On Mon, 2012-09-17 at 16:49 -0700, Andy Lutomirski wrote: > I haven't looked in any great detail, but the approach looks sensible > and should slow down the vsyscall code. > > That being said, as long as you're playing with this, here are a > couple thoughts: > > 1. The TSC-reading code does this: > > ret = (cycle_t)vget_cycles(); > > last = VVAR(vsyscall_gtod_data).clock.cycle_last; > > if (likely(ret >= last)) > return ret; > > I haven't specifically benchmarked the cost of that branch, but I > suspect it's a fairly large fraction of the total cost of > vclock_gettime. IIUC, the point is that there might be a few cycles > worth of clock skew even on systems with otherwise usable TSCs, and we > don't want a different CPU to return complete garbage if the cycle > count is just below cycle_last. > > A different formulation would avoid the problem: set cycle_last to, > say, 100ms *before* the time of the last update_vsyscall, and adjust > the wall_time, etc variables accordingly. That way a few cycles (or > anything up to 100ms) or skew won't cause an overflow. Then you could > kill that branch. >
I'm curious... If the task gets preempted after reading ret, and doesn't get to run again for another 200ms, would that break it? -- Steve -- 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/
