Jan Kiszka wrote:
Philippe Gerum wrote:
Jan Kiszka wrote:
Petr Cervenka wrote:
The problem was in my calling "RT" task. I forgot to switch the new
created task to PRIMARY MODE, because I didn't know it's neccessary.
As Philippe said, it isn't usually required - with exceptions. Let me
first sketch the standard situation: You registered a read_rt handler
with your device, but no read_nrt. If your application now calls
rt_dev_read (or just read() for the POSIX skin) from the "wrong"
context, RTDM will detect the missing _nrt handler and trigger an
automatic switch to primary mode (and vice versa). So, no need for
manual switching in the standard case.
But if you had registered handlers for both contexts, RTDM will always
invoke the one for your current mode.
I wonder if marking the call as "conforming" instead of leaving it to
the current domain would make such handling more intuitive? RT threads
would automatically switch to primary before always going to the _rt
routine, while plain Linux tasks would just go to the _nrt one. Am I
off-base wrt RTDM's logic here?
Its the difference between a RT thread issuing some service request
during its init phase (where it may not need hard guarantees, rather
wants to save short RT resources) and from inside its critical loop
(where no mode switch is desired). The second case would be ok with
"default-to-RT", but the first one would require to move the job into a
plain linux thread in order to address the _nrt variant of the invoked
service.
A concrete example: RTnet can create sockets with strict RT guarantees.
In case the socket creation is invoked from primary mode, the required
buffers for the socket are taken from a limited, user-tuned global pool.
But the standard, far more convenient case is that socket creation does
not have to happen in RT. In this case we can allocate the buffers from
Linux. This may lead to swapping or may even fail if the system is
generally low on memory. But that's non-RT. An RT task can now select
the allocation strategy by setting its execution mode either to primary
or secondary.
Selecting the strategy via function arguments may appear as a better
way, but this cannot easily be done where conformance to existing APIs
(like POSIX) is desired.
I'm worried by the fact that mode switching needs to be exposed to the
application layer in this case. Actually, it has always been seen as an
internal request, but never as part of the recommended API, because one
might just do utterly wrong things with this syscall (useless eager
switch when none is due etc). My worst nightmare waking me up in cold
sweat is seeing Xenomai-based applications litterally stuffed with
rt_task_set_mode(...T_PRIMARY...) calls, breaking the lazy switch scheme
without any upside, but additional latencies. Actually, a lot of work
has been done to make those mode switches as transparent/invisible as
possible. My fear is that people having problems with their application
would start adding mode switches everywhere "just in case", without
really understanding the logic behind it. Gah...! cold sweat again...
The other issue which bothers me is that applications would need to know
the actual implementation of the syscall to pick the right mode, i.e.
whether rtdm_socket wants to get memory from the Linux pool, or from a
predefined local pool, and so on. Sounds ok for a low-level library
which must know about RTDM's internals, but might be error-prone for
writing regular apps.
--
Philippe.
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