Re: [PATCH 00/18] sched: balance callbacks v4

Byungchul Park
2015-12-29T05:41:42Z

On Thu, Jun 11, 2015 at 02:46:36PM +0200, Peter Zijlstra wrote:
> Mike stumbled over a cute bug where the RT/DL balancing ops caused a bug.

Hello.

We also faced this bug in our embedded product using stable tree. Eventually,
we found that these patch set exactly dealt with the problem. Now, I am just
curious about if you have any plans to merge this to stable trees to make
the stable trees stabler.

Hm?

>
> The exact scenario is __sched_setscheduler() changing a (runnable) task from
> FIFO to OTHER. In swiched_from_rt(), where we do pull_rt_task() we temporarity
> drop rq->lock. This gap allows regular cfs load-balancing to step in and
> migrate our task.
>
> However, check_class_changed() will happily continue with switched_to_fair()
> which assumes our task is still on the old rq and makes the kernel go boom.
>
> Instead of trying to patch this up and make things complicated; simply disallow
> these methods to drop rq->lock and extend the current post_schedule stuff into
> a balancing callback list, and use that.
>
> This survives Mike's testcase.
>
> Changes since -v3:
> - reworked the hrtimer stuff, again. -- Kirill, Oleg
> - small changes to the new lockdep stuff
>
> Changes since -v2:
> - reworked the hrtimer patch. -- Kirill, tglx
> - added lock pinning
>
> Changes since -v1:
> - make SMP=n build,
> - cured switched_from_dl()'s cancel_dl_timer().
>
> no real tests on the new parts other than booting / building kernels.
>
>
>
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