Jonathan Larmour <j...@ecoscentric.com> writes: > On 21/06/12 15:57, Sergei Organov wrote: >> Nick Garnett <ni...@calivar.com> writes: >> >>> On 21/06/12 12:51, Sergei Organov wrote: >>>> Nick Garnett <ni...@calivar.com> writes: >>> >>>>> Essentially locking the scheduler yourself is like running in a DSR, >>>>> there is only a limited number of calls it makes sense to use, and their >>>>> effects may be deferred until the scheduler lock returns to zero. >>>> >>>> So, there is no way in eCos to just disable _preemption_? I.e., >>>> (temporarily) switch from preemptive to cooperative multi-threading? >>> >>> Not really. You could try implementing a new scheduler that only did >>> context switches when a yield function is called. If you disable >>> timeslicing and run all your threads at the same priority they will >>> essentially run round-robin and only lose the CPU when they explicitly >>> suspend, although higher priority threads will still preempt. >> >> No, I need full-featured preemptive scheduling with multiple real-time >> thread priorities. I thought "disable preemption" feature is pretty >> standard among RTOSes and have it in my RTOS abstraction layer, >> implemented as a call to cyg_scheduler_lock() in eCos as disabling >> preemption is what description of the call claims. > > The problem with any sort of abstraction layer is matching what you mean with > what exactly is provided. In particular does disabling pre-emption mean that > DSRs should be disabled too? From what you're saying, I think you mean that > DSRs should still be allowed.
No, in fact I didn't care. For 2-level interrupts servicing eCos implements, complete set of features would be: 1. Disable/restore ISRs (also effectively does 2 and 3) 2. Disable/restore DSRs (also effectively does 3) 3. Disable/restore threads switching all these states being thread-local. Current scheduler lock feature is close to (2), except that it introduces surprising behavior I was referring to. The main idea behind disabling preemption is that as soon as system function that could lead to DSRs invokation and/or task switch is called explicitly, DSRs are run and taks will be switched. This is what does not happen in some of current eCos system functions. Note that even though DSRs are disabled when scheduler is locked, given scheduler lock is thread-local, even timed waits should work out of the box for the thread that locks scheduler as scheduler is still enabled when this thread sleeps. > So if you just want to make sure no other threads can pre-empt this one, then > surely the answer is to temporarily change its thread priority to 1, and > change it back when you're done. And reserve priority 1 for threads in this > situation. Yes, reserving highest priority for that purpose would work, provided scheduler supports dynamic priority change. > If multiple threads can disable pre-emption at the same time, but still be > able to suspend themselves, then you would have to work out what that really > is meant to mean. It's not clear what would be correct behaviour in that > situation in general, so you would have to decide what that means in your > specific case. > >> In fact I use this feature for very special purpose and can re-design my >> code to avoid it. > > Good, but I do wonder if that could have been avoided... I believe it could. Hopefully it's just a matter of adding a few explicit calls to reschedule() where appropriate in the eCos kernel code as waits on primitives such as semaphores do work as expected, so all the required functionality is apparently already there. -- Sergei. -- Before posting, please read the FAQ: http://ecos.sourceware.org/fom/ecos and search the list archive: http://ecos.sourceware.org/ml/ecos-discuss
