On Fri, 2012-11-09 at 18:04 -0800, John Stultz wrote: > On 10/16/2012 10:23 AM, Peter Zijlstra wrote:
> > I've no problem with adding CLOCK_PERF (or another/better name). > Hrm. I'm not excited about exporting that sort of internal kernel > details to userland. > > The behavior and expectations from sched_clock() has changed over the > years, so I'm not sure its wise to export it, since we'd have to > preserve its behavior from then on. > > Also I worry that it will be abused in the same way that direct TSC > access is, where the seemingly better performance from the more > careful/correct CLOCK_MONOTONIC would cause developers to write fragile > userland code that will break when moved from one machine to the next. > > I'd probably rather perf output timestamps to userland using sane clocks > (CLOCK_MONOTONIC), rather then trying to introduce a new time domain to > userland. But I probably could be convinced I'm wrong. I'm surprised that perf has its own clock anyway. But I would like to export the tracing clocks. We have three (well four) of them: trace_clock_local() which is defined to be a very fast clock but may not be synced with other cpus (basically, it just calls sched_clock). trace_clock() which is not totally serialized, but also not totally off (between local and global). This uses local_clock() which is the same thing that perf_clock() uses. trace_clock_global() which is a monotonic clock across CPUs. It's much slower than the above, but works well when you require synced timestamps. There's also trace_clock_counter() which isn't even a clock :-) It's just a incremental atomic counter that goes up every time it's called. This is the most synced clock, but is absolutely meaningless for timestamps. It's just a way to show ordered events. -- 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/
