Re: floppy.c soft lockup

Mark Hounschell
2007-06-04T14:01:15Z

Oleg Nesterov wrote:
> On 06/02, Mark Hounschell wrote:
>> Jun 2 16:36:11 harley kernel: ERR!! events/1 flush hang: c201dbc0
>> c201dbc0 10012 10012
>> Jun 2 16:36:11 harley kernel: CURR: 7974 7974 vrsx 93 26
>> Jun 2 16:36:11 harley kernel: wq_barrier_func+0x0/0x8
>> Jun 2 16:36:11 harley kernel: vmstat_update+0x0/0x24
>> Jun 2 16:36:11 harley kernel: ----
>> Jun 2 16:36:11 harley kernel: cache_reap+0x0/0xf4
>
> As expected.
>
> Note that ->nivcsw/->nvcsw doesn't change. There is no "spare time"
> on CPU 1, "vrsx" monopolizes CPU. events/1->cache_reap() was preempted
> by vrsx, it had no chance to run since then. Note that jobs == 7974
> doesn't change too. I forgot to print cwq->thread->state, but it should
> be TASK_RUNNING. It would not be possible to kill vrsx if cache_reap()
> stalled.
>
> I don't think this is a kernel problem, vrsx breaks flush_workqueue().
> Ingo can answer authoritatively, but I think SCHED_RR/SCHED_FIFO were
> not designed to be 100% cpu-bound.
>
> That said, I think it makes sense to get rid of flush_scheduled_work()
> in floppy.c.
>

Oleg, thanks for your time in diagnosing this.

As far as a 100% CPU bound task being a valid thing to do, it has been
done for many years on SMP machines. Any kernel limitation on this
surely must be considered a bug?

Thanks again
Regards
Mark