Re: Query regarding 2.6.335 RT and Non-RT performance

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

  On 08/19/2010 11:15 AM, Manikandan Ramachandran wrote:
> On Thu, Aug 19, 2010 at 9:06 AM, Sven-Thorsten Dietrich
> <thebigcorporation@gmail.com>  wrote:
>> On Thu, 2010-08-19 at 07:59 -0700, Nivedita Singhvi wrote:
>>> On 08/17/2010 03:22 PM, Manikandan Ramachandran wrote:
>>>
>>>
>>>> I see a scenario where higher priority task is ready to run but lower
>>>> priority IRQ thread is hogging the cpu. Please see the timeline log
>>>> below:
>>>>
>>>> GIVE ffff0c1c [Comment: High priority IRQ thread wakes up high
>>>> priority process at jiffy 0xffff0c1c]
>>>> **ffff0c21**##ffff0c21##**ffff0c21**##ffff0c21##**ffff0c21**##ffff0c21##**ffff0c21**##ffff0c21##**ffff0c21**##ffff0c22##**ffff0c22**##ffff0c22##**ffff0c22**##ffff0c22##**ffff0c22**##ffff0c22##**ffff0c22**##ffff0c22##**ffff0c22**##ffff0c22##**ffff0c22**##ffff0c22##**ffff0c22**##ffff0c22##**ffff0c23**##ffff0c23##**ffff0c23**##ffff0c23##**ffff0c23**##ffff0c24##**ffff0c24**##ffff0c24##**ffff0c24**##ffff0c24##
>>>> [Comment: Low priority IDE[ide_intr] handler hogs CPU for almost 3 ms]
>>>> GOTffff0c26 [Comment: High priority task wakes up after 10 ms!!]
>>>>
>>>> I fail to understand why scheduler failed to schedule high priority
>>>> task 10 times in a row. FYI, I have put IDE thread to SCHED_NORMAL
>>>> while high priority thread and task to SCHED_FIFO.
>>> Not really sure from the above what's going on, what priorities
>>> you are running at, etc.
>>>
>>> You might want look at the output of "ps -eLo pid,rtprio,policy,comm"
>>> to see if you can confirm your scheduler attributes.
>>>
>> Also, use the tracing facilities, as its hard to understand your time
>> line and the source of it.
>>
>> I think if your priorities are right, your kernel config is right, and
>> your measurement tool doesn't interfere, 10 ms latency is incredibly
>> unlikely.
>>
>>
>>> thanks,
>>> Nivedita
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>>
>>
> I see many tracing options. Any trace config in particular?
>
>

Steve's latency tracing can generate a time-stamped stack dump of the 
Kernel code-path that blocks the RT task you are trying to run.