Gilles Chanteperdrix wrote: > Jan Kiszka wrote: >> Jan Kiszka wrote: >>> Gilles Chanteperdrix wrote: >>>> Jan Kiszka wrote: >>>>> Gilles Chanteperdrix wrote: >>>>>> Jan Kiszka wrote: >>>>>>> Andreas Glatz wrote: >>>>>>>> Hi Philippe, >>>>>>>> >>>>>>>> At the Xenomai Users Meeting last year I asked you if Xenomai would >>>>>>>> offer a possibility to lower the priority of certain Xenomai tasks >>>>>>>> below that of Linux. We need this feature since we have tasks in our >>>>>>>> RT application which should only run when Linux is idle (A statistics >>>>>>>> collection task which part of the RT application and hard to isolate >>>>>>>> from this application). >>>>>>>> >>>>>>> What prevents using a borderline thread (if you need to interact with >>>>>>> blocking Xenomai services) with SCHED_OTHER and a Linux nice level of >>>>>>> 19? >>>>>> Well, this does not really guarantee that the thread will run only when >>>>>> linux is idle. The thread will eat some cpu time, the nice level is not >>>>>> a strict priority, as you know. >>>>> Where do you really need anything stricter? It's the opposite of "I need >>>>> true 100% CPU for my task, and that forever." >>>>> >>>>>> But in fact, I wonder why Andreas wants >>>>>> a new scheduling policy for xenomai, what is needed, is simply a >>>>>> SCHED_IDLE (maybe it exists ?!) policy for Linux. >>>>>> >>>>> There is no such thing AFAIK. If you are concerned that some CPU >>>>> intensive low prio job eats too much CPU, you normally reduce its >>>>> nice-level and/or confine its CPU bandwidth via cgroups. >>>> SCHED_IDLE exists. >>>> >>> Ah, as "nice 20". Same mechanism, just another level. >>> >> ...and before trying something else: >> >> "SCHED_IDLE: This is even weaker than nice 19, but its not a true >> idle timer scheduler in order to avoid to get into priority >> inversion problems which would deadlock the machine." > > Mmmm, priority inversion? I thought the kernel had priority inheritance? >
I do not recall the details, but I don't think non-RT priorities are inherited. Moreover, user space locks without explicit inheritance set (ie. the majority of locks) would still cause deadlocks, though not for the kernel. Jan
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