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.
--
Gilles Chanteperdrix
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