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) > > And NTP limits the slew rate to 500 PPM, so even if you would get a Assuming it is not broken. > 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. > > 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. > > 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. -- 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/
