Re: Scheduler: Spinning until tasks are STOPPED

[Nick Piggin]

Yuly Finkelberg wrote:
>                                                                       
>                                                                       
>                                                                       
>                                           Hi,
>
> I sent a message regarding this issue earlier, but after re-reading
> it, I realized that it wasn't very clear.  Hopefully, this will
> clarify things a little bit:
>
> I have a strange scheduling issue: a bunch of worker tasks are all waiting 
> on a wait queue. Each task is woken up by the preceeding, does some work, 
> wakes up the next one, and then sends a SIGSTOP to itself. The last task however
> does not stop itself, but instead yield()s until all tasks have reached state
> TASK_STOPPED.
>
> The code looks like this (irrelevant parts cut out):
> 	...
>         ret = wait_event_interruptible(waitq, next_in_line == myself);
> 	...
> 	(some work)
> 	...
> 	next_in_line = next;	
>         ret = wakeup_next_one();
> 	if (!last_one)
> 		send_sig(SIGSTOP, current, 1);
> 	else
> 		spin_until_all_stopped()
>
> When run with 50 tasks, normally this works well. However sometimes one of the
> tasks (never the last one) gets stuck between calling wakeup_next_one() and 
> between sending the signal. It accumulates system time, and its stack looks
> like (no pending signals, ti_flags is clear):
>
> c55e7ad0 00000086 c55e6000 c55e7a94 00000046 c55e6000 c55e7ad0 c0109c2d
>          00000000 c0497800 00000001 d38da344 0013bc9c c5632840 00071931 d3d93161
>          0013bc9c c55d546c c05d3960 0000270f c05d3960 c55e6000 c0106f25 c05d3960
>
> Call Trace:
> [<c0106f25>] need_resched+0x27/0x32
>
> (yes, this is not a mistake: this is ALL the stack reported by show_stack())
>
> Normally the spinning task will magically get released after "a while", where 
> few seconds < "a while" < 10 minutes and sometimes even longer. 
> So the mystery is -
> 1. Why does the task spin for so long ?
> 2. Where does it spin ?  (the kernel stack doesn't hint on anything...)
> 3. How can I find out #2 ?
> 4. How to fix it ?
> 5. Is there a better way to make sure a specific task is STOPPED ?
>
> Currently running 2.6.8.1 and 2.6.9 (UP, PREEMPT).  I'd appreciate any
> help here...

You're doing this in the *kernel*? It sounds like it should be done
in userspace or done a different way (ie. not with 50 tasks).

And using signals and spinning on yield for synchronisation and
process control in the kernel like this is fairly crazy.

Can't you use a semaphore or something?

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