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. 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
