On Mon, Nov 3, 2014 at 5:11 PM, John Stultz <john.stultz@xxxxxxxxxx> wrote: > On Mon, Nov 3, 2014 at 4:58 PM, Andy Lutomirski <luto@xxxxxxxxxxxxxx> wrote: >> On Mon, Nov 3, 2014 at 4:28 PM, Pawel Moll <pawel.moll@xxxxxxx> wrote: >>> From: Pawel Moll <mail@xxxxxxxxxxxxx> >>> Thomas suggested solution which gets down to my original proposal for >>> sched/monotonic clock correlation - an additional sample type so events >>> can be "double stamped" using different clock sources providing >>> synchronisation points for later time approximation. I've just extended >>> the implementation with configuration value to select the clock source. >>> If the first patch (making perf timestamps monotonic) gets accepted, >>> there will be no immediate need for this one, but I'd like to gain some >>> feedback anyway. >>> >> >> I have nothing intelligent to add to the potentional Thomas/Ingo >> showdown, but I do have a related thought. :) >> >> If you're going to add double-stamped packets, can you also add a >> syscall to read multiple clocks at once, atomically? Or can you >> otherwise add a non-perf mechanism to get at this data? >> >> Because the realtime to monotonic offset is really quite useful for >> things like this, and it seems silly to make people actually open a >> perf_event to get at it. > > So this comes up periodically, but I don't think I've seen a interface > proposal that was decent yet. > > Also, if you want to read multiple clocks at once, do you stop at two, > or three, or... there's possibly quite a few. Additionally some > clocks may not be possible to read atomically (perf/sched clock and > system time for example may be based on different underlying > clocksources). The general idea feels like its creeping towards some > "atomically expose all timekeeping state" mega-interface. > > I've got some thoughts on what a possible interface that wouldn't be > awful could look like, but I'm still hesitant because I don't really > know if exposing this sort of data is actually a good idea long term. My only real thought here is that, if perf is going to try to do this, then presumably it should be reasonably integrated w/ the core timing code. I.e. if perf does this, then presumably the core code should know about it and there should be a core interface to it. --Andy > > thanks > -john -- Andy Lutomirski AMA Capital Management, LLC -- 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
