Gilles Chanteperdrix wrote:
> On Jan 17, 2008 3:16 PM, Jan Kiszka <[EMAIL PROTECTED]> wrote:
>> Gilles Chanteperdrix wrote:
>>> On Jan 17, 2008 12:55 PM, Jan Kiszka <[EMAIL PROTECTED]> wrote:
>>>> Gilles Chanteperdrix wrote:
>>>>> On Jan 17, 2008 11:42 AM, Jan Kiszka <[EMAIL PROTECTED]> wrote:
>>>>>> Gilles Chanteperdrix wrote:
>>>>>>> Hi,
>>>>>>>
>>>>>>> after some (unsuccessful) time trying to instrument the code in a way
>>>>>>> that does not change the latency results completely, I found the
>>>>>>> reason for the high latency with latency -t 1 and latency -t 2 on ARM.
>>>>>>> So, here comes an update on this issue. The culprit is the user-space
>>>>>>> context switch, which flushes the processor cache with the nklock
>>>>>>> locked, irqs off.
>>>>>>>
>>>>>>> There are two things we could do:
>>>>>>> - arrange for the ARM cache flush to happen with the nklock unlocked
>>>>>>> and irqs enabled. This will improve interrupt latency (latency -t 2)
>>>>>>> but obviously not scheduling latency (latency -t 1). If we go that
>>>>>>> way, there are several problems we should solve:
>>>>>>>
>>>>>>> we do not want interrupt handlers to reenter xnpod_schedule(), for
>>>>>>> this we can use the XNLOCK bit, set on whatever is
>>>>>>> xnpod_current_thread() when the cache flush occurs
>>>>>>>
>>>>>>> since the interrupt handler may modify the rescheduling bits, we need
>>>>>>> to test these bits in xnpod_schedule() epilogue and restart
>>>>>>> xnpod_schedule() if need be
>>>>>>>
>>>>>>> we do not want xnpod_delete_thread() to delete one of the two threads
>>>>>>> involved in the context switch, for this the only solution I found is
>>>>>>> to add a bit to the thread mask meaning that the thread is currently
>>>>>>> switching, and to (re)test the XNZOMBIE bit in xnpod_schedule epilogue
>>>>>>> to delete whatever thread was marked for deletion
>>>>>>>
>>>>>>> in case of migration with xnpod_migrate_thread, we do not want
>>>>>>> xnpod_schedule() on the target CPU to switch to the migrated thread
>>>>>>> before the context switch on the source CPU is finished, for this we
>>>>>>> can avoid setting the resched bit in xnpod_migrate_thread(), detect
>>>>>>> the condition in xnpod_schedule() epilogue and set the rescheduling
>>>>>>> bits so that xnpod_schedule is restarted and send the IPI to the
>>>>>>> target CPU.
>>>>>>>
>>>>>>> - avoid using user-space real-time tasks when running latency
>>>>>>> kernel-space benches, i.e. at least in the latency -t 1 and latency -t
>>>>>>> 2 case. This means that we should change the timerbench driver. There
>>>>>>> are at least two ways of doing this:
>>>>>>> use an rt_pipe
>>>>>>>  modify the timerbench driver to implement only the nrt ioctl, using
>>>>>>> vanilla linux services such as wait_event and wake_up.
>>>>>> [As you reminded me of this unanswered question:]
>>>>>> One may consider adding further modes _besides_ current kernel tests
>>>>>> that do not rely on RTDM & native userland support (e.g. when
>>>>>> CONFIG_XENO_OPT_PERVASIVE is disabled). But the current tests are valid
>>>>>> scenarios as well that must not be killed by such a change.
>>>>> I think the current test scenario for latency -t 1 and latency -t 2
>>>>> are a bit misleading: they measure kernel-space latencies in presence
>>>>> of user-space real-time tasks. When one runs latency -t 1 or latency
>>>>> -t 2, one would expect that there are only kernel-space real-time
>>>>> tasks.
>>>> If they are misleading, depends on your perspective. In fact, they are
>>>> measuring in-kernel scenarios over the standard Xenomai setup, which
>>>> includes userland RT task activity these day. Those scenarios are mainly
>>>> targeting driver use cases, not pure kernel-space applications.
>>>>
>>>> But I agree that, for !CONFIG_XENO_OPT_PERVASIVE-like scenarios, we
>>>> would benefit from an additional set of test cases.
>>> Ok, I will not touch timerbench then, and implement another kernel module.
>>>
>> [Without considering all details]
>> To achieve this independence of user space RT thread, it should suffice
>> to implement a kernel-based frontend for timerbench. This frontent would
>> then either dump to syslog or open some pipe to tell userland about the
>> benchmark results. What do yo think?
> 
> My intent was to implement a protocol similar to the one of
> timerbench, but using an rt-pipe, and continue to use the latency
> test, adding new options such as -t 3 and t 4. But there may be
> problems with this approach: if we are compiling without
> CONFIG_XENO_OPT_PERVASIVE, latency will not run at all. So, it is
> probably simpler to implement a klatency that just reads from the
> rt-pipe.

But that klantency could perfectly reuse what timerbench already
provides, without code changes to the latter, in theory.

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