Jan Kiszka 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?

(That is only in case you meant "reimplementing timerbench" with
"implement another kernel module". Just write a kernel-hosted RTDM user
of timerbench.)

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