Re: [Adeos-main] latency fix for 2.6.25?

[Philippe Gerum <rpm@xenomai.org>]

Pierangelo Masarati wrote:
> Philippe Gerum wrote:
> 
>>>> Ok, you were talking about the _previous_ implementation, right?
>>> Yes.  It's been running for a few hours today, so your fix seems to be 
>>> fine; in any case, the tests will run overnight, and I'll let you know 
>>> tomorrow.  I believe this issue could not be easily pointed out by just 
>>> a simple latency check.
>>>
>> Agreed, we need to create a load that makes races on seqlocks more likely, and
>> ideally with lots of readers delaying a single writer.
> 
> The testing of your fix ran overnight without problems, so we'd consider 
> it fine.

Ok, thanks. Will issue -03 asap.

  Another possible fix to preserve linux timings is to use
> local_irq_disable_hw() after halt.  It seems to create no significant 
> latency.
> 

The only way to fully preserve the timings would involve running
sched_clock_idle_wakeup_event() with hw interrupts off, which grabs a spinlock
on the current runqueue; I'm worried about latencies introduced by sporadic
contention on that lock due to task migrations.

OTOH, the scheduler tick handler implements a counter-measure against underflows
of the per-runqueue clocks, so even if the RTOS preempts often or for a long
time the idling code, it should resync reasonably well. Hopefully.

-- 
Philippe.