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    SubjectRe: [Next] CPU Hotplug test failures on powerpc
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    On Tue, 2009-12-15 at 15:14 +0530, Sachin Sant wrote:
    > Benjamin Herrenschmidt wrote:
    > >> static void move_task_off_dead_cpu(int dead_cpu, struct task_struct *p)
    > >> {
    > >> int dest_cpu;
    > >> const struct cpumask *nodemask = cpumask_of_node(cpu_to_node(dead_cpu));
    > >>
    > >> again:
    > >> /* Look for allowed, online CPU in same node. */
    > >> for_each_cpu_and(dest_cpu, nodemask, cpu_active_mask)
    > >> if (cpumask_test_cpu(dest_cpu, &p->cpus_allowed))
    > >> goto move;
    > >>
    > >> /* Any allowed, online CPU? */
    > >> dest_cpu = cpumask_any_and(&p->cpus_allowed, cpu_active_mask);
    > >> if (dest_cpu < nr_cpu_ids)
    > >> goto move;
    > >>
    > >> /* No more Mr. Nice Guy. */
    > >> if (dest_cpu >= nr_cpu_ids) {
    > >> cpuset_cpus_allowed_locked(p, &p->cpus_allowed);
    > >> ====> dest_cpu = cpumask_any_and(cpu_active_mask, &p->cpus_allowed);
    > >>
    > >> /*
    > >> * Don't tell them about moving exiting tasks or
    > >> * kernel threads (both mm NULL), since they never
    > >> * leave kernel.
    > >> */
    > >> if (p->mm && printk_ratelimit()) {
    > >> pr_info("process %d (%s) no longer affine to cpu%d\n",
    > >> task_pid_nr(p), p->comm, dead_cpu);
    > >> }
    > >> }
    > >>
    > >> move:
    > >> /* It can have affinity changed while we were choosing. */
    > >> if (unlikely(!__migrate_task_irq(p, dead_cpu, dest_cpu)))
    > >> goto again;
    > >> }
    > >>
    > >> Both masks, p->cpus_allowed and cpu_active_mask are stable in that p
    > >> won't go away since we hold the tasklist_lock (in migrate_list_tasks),
    > >> and cpu_active_mask is static storage, so WTH is it going funny on?
    > >>
    > I added some debug statements within the above code.
    > This is a 2 cpu machine.
    >
    > XMON dest_cpu = 1024 . dead_cpu = 1 . nr_cpu_ids = 2
    > XMON dest_cpu = 1024
    > XMON dest_cpu = 1024 . dead_cpu = 1
    > XMON dest_cpu = 1024 . dead_cpu = 1 . nr_cpu_ids = 2
    > XMON dest_cpu = 1024
    > XMON dest_cpu = 1024 . dead_cpu = 1
    > XMON dest_cpu = 1024 . dead_cpu = 1 . nr_cpu_ids = 2
    > XMON dest_cpu = 1024
    > XMON dest_cpu = 1024 . dead_cpu = 1
    >
    > Seems to me that the control is stuck in an infinite loop and hence the
    > machine appears to be in hung state. The dest_cpu value is always 1024
    > and never changes, which result in an infinite loop.
    >
    > In working scenario the o/p is something on the following lines
    >
    > XMON dest_cpu = 1024 . dead_cpu = 1 . nr_cpu_ids = 2
    > XMON dest_cpu = 0
    > XMON dest_cpu = 1024 . dead_cpu = 1 . nr_cpu_ids = 2
    > XMON dest_cpu = 0
    > XMON dest_cpu = 1024 . dead_cpu = 1 . nr_cpu_ids = 2
    > XMON dest_cpu = 0
    >
    > Let me know if i should try to record any specific value ?

    Could you possibly print the two masks themselves? cpumask_scnprintf()
    and friend come in handy for this.

    The dest_cpu=1024 thing seem to suggest the intersection between
    p->cpus_allowed and cpu_active_mask is empty for some reason, even
    though we forcefully reset p->cpus_allowed to the full set using
    cpuset_cpus_allowed_locked().

    /me goes re-read the cpu_active_map code, this really shouldn't happen.


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