linux-kernel - Re: floppy.c soft lockup

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Date:	Wed, 06 Jun 2007 09:12:04 -0400
From:	Mark Hounschell <markh@...pro.net>
To:	linux-kernel@...r.kernel.org
CC:	markh@...pro.net, Oleg Nesterov <oleg@...sign.ru>,
	Mark Hounschell <dmarkh@....rr.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Ingo Molnar <mingo@...e.hu>
Subject: Re: floppy.c soft lockup

Mark Hounschell wrote:
> 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? 
> 

Could someone authoritatively comment on this? Is a SCHED_RR/SCHED_FIFO
100% Cpu bound process supported in an SMP env on Linux? (vanilla or -rt)

Thanks and Regards
Mark
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