Wolfgang Grandegger wrote: > Philippe Gerum wrote: >> Jan Kiszka wrote: >>> Hi, >>> >>> [Daniel, I put you in the CC as you showed some interest in this topic.] >>> >>> as I indicated a some weeks ago, I had a closer look at the code the >>> user space libs currently produce (on x86). The following considerations >>> are certainly not worth noticeable microseconds on GHz boxes, but they >>> may buy us (yet another) few micros on low-end. >>> >>> First of all, there is some redundant code in the syscall path of each >>> skin service. This is due to the fact that the function code is >>> calculated based on the the skin mux id each time a service is invoked. >>> The mux id has to be shifted and masked in order to combine it with the >>> constant function code part - this could also easily happen >>> ahead-of-time, saving code and cycles for each service entry point. >>> >>> Here is a commented disassembly of some simple native skin service which >>> only takes one argument. >>> >>> >>> Function prologue: >>> 460: 55 push %ebp >>> 461: 89 e5 mov %esp,%ebp >>> 463: 57 push %edi >>> 464: 83 ec 10 sub $0x10,%esp >>> >>> Loading the skin mux-id: >>> 467: a1 00 00 00 00 mov 0x0,%eax >>> >>> Loading the argument (here: some pointer) >>> 46c: 8b 7d 08 mov 0x8(%ebp),%edi >>> >>> Calculating the function code: >>> 46f: c1 e0 10 shl $0x10,%eax >>> 472: 25 00 00 ff 00 and $0xff0000,%eax >>> 477: 0d 2b 02 00 08 or $0x800022b,%eax >>> >>> Saving the code: >>> 47c: 89 45 f8 mov %eax,0xfffffff8(%ebp) >>> >>> 47f: 53 push %ebx >>> >>> Loading the arguments (here only one): >>> 480: 89 fb mov %edi,%ebx >>> >>> Restoring the code again, issuing the syscall: >>> 482: 8b 45 f8 mov 0xfffffff8(%ebp),%eax >>> 485: cd 80 int $0x80 >>> >>> 487: 5b pop %ebx >>> >>> Function epilogue: >>> 488: 83 c4 10 add $0x10,%esp >>> 48b: 5f pop %edi >>> 48c: 5d pop %ebp >>> 48d: c3 ret >>> >>> >>> Looking at this code, I also started thinking about inlining short and >>> probably heavily-used functions into the user code. This would save the >>> function prologue/epilogue both in the lib and the user code itself. For >>> sure, it only makes sense for time-critical functions (think of >>> mutex_lock/unlock or rt_timer_read). But inlining could be made optional >> >> The best optimization for rt_timer_read() would be to do the >> cycles-to-ns conversion in user-space from a direct TSC reading if the >> arch supports it (most do). Of course, this would only be possible for >> strictly aperiodic timing setups (i.e. CONFIG_XENO_OPT_PERIODIC_TIMING >> off). >> >> For the rt_mutex_lock()/unlock(), we still need to refrain from >> calling the kernel for uncontended access by using some Xeno >> equivalent of the futex approach, which would suppress most of the >> incentive to micro-optimize the call itself.
Ack. That's a bit more complex to realise but should be put on our to-do list as well. >> >>> for the user by providing both the library variant and the inlined >>> version. The users could then select the preferred one by #defining some >>> control switch before including the skin headers. >>> >>> Any thoughts on this? And, almost more important, anyone around willing >>> to work on these optimisations and evaluate the results? I can't ATM. >>> >> >> Quite frankly, I remember that I once had to clean up the LXRT >> inlining support in RTAI 3.0/3.1, and this was far from being fun >> stuff to do. Basically, AFAICT, having both inline and out-of-line >> support for library calls almost invariably ends up to a maintenance >> nightmare of some sort, e.g. depending whether to compile with gcc's >> optimization on or not, which might be dictated by the fact that one >> also wants (exploitable) debug information or not, and so on. Not to >> speak of the fact that you end up having two implementations to >> maintain separately. >> >> This said, only the figures would tell us if such inlining brings >> something significant or not to the picture performance-wise on >> low-end hw, so I'd be interested to see those first. > > I agree! I have also doubts that using inline function will improve > latencies. Larger code also results in more TBL misses and cache refills. > I'm definitely not arguing for the aggressive inlining RTAI provides (all or nothing). I'm rather suggesting selective optional inlining of trivial and heavily-used stubs. Think of something like rt_sem_v() for the library call vs. rt_sem_v_inlined() for an unrolled variant. rt_sem_v() could simply be implemented by calling rt_sem_v_inlined(), thus there should also be no reason why the maintenance hell might get reopened. I think the order I presented should be kept when looking at these optimisations: 1. reduce the complexity of existing syscall code, 2. consider if we can provide any benefit to the user by offering /some/ functions as optional inlines. Jan
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