Re: [patch] Real-Time Preemption, -RT-2.6.13-rc4-V0.7.53-01, High Resolution Timers & RCU-tasklist features

[George Anzinger <george@mvista.com>]

Ingo Molnar wrote:
> * Ingo Molnar <mingo@elte.hu> wrote:
> 
> 
>>* George Anzinger <george@mvista.com> wrote:
>>
>>
>>>Ingo, all
>>>
>>>I, silly person that I am, configured an RT, SMP, PREEMPT_DEBUG system. 
>>> Someone put code in the NMI path to modify the preempt count which, 
>>>often as not will generate a PREEMPT_DEBUG message as there is no tell 
>>>what state the preempt count is in on an NMI interrupt.  I have sent 
>>>the attached patch to Andrew on this, but meanwhile, if you want RT, 
>>>SMP, PREEMPT_DEBUG you will be much better off with this.
>>
>>ah - thanks, applied. Might explain some of the recent SMP weirdnesses 
>>i'm seeing. Attributed them to the HRT patch ;-)
> 
> 
> i'm still seeing weird crashes under SMP, which go away if i disable 
> CONFIG_HIGH_RES_TIMERS. (this after i fixed a couple of other SMP bugs 
> in the HRT code) It happens sometime during the bootup, after enabling 
> the network but before users can log in. There's no good debug info, 
> just a hang that comes from all CPUs trying to get some debug info out 
> but crashing deeply.
> 
I haven't looked at this new code all that closely as yet.  One thing I 
did notice is that there is an assumption that the "timer being 
delivered flag" can be shared between LR timers and HR timers.  I 
suspect this is wrong as the delivery code is in seperate threads (I 
assume).  This could lead to del_timer_async missing a timer.

In the prior patch we just ignored the del_timer_async issue for HR 
timers (code I plan to do soon).  This WAS taken care of in earlier 
kernels by a reuse of one of the list link fields, but Andrew convince 
me that this was _not_ good.

So, my guess, a nanosleep for an RT task (I think you said these are 
promoted to HR) is completing and over writing the deliver in progress 
flag for a LR timer which just happens to have a del_timer_sync going on 
at the same time.
-- 
George Anzinger   george@mvista.com
HRT (High-res-timers):  http://sourceforge.net/projects/high-res-timers/