Linux has High-Resolution Timers (HRTs) that may be appropriate. See the manpage for clock_gettime(), which defines these HRTs: [...] CLOCK_MONOTONIC, in particular, looks suitable. Using it could be a matter of just a few quick likes in FFI.
I don't know if Windows has similar features.
http://msdn.microsoft.com/en-us/library/ms632592(VS.85).aspx and QueryPerformanceCounter, in detail.
Going from GetProcessTimes, which I found in ghc code in libraries/base/System/CPUTime.hsc, I was just able to find this in that same site: http://msdn.microsoft.com/en-us/library/ms724408(VS.85).aspx Seems simpler. Since linux (and unix?) have clock_gettime and windows has this GetTickCount, maybe it could be possible to add getSystemCPUTime to System.CPUTime. Thanks, Maurício