netdev - Re: net_timedelta() affected by settimeofday() (was: [patch 12/13] net: mac80211: Remove silly timespec dance)

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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



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