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? Jan -- Siemens AG, Corporate Technology, CT SE 2 Corporate Competence Center Embedded Linux _______________________________________________ Xenomai-core mailing list Xenomai-core@gna.org https://mail.gna.org/listinfo/xenomai-core
