Re: floppy.c soft lockup

[Mark Hounschell <dmarkh@cfl.rr.com>]

Matt Mackall wrote:
> On Thu, Jun 07, 2007 at 06:18:52AM -0400, Mark Hounschell wrote:
>> 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@compro.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?
> 
> The kernel's internal event API doesn't track any of this stuff and
> it's not clear we'd want it to. It'd be a bit simpler perhaps to
> simply allow SIGSTOPing events/0. This might even work today from
> userspace.
> 
> In general, it's considered a mistake to mark CPU hogs as RT precisely
> because they present a starvation risk to everything else in the
> system, not just kernel threads. We could add kernel infrastructure to
> make events survive this sort of thing, but that will very likely just
> expose another kernel or userspace livelock.
> 

In general maybe, but we are not really talking in general terms here.
For everything other than kernel stuff, I (userland) would assume
responsibility. That is why I force all other userland tasks that I want
to run onto all the other processors before my RT-HOG starts.

>From a userland point of view, if I have an 8 processor box, I should be
able to have 7 RT-HOGS running on it while all other proceses are on the
8th. Yet in Linux that I can't have even one because it breaks the
kernel. I'm sorry, this does not sound right to me. To me and people in
my world, this is clearly a kernel deficiency.

Regards
Mark