On Thu, Feb 19, 2015 at 04:38:57PM +0200, Adrian Hunter wrote: > On 19/02/15 15:50, Peter Zijlstra wrote: > > On Thu, Feb 19, 2015 at 02:11:08PM +0200, Adrian Hunter wrote: > >> Hi > >> > >> With the advent of switching perf_clock to CLOCK_MONOTONIC, > >> it will not be possible to convert perf_clock directly to/from > >> TSC. So add the ability to sample TSC instead. > > > > Well, you can, mostly. MONOTONIC is only affected by NTP slew rate > > changes, not offset changes. > > man page says is also subject to adjtime(3)
which is slew adjustment; read the adjtime manpage :-) > > And NTP limits the slew rate to 500 PPM, so even if you would get a > > Assuming it is not broken. NTP people are a cautious crowd, sure they get it wrong just like the rest of us, but mostly it needs to work. > > slew change and then not update the userpage data for a second you'd be > > maximally off by 0.0005 seconds. > > That could still be enough to break the decoder. It will certainly > misrepresent the order of events, which is a big loss of information. What decoder? perf report is already subject to much larger shifts in time if you run it on say a core2 machine. > > And that is way below what the current perf clock guarantees on funny > > hardware. > > > > If you're really worried about this; we could maybe get John and Thomas > > to allow us a callback on every slew change so we can update the > > userpage data ASAP, much reducing the max error. > > > > Say it takes a 10e5 cycles to update your userpage, then you're never > > further off than 50 cycles, which is below your ART multiplier. > > You still need to wake up user space to read the userpage. Uhm what? Userspace is already awake. > > Does that really matter? Also, if you have a stable crystal, the slew > > rate change should be minimal and infrequent, never getting you close to > > these numbers. > > > > So no, I'm not convinced we need this. > > Adding TSC to the sample is a lot simpler and more accurate. Finding multiple samples and interpolating between them is much simpler than reading tsc and doing the mult, shift and offset addition? I suspect you're talking about something else entirely; your changelogs are inadequate for they tell ntohing of your usecase and have me guessing. Don't do that. -- 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/
