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    SubjectRe: [PATCH -tip] trace_workqueue: use percpu data for workqueue stat
    On Fri, Jan 16, 2009 at 04:11:51PM +0800, Lai Jiangshan wrote:
    > 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().


    Oh ok I see now. If the cpu_possible_mask is 101, there is a risk I use cpu number 3
    later to directly find the index into the table. But I allocated the array in a
    contiguous way.

    Thanks I better understand now.

    > but percpu data as my patch shows is better than nr_cpu_ids.
    >
    > Thanks, Lai.
    >
    >
    >

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