linux-kernel - Re: floppy.c soft lockup

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Date:	Mon, 04 Jun 2007 10:00:54 -0400
From:	Mark Hounschell <markh@...pro.net>
To:	Oleg Nesterov <oleg@...sign.ru>
CC:	Mark Hounschell <dmarkh@....rr.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	linux-kernel@...r.kernel.org, Ingo Molnar <mingo@...e.hu>
Subject: Re: floppy.c soft lockup

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

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