On Tue, 16 Oct 2007, Andrew Morton wrote:

quoted text
> On Mon, 24 Sep 2007 14:08:53 -0700 travis@sgi.com wrote:

quoted text
> > cpu_sibling_map and cpu_core_map have been taken care of in
> > a prior patch.  This patch deals with the cpu_data array of
> > cpuinfo_x86 structs.  The model that was used in sparc64
> > architecture was adopted for x86.
> 
> This has mysteriously started to oops on me, only on x86_64.
> 
> http://userweb.kernel.org/~akpm/config-x.txt
> http://userweb.kernel.org/~akpm/dsc00001.jpg
> 
> which is a bit strange since this patch doesn't touch sched.c.  Maybe
> there's something somewhere else in the -mm lineup which when combined with
> this prevents it from oopsing, dunno.  I'll hold it back for now and will
> see what happens.

The config that you are using has

	CONFIG_SCHED_MC

and 

	CONFIG_SCHED_MT

set.

So we use cpu_corecroup_map() from arch/x86_64/kernel/smpboot.c in
cpu_to_phys_group that has these nice convoluted ifdefs:

static int cpu_to_phys_group(int cpu, const cpumask_t *cpu_map,
                             struct sched_group **sg)
{
        int group;
#ifdef CONFIG_SCHED_MC
        cpumask_t mask = cpu_coregroup_map(cpu);
        cpus_and(mask, mask, *cpu_map);
        group = first_cpu(mask);
#elif defined(CONFIG_SCHED_SMT)
        cpumask_t mask = per_cpu(cpu_sibling_map, cpu);
        cpus_and(mask, mask, *cpu_map);
        group = first_cpu(mask);
#else
        group = cpu;
#endif
        if (sg)
                *sg = &per_cpu(sched_group_phys, group);
        return group;
}

and I guess that some sched domain patches resulted in an empty
nodemask so that we end up with an invalid group number for the sched 
group?


/* maps the cpu to the sched domain representing multi-core */
cpumask_t cpu_coregroup_map(int cpu)
{
        struct cpuinfo_x86 *c = &cpu_data(cpu);
        /*
         * For perf, we return last level cache shared map.
         * And for power savings, we return cpu_core_map
         */
        if (sched_mc_power_savings || sched_smt_power_savings)
                return per_cpu(cpu_core_map, cpu);
        else
                return c->llc_shared_map;
}

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