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

[Philippe Gerum <rpm@xenomai.org>]

Philippe Gerum wrote:
> Pierangelo Masarati wrote:
>> Philippe Gerum wrote:
>>> Pierangelo Masarati wrote:
>>>> Dear ADEOS users,
>>>>
>>>> 2.6.25 seems to show abnormal latencies on hardware that showed good
>>>> performances up to 2.6.24.  We think we traced down the issue to x86's
>>>> process_xx.c, which disappeared after regressing default_idle() to
>>>> 2.6.24.  The related changes are described in the attached patch.
>>>>
>>> This patch would badly break the runqueue statistics, and likely the Linux
>>> scheduler tick engine too.
>>>
>>> Actually, the hunk in default_idle() seems useless, since co-kernel activity
>>> should be accounted as Linux idle time anyway. Does this patch also fixes
>>> the issue you tracked down?
>> Dear Philippe,
>>
>> we'll try it.  It will require some time to empirically let it run for a 
>> while to be sure.  Usually, the weird latency effect occurs within half 
>> a hour, but we'd like to wait a little longer.
>>
>> Apart from not disturbing Linux, your fix should work, since it doesn't 
>> touch the hw flag.  The problem with the patch is likely that: CPU i 
>> gets the seqlock after hlt,
>

Ok, you were talking about the _previous_ implementation, right? Ok, I must be a
bit slow today; Yes, the issue was very likely the one you described.

> I don't see any seqlock in the idling code. Did you mean the spinlock in
> sched_clock_idle_wakeup_event()?
> 
>  and can be preempted by the RTOS; CPU k
>> tries to acquire the lock before hlt, i.e. with hw flags disabled,
> 
> Hw flags are still on actually -- local_irq_disable() won't switch them off.
> 
>  so it
>> cannot be preempted by the RTOS.  If the RTOS after preempting CPU i 
>> does a bit of work, the RTOS on CPU k is stalled until the RTOS finishes 
>> working on CPU i.
> 
>> In any case, it is now running on two machines: a 32 and a 64 bit.  I'll 
>> let you know.
>>
>> Sincerely, p.
>>
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> 
> 


-- 
Philippe.