On Fri, 2014-09-26 at 20:25 +0100, David Ahern wrote: > On 9/26/14, 9:05 AM, Pawel Moll wrote: > > To do the correlation you need both timestamps to be "taken" > > simultaneously: > > > > perf event user event > > -----O--------------+-------------O------> t_mono > > : | : > > : V : > > -----O----------------------------O------> t_perf > > > > Of course it's not possible get both values literally at the same time, > > but placing them in a atomic context a couple of instructions from each > > other still gives pretty good results. The larger this distance is, the > > An early patchset on this topic added the realtime clock as an event and > an ioctl was used to push a sample into the event stream. Yeah, I remember. If I remember correctly correctly the pushback was on a custom event type, right? Generally speaking I don't mind any solution that we'll get us to the place both you and I want to be (just being able to time stamp some performance data in userspace, how difficult can this be! ;-) but I like the flexibility of an extra sample - one can pick and mix events and samples at one's leisure. > In that case > you have wall clock and perf-clock samples taken in the same kernel > context and about as close together as you can get. Yep, that's what I was saying - we can't quite get two timestamps at the *same*, but getting them within a single atomic block of instructions gives reasonable accuracy. Thanks! Pawel -- To unsubscribe from this list: send the line "unsubscribe linux-api" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
