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    SubjectRe: [BUG] kernel freezes with latest tree
    * Peter Zijlstra <peterz@infradead.org> wrote:

    > On Wed, 2012-01-11 at 10:04 +0100, Peter Zijlstra wrote:
    > > Maybe adding a few more NEED_BREAK bits
    > > and making it a counter and overflowing it into ABORT might be good.
    > >
    > >
    >
    > I could reproduce and confirm something like the below makes
    > the hang go-away. I haven't managed to fully understand why
    > we're stuck though because we do release the runqueue locks
    > and re-enable IRQs on this lock-break.

    Well, what happens if every CPU runs load_balance() and we keep
    triggering:

    if (loops++ > sysctl_sched_nr_migrate) {
    *lb_flags |= LBF_NEED_BREAK;
    break;
    }

    in this case load_balance() will do the retry:

    if (lb_flags & LBF_NEED_BREAK) {
    lb_flags &= ~LBF_NEED_BREAK;
    goto redo;
    }

    but the retry starts the loop again:

    list_for_each_entry_safe(p, n, &busiest_cfs_rq->tasks, se.group_node) {

    so nobody is able to make progress: livelock/lockup.

    ( This also explains why i was unable to see this in my
    randomized testing: my tests never extreme enough to trigger
    the sysctl_sched_nr_migrate threshold. )

    Thanks,

    Ingo


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