On Jan 17, 2008 3:16 PM, Jan Kiszka <[EMAIL PROTECTED]> 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?
My intent was to implement a protocol similar to the one of
timerbench, but using an rt-pipe, and continue to use the latency
test, adding new options such as -t 3 and t 4. But there may be
problems with this approach: if we are compiling without
CONFIG_XENO_OPT_PERVASIVE, latency will not run at all. So, it is
probably simpler to implement a klatency that just reads from the
rt-pipe.
--
Gilles Chanteperdrix
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