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    SubjectRe: [PATCH] sched: Simplify cpu-hot-unplug task migration
    On 11/17, Peter Zijlstra wrote:
    >
    > On Wed, 2010-11-17 at 20:27 +0100, Oleg Nesterov wrote:
    >
    > > > -static void migrate_dead_tasks(unsigned int dead_cpu)
    > > > -{
    > > > - struct rq *rq = cpu_rq(dead_cpu);
    > > > - struct task_struct *next;
    > > > + rq->stop = NULL;
    > >
    > > (or we could do current->state = TASK_INTERRUPTIPLE, afaics)
    >
    > Ah, you missed a patch that made pick_next_task_stop() look like:
    >
    > static struct task_struct *pick_next_task_stop(struct rq *rq)
    > {
    > struct task_struct *stop = rq->stop;
    >
    > if (stop && stop->se.on_rq)

    Yes, thanks.

    > > > for ( ; ; ) {
    > > > - if (!rq->nr_running)
    > > > + /*
    > > > + * There's this thread running, bail when that's the only
    > > > + * remaining thread.
    > > > + */
    > > > + if (rq->nr_running == 1)
    > > > break;
    > >
    > > I was very much confused, and I was going to say this is wrong.
    > > However, now I think this is correct, just the comment is not
    > > right.
    > >
    > > There is another running thread we should not migrate, rq->idle.
    > > If nothing else, dequeue_task_idle() should be never called.
    >
    > In fact, dequeue_task_idle() will yell if you try that ;-)
    >
    > > But, if I understand correctly, ->nr_running does not account
    > > the idle thread, and this is what makes this correct.
    > >
    > > Correct?
    >
    > Right, I can add: (the idle thread is not counted in nr_running), if
    > that makes things clearer for you; however its a quite fundamental
    > property,

    Yes, I see now.

    OK, this also explains my previous questions. I greatly misunderstood
    this "small detail", starting from your initial patch. Every time I
    thought you are trying to migrate rq->idle as well.

    Thanks Peter. Only one question,

    > @@ -253,9 +246,12 @@ static int __ref _cpu_down(unsigned int
    > }
    > BUG_ON(cpu_online(cpu));
    >
    > - /* Wait for it to sleep (leaving idle task). */
    > - while (!idle_cpu(cpu))
    > - yield();
    > + /*
    > + * The migration_call() CPU_DYING callback will have removed all
    > + * runnable tasks from the cpu, there's only the idle task left now
    > + * that the migration thread is done doing the stop_machine thing.
    > + */
    > + BUG_ON(!idle_cpu(cpu));

    I am not sure.

    Yes, we know for sure rhat the only runnable task is rq->idle.
    But only after migration thread calls schedule() and switches to the
    idle thread.

    However, I see nothing which can guarantee this. Migration thread
    running on the dead cpu wakes up the caller of stop_cpus() before
    it calls schedule(), _cpu_down() can check rq->curr before it was
    changed.

    No?



    Hmm. In fact, I think it is possible that cpu_stopper_thread() can
    have more cpu_stop_work's queued when __stop_machine() returns.
    This has nothing to do with this patch, but I think it makes sense
    to clear stopper->enabled at CPU_DYING stage as well (of course,
    this needs a separate patch).

    Oleg.



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