Philippe Gerum wrote: > Jan Kiszka wrote: > >> Philippe Gerum wrote: >> >>> Jan Kiszka wrote: >>> >>> >>>> Hi, >>>> >>>> I happened to stumble over this comment. It made me curious, >>>> especially as it is not totally correct (the loop is executed in >>>> IRQ-off >>>> context, thus it *is* timecritical). >>>> >>> >>> Critical should be understood here in the sense that IRQs are off while >>> the loop workload is high, which is fortunately not the case. Hence the >>> comment. >> >> >> >> Sure, there is not much to do inside the loop. But it does not scale >> very well in case a significant number of elements are registered - and >> they are scattered over a larger memory area so that cache missed >> strike us. >> > > Compared to what it costs to actually call Linux to release the system > memory which is an operation the syscall will do anyway, those cache > misses account for basically nothing.
I don't have the function caller's cost in mind here (which is likely either starting up or on the way to termination anyway), I just worry about the rest of the system which may want to continue it's operation undisturbed. > >> It's a bit theoretical, but I also think we can easily resolve it by >> using Linux locks as soon as we can sanely sleep inside >> xnheap_init/destroy_shared and xnheap_ioctl. >> >> >>>> While thinking about the possibility to convert the hard IRQ lock >>>> protection of kheapq into some Linux mutex or whatever, I analysed the >>>> contexts the users of this queue (__validate_heap_addr/xnheap_ioctl, >>>> xnheap_init_shared, xnheap_destroy_shared) execute in. Basically, it is >>>> Linux/secondary mode, but there are unfortunate exceptions: >>>> >>>> rt_heap_delete(): take nklock, then call xnheap_destroy_shared(). >>>> The latter will call __unreserve_and_free_heap() which calls Linux >>>> functions like vfree() or kfree() -- I would say: not good! At >>>> least on SMP we could easily get trapped by non-deterministic >>>> waiting on >>>> Linux spinlocks inside those functions. >>>> >>>> The same applies to rt_queue_delete(). >>>> >>> >>> Good spot. Better not calling the heap deletion routines under nklock >>> protection in the first place. The committed fix does just that for both >>> rt_heap_delete and rt_queue_delete. >> >> >> >> Ok, we no longer have IRQs locked over vfree/kfree, but task scheduling >> is still suffering from potential delays. Wouldn't it be better to defer >> such operations to an asynchronous Linux call? >> > > Do we really want heap creation/deletion to be short time bounded > operations at the expense of added complexity? > Again, the side effects on other real-time programs are my concern. There are quite a lot of scenarios where only parts of the real-time programs are started or stopped while others keep on working as usual. The caller's cost is more or less irrelevant in that case. Jan
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