Re: Interrupt Bottom Half Scheduling

[Sven-Thorsten Dietrich <thebigcorporation@gmail.com>]

On 02/14/2011 03:42 PM, Peter LaDow wrote:
> Pseudo-code. I can post the full code if it helps.
>

My first thought was whether the compiler optimizes away the for loop.

Either way you would be hammering on the clock_gettime in a narly way, 
it might be better just to hand-code a register read, if this is just 
some kind of benchmark.

It would be good to know the time source chipset and its resolution.

what is the distribution for t2 - t1 btw?

for starters ;)


On Feb 14, 2011, at 3:35 PM, Sven-Thorsten Dietrich 
<thebigcorporation@gmail.com> wrote:
>> On Mon, 2011-02-14 at 15:08 -0800, Peter LaDow wrote:
>>> Note sure how that is possible.  This is related to my earlier posting
>>> about timing jitter.  Our code is basically this:
>>>
>> Where is the semicolon ending the for loop?
>>
>>
>>> while(1)
>>> {
>>>   t1 = clock_gettime()
>>>   for(i=0; i<  10000; i++)
>>>   t2 = clock_gettime()
>>>
>>>   diff = t2 - 1
>>> }
>>>
>>> This task is pending on nothing.  Unless clock_gettime() causes some
>>> sort of priority inversion, I don't see the problem.  We see
>>> significant jitter on the for-loop when there are a significant number
>>> of other kernel timers.  Now, as we understand it, the hrtimers run in
>>> the softirq.  But if the softirq is priority 50, and this for-loop is
>>> priority 99, it shouldn't be affected by the softirq thread.
>>>
>>> Pete
>>>
>>> On Mon, Feb 14, 2011 at 3:04 PM, Sven-Thorsten Dietrich
>>> <thebigcorporation@gmail.com>  wrote:
>>>> On Mon, 2011-02-14 at 14:31 -0800, Peter LaDow wrote:
>>>>> How is the scheduling of the hrtimers softirq thread handled?
>>>>>
>>>>> When querying the RT priority of the hrtimer softirq, I get a priority
>>>>> of 50.  But when running a priority 99 thread, we still seem to be
>>>>> getting interrupted.  Shouldn't the hrtimer softirq be put off until
>>>>> the CPU is idle or a lower priority task is running?
>>>> Does your prio 99 thread perhaps encounter a prio inversion dependency
>>>> on one of the softriq threads?
>>>>
>>>>> Thanks,
>>>>> Pete
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>>>>
>>