Re: [PATCH -tip] trace_workqueue: use percpu data for workqueue stat

Lai Jiangshan
2009-01-16T08:12:42Z

Frédéric Weisbecker wrote:
> Hi Lai,
>
> 2009/1/15 Lai Jiangshan <laijs@cn.fujitsu.com>:
>> Impact: make trace_workqueue works well on NUMA
>>
>> It's not correct when (num_possible_cpus() < nr_cpumask_bits):
>> all_workqueue_stat = kmalloc(sizeof(struct workqueue_global_stats)
>> * num_possible_cpus(), GFP_KERNEL);
>
>
> What is the difference between num_possible_cpus() and nr_cpumask_bits actually?
> It looks like nr_cpumask_bits binds to NR_CPUS on early time and after
> it is set to
> num_possible_cpus() , right?
> In this case num_possible_cpus() seems more relevant...no?
>
> (I'm pretty sure I'm wrong.... :-)
>

I wanted to reference to nr_cpu_ids, not nr_cpumask_bits(I made mistake yesterday)

init/main.c
static void __init setup_nr_cpu_ids(void)
{
nr_cpu_ids = find_last_bit(cpumask_bits(cpu_possible_mask),NR_CPUS) + 1;
}
setup_nr_cpu_ids() is called directly in main.c, it's earlier than early_initcall.

So nr_cpu_ids is better than num_possible_cpus(), for maybe cpu_possible_mask=101B
nr_cpu_ids=3, num_possible_cpus()=2, We will access to invalid memory when we use
num_possible_cpus().

but percpu data as my patch shows is better than nr_cpu_ids.

Thanks, Lai.