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.

-- 
                                               Gilles Chanteperdrix

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