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Date: Thu, 12 Jun 2014 10:57:57 +0200 (CEST) From: Thomas Gleixner <tglx@...utronix.de> To: Johannes Berg <johannes@...solutions.net> cc: LKML <linux-kernel@...r.kernel.org>, John Stultz <john.stultz@...aro.org>, Peter Zijlstra <peterz@...radead.org>, Ingo Molnar <mingo@...nel.org>, "John W. Linville" <linville@...driver.com>, linux-wireless@...r.kernel.org, Stephen Hemminger <shemminger@...ux-foundation.org>, netdev <netdev@...r.kernel.org> Subject: Re: net_timedelta() affected by settimeofday() (was: [patch 12/13] net: mac80211: Remove silly timespec dance) On Thu, 12 Jun 2014, Johannes Berg wrote: > On Thu, 2014-06-12 at 10:35 +0200, Johannes Berg wrote: > > +netdev, Stephen > > Well, stupid me. Fixing that netdev address. > > > On Thu, 2014-06-12 at 10:19 +0200, Thomas Gleixner wrote: > > > On Thu, 12 Jun 2014, Johannes Berg wrote: > > > > > > > On Wed, 2014-06-11 at 23:59 +0000, Thomas Gleixner wrote: > > > > > > > > > + msrmnt = ktime_to_ms(net_timedelta(skb_arv)); > > > > > > > > This is probably more of a question about net_timedelta(), but is > > > > ktime_get_real() really appropriate for duration measurements? Isn't > > > > that non-monotonic? > > > > > > Well, it's monotonic, but might be affected by settimeofday(). > > > > Right, but isn't that odd? Suddenly your delay measurement here might be > > minutes, hours, or years if you settimeofday() between timestamping and > > calculating the delta. That seems very strange to me, why would that be > > the right behaviour in any way? Indeed. clock monotonic is the appropriate one for measurements. > > Now, it seems that there are only two current users of net_timedelta() > > (in DCCP) so perhaps it's not too late to change some of this? > > > > Maybe in general the skb timestamp should be based on a different clock > > and only adjusted to real time when used in userspace? You have the same problem then, just at a different place: ts = ktime_get(); settimeofday() offset_mono_to_real = new value; userts = mono_to_real(ts); But maybe that's not a real issue, as ktime_get_real() can race with settimeofday() or NTP as well. ts = ktime_get_real(); settimeofday(); userts = ts; So the user might see a weird timestamp for a packet, which cannot be correlated with the user space gettimeofday(). Thanks, tglx -- To unsubscribe from this list: send the line "unsubscribe netdev" in the body of a message to majordomo@...r.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html
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