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    SubjectRe: [regression] cpuset,mm: update tasks' mems_allowed in time (58568d2)
    On Tue, 23 Feb 2010, Miao Xie wrote:

    > > Cpu hotplug sets top_cpuset's cpus_allowed to cpu_active_mask by default,
    > > regardless of what was onlined or offlined. cpus_attach in the context of
    > > your patch (in cpuset_attach()) passes cpu_possible_mask to
    > > set_cpus_allowed_ptr() if the task is being attached to top_cpuset, my
    > > question was why don't we pass cpu_active_mask instead? In other words, I
    > > think we should do
    > >
    > > cpumask_copy(cpus_attach, cpu_active_mask);
    > >
    > > when attached to top_cpuset like my patch did.
    >
    > If we pass cpu_active_mask to set_cpus_allowed_ptr(), task->cpus_allowed just contains
    > the online cpus. In this way, if we do cpu hotplug(such as: online some cpu), we must
    > update cpus_allowed of all tasks in the top cpuset.
    >
    > But if we pass cpu_possible_mask, we needn't update cpus_allowed of all tasks in the
    > top cpuset. And when the kernel looks for a cpu for task to run, the kernel will use
    > cpu_active_mask to filter out offline cpus in task->cpus_allowed. Thus, it is safe.
    >

    That is terribly inconsistent between top_cpuset and all descendants; all
    other cpusets require that task->cpus_allowed be a subset of
    cpu_online_mask, including those descendants that allow all cpus (and all
    mems).


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