Messages in this thread | | | Date | Tue, 11 Feb 2014 16:34:38 +0100 (CET) | From | Thomas Gleixner <> | Subject | Re: [PATCH 1/6 v2] sched: Init idle->on_rq in init_idle() |
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On Tue, 11 Feb 2014, Peter Zijlstra wrote:
> On Tue, Feb 11, 2014 at 10:17:58AM +0100, Sebastian Andrzej Siewior wrote: > > The init task is state TASK_RUNNING and on_irq should be set to 1. It won't > > ^^^ irq? :-) > > > be set by scheduler because the idle task is never woken up, it is always the > > task we fall back to if there is no other task pending. > > But why? Who cares? I mean, its true.. but what problem does it solve. > Why did Thomas write this patch.
I could slap myself for not writing a proper changelog right away. It took me some time to figure out why it was added in the first place, why it's not longer necessary and why I kept it.
We stumbled in RT over a SMP bringup issue on ARM where the idle->on_rq == 0 was causing try_to_wakeup() on the other cpu to run into nada land.
After adding that idle->on_rq = 1; I was able to find the root cause of the lockup: the idle task on the newly woken up cpu was fiddling with a sleeping spinlock, which is a nono.
I kept the init of idle->on_rq to keep the state consistent and to avoid another long lasting debug session.
As a side note, the whole debug mess could have been avoided if might_sleep() would have yelled when called from the idle task. That's fixed with patch 2/6 - and that one actually has a changelog :)
Thanks,
tglx
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