Messages in this thread | | | Date | Wed, 11 Jan 2012 16:56:59 +0100 | From | Ingo Molnar <> | Subject | Re: [BUG] kernel freezes with latest tree |
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* 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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