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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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