Re: [patch] Real-Time Preemption, -RT-2.6.12-rc1-V0.7.43-00

[Ingo Molnar <mingo@elte.hu>]

* Steven Rostedt <rostedt@goodmis.org> wrote:

> > actually, what priorities do the yielding tasks have? sched_yield() does 
> > not guarantee that the CPU will be given up, of if a highest-prio 
> > SCHED_FIFO task is in a yield() loop it will livelock the system.
> 
> What scares me is the code in fs/inode.c with that 
> __wait_on_freeing_inode.  Look at the code in find_inode and 
> find_inode_fast.  Here you will see that they really are busy loops 
> with a yield in them, if the inode they are waiting on is I_FREEING or 
> I_CLEAR and the process doing this hasn't set I_LOCK.  I haven't 
> looked much at this, but my kernel has livelocked on it.

ok, makes sense.

> Currently my fix is in yield to lower the priority of the task calling 
> yield and raise it after the schedule.  This is NOT a proper fix. It's 
> just a hack so I can get by it and test other parts.

yeah, yield() is a quite RT-incompatible concept, which could livelock 
an upstream kernel just as much - if the task in question is SCHED_FIFO.  
Almost all yield() uses should be eliminated from the upstream kernel, 
step by step.

	Ingo