linux-kernel - Re: floppy.c soft lockup

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Date:	Thu, 07 Jun 2007 06:18:52 -0400
From:	Mark Hounschell <dmarkh@....rr.com>
To:	Matt Mackall <mpm@...enic.com>
CC:	Andrew Morton <akpm@...ux-foundation.org>, markh@...pro.net,
	linux-kernel@...r.kernel.org, Oleg Nesterov <oleg@...sign.ru>,
	Ingo Molnar <mingo@...e.hu>
Subject: Re: floppy.c soft lockup

Matt Mackall wrote:
> On Wed, Jun 06, 2007 at 10:28:28AM -0700, Andrew Morton wrote:
>> On Wed, 06 Jun 2007 09:12:04 -0400 Mark Hounschell <markh@...pro.net> wrote:
>>
>>>> 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)
>> It will kill the kernel, sorry.
>>
>> The only way in which we can fix that is to allow kernel threads to preempt
>> rt-priority userspace threads.  But if we were to do that (to benefit the
>> few) it would cause _all_ people's rt-prio processes to experience glitches
>> due to kernel activity, which we believe to be worse.
>>
>> So we're between a rock and a hard place here.
>>
>> If we really did want to solve this then I guess the kernel would need some
>> new code to detect a 100%-busy rt-prio process and to then start premitting
>> preemption of it for kernel thread activity.  That detector would need to
>> be smart enough to detect a number of 100%-busy rt-prio processes which are
>> yielding to each other, and one rt-prio process which keeps forking others,
>> etc.  It might get tricky.
> 
> The usual alternative is to manually chrt the relevant kernel threads
> to RT priority and adjust the priority scheme of their processes appropriately.
> 

>From an earlier thread member:

>> Mark writes:
>> Again I don't understand why flush_scheduled_work() running on behalf
>> of a process affinitized to processor-1 requires cooperation from
>> events/2 (affinitized to processor-2)
>> when there is an events/1 already affinitized to processor 1?

>Oleg write:
>flush_workqueue() blocks until any scheduled work on any CPU has run to
>completion. If we have some work_struct pending on CPU 2, it can be
>completed only when events/2 executes it.

Could not flush_scheduled_work() just follow the affinity mask of the
task that caused the call to begin with. If calling task had a cpu-mask
of 3 then flush_scheduled_work() would do the events/0 and events/1
thing and if the calling task had an affinity mask of 1 then only
events/0 would be done?

In other words changing what Oleg says above just slightly:

flush_workqueue() blocks until any scheduled work on any CPU in the
calling tasks affinity mask has run to completion?

Thanks
Mark

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