Am Samstag, den 01.03.2014, 14:56 -0800 schrieb H. Peter Anvin: > On 02/28/2014 06:00 PM, Andy Lutomirski wrote: > > > > This leads to a potentially interesting question: is rdtsc_barrier() > > actually necessary on UP? IIRC the point is that, if an > > rdtsc_barrier(); rdtsc in one thread is "before" (in the sense of > > being synchronized by some memory operation) an rdtsc_barrier(); rdtsc > > in another thread, then the first rdtsc needs to return an earlier or > > equal time to the second one. > > > > I assume that no UP CPU is silly enough to execute two rdtsc > > instructions out of order relative to each other in the absence of > > barriers. So this is a nonissue on UP. > > > > On the other hand, suppose that some code does: > > > > volatile long x = *(something that's not in cache) > > clock_gettime > > > > I can imagine a modern CPU speculating far enough ahead that the rdtsc > > happens *before* the cache miss. This won't cause visible > > non-monotonicity as far as I can see, but it might annoy people who > > try to benchmark their code. > > > > Note: actually making this change might be a bit tricky. I don't know > > if the alternatives code is smart enough. > > > > Let's put it this way... this is at best a third-order optimization... > let's not worry about it right now. >
IMHO it is the behaviour that most developer expect. It would a bad idea to get a time value before the previous operations are not finished. In some use case this will result in a fail. Imagine a HW where two designated register can only consecutively accessed after a given time period is elapsed. This would be normally done by a busy loop for very short periods. It would be okay when the time period to wait is exceeded, but will maybe fail when the wait time is to short. - Stefani -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/