.
>>
>> I'll resend with the extra return.
> 
> After looking at the sched domains code it's not clear to me that
> returning 0 is necessarily the right thing to do -
> partition_sched_domains() says that 0 is a special case used for
> destroying existing domains? Would returning 1 and setting up a single
> dummy domain be better?
> 

Yes, return 1 seems more reasonable. And if we do this, should we also set
*domains to NULL like this?

static int generate_sched_domains(struct cpumask **domains,
                     struct sched_domain_attr **attributes)
{
	*domains = NULL;
	return 1;
}

because otherwise partition_sched_domains() will access invalid memory:

void partition_sched_domains(int ndoms_new, struct cpumask *doms_new,
			     struct sched_domain_attr *dattr_new)
{
	...
	n = doms_new ? ndoms_new : 0;

	for (i = 0; i < ndoms_cur; i++) {
		for (j = 0; j < n && !new_topology; j++) {
			// *****here*****/
			if (cpumask_equal(&doms_cur[i], &doms_new[j])
			    && dattrs_equal(dattr_cur, i, dattr_new, j))
				goto match1;
		}
	...
	}

> Given that this return code only matters when CONFIG_HOTPLUG_CPU &&
> !CONFIG_SMP it's unlikely to ever be used

That's why I didn't comment on this.

> but I guess it's better to get it right.
> 

But I agree with you. :)