On 20/02/15 16:29, Peter Zijlstra wrote: > While thinking on the whole clock discussion it occured to me we have > two distinct uses of time: > > 1) the tracking of event/ctx/cgroup enabled/running/stopped times > which includes the self-monitoring support in struct > perf_event_mmap_page. > > 2) the actual timestamps visible in the data records. > > And we've been conflating them. > > The first is all about tracking time deltas, nobody should really care > in what time base that happens, its all relative information, as long > as its internally consistent it works. > > The second however is what people are worried about when having to > merge their data with external sources. And here we have the > discussion on MONOTONIC vs MONOTONIC_RAW etc.. > > Where MONOTONIC is good for correlating between machines (static > offset), MONOTNIC_RAW is required for correlating against a fixed rate > hardware clock. > > This means configurability; now 1) makes that hard because it needs to > be internally consistent across groups of unrelated events; which is > why we had to have a global perf_clock(). > > However, for 2) it doesn't really matter, perf itself doesn't care > what it writes into the buffer. > > The below patch makes the distinction between these two cases by > adding perf_event_clock() which is used for the second case. It > further makes this configurable on a per-event basis, but adds a few > sanity checks such that we cannot combine events with different clocks > in confusing ways. > > And since we then have per-event configurability we might as well > retain the 'legacy' behaviour as a default.
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