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From: Thomas Gleixner <tglx@linutronix.de>
To: Johannes Berg <johannes@sipsolutions.net>
Cc: LKML <linux-kernel@vger.kernel.org>,
	John Stultz <john.stultz@linaro.org>,
	Peter Zijlstra <peterz@infradead.org>,
	Ingo Molnar <mingo@kernel.org>,
	"John W. Linville" <linville@tuxdriver.com>,
	linux-wireless@vger.kernel.org,
	Stephen Hemminger <shemminger@linux-foundation.org>,
	netdev <netdev@vger.kernel.org>
Subject: Re: net_timedelta() affected by settimeofday() (was: [patch 12/13] net: mac80211: Remove silly timespec dance)
Date: Thu, 12 Jun 2014 10:57:57 +0200 (CEST)	[thread overview]
Message-ID: <alpine.DEB.2.10.1406121047280.5170@nanos> (raw)
In-Reply-To: <1402562399.8095.5.camel@jlt4.sipsolutions.net>

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




  reply	other threads:[~2014-06-12  8:58 UTC|newest]

Thread overview: 55+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2014-06-11 23:59 [patch 00/13] time: Tree wide cleanup of interfaces and crap Thomas Gleixner
2014-06-11 23:59 ` [patch 01/13] acct: Use ktime_get_ts() Thomas Gleixner
2014-06-21 20:36   ` [tip:timers/core] " tip-bot for Thomas Gleixner
2014-06-11 23:59 ` [patch 02/13] tsacct: " Thomas Gleixner
2014-06-21 20:37   ` [tip:timers/core] " tip-bot for Thomas Gleixner
2014-06-11 23:59 ` [patch 03/13] delayacct: " Thomas Gleixner
2014-06-21 20:36   ` [tip:timers/core] " tip-bot for Thomas Gleixner
2014-06-11 23:59 ` [patch 05/13] sound: " Thomas Gleixner
2014-06-12 10:42   ` Takashi Iwai
2014-06-12 10:51     ` Thomas Gleixner
2014-06-12 10:59       ` Takashi Iwai
2014-06-11 23:59 ` [patch 04/13] net: mac80211: " Thomas Gleixner
2014-06-11 23:59   ` Thomas Gleixner
2014-06-12  6:53   ` Johannes Berg
2014-06-12  9:03     ` Luis R. Rodriguez
2014-06-12  9:03       ` [Cocci] " Luis R. Rodriguez
2014-06-12  9:51       ` Julia.Lawall
2014-06-12  9:51         ` Julia.Lawall at lip6.fr
2014-06-12 10:49         ` Thomas Gleixner
2014-06-12 10:49           ` Thomas Gleixner
2014-06-11 23:59 ` [patch 06/13] sound: intel8x0: Use ktime and ktime_get() Thomas Gleixner
2014-06-11 23:59 ` [patch 08/13] firewire: Use ktime_get_ts() Thomas Gleixner
2014-06-12 12:35   ` Stefan Richter
2014-06-12 14:12     ` Thomas Gleixner
2014-06-21 20:37   ` [tip:timers/core] " tip-bot for Thomas Gleixner
2014-06-11 23:59 ` [patch 07/13] kdb: " Thomas Gleixner
2014-06-21 20:37   ` [tip:timers/core] " tip-bot for Thomas Gleixner
2014-06-11 23:59 ` [patch 10/13] time: Remove do_posix_clock_monotonic_gettime() Thomas Gleixner
2014-06-11 23:59 ` [patch 09/13] fork: Use ktime_get_ts() Thomas Gleixner
2014-06-21 20:37   ` [tip:timers/core] " tip-bot for Thomas Gleixner
2014-06-11 23:59 ` [patch 11/13] wireless: mwifiex: Use the proper interfaces Thomas Gleixner
2014-06-11 23:59   ` Thomas Gleixner
2014-06-12  3:22   ` Bing Zhao
2014-06-12  8:31     ` [patch V2] " Thomas Gleixner
2014-06-12  8:38       ` Johannes Berg
2014-06-13 18:28         ` Bing Zhao
2014-06-13 18:28           ` Bing Zhao
2014-06-11 23:59 ` [patch 12/13] net: mac80211: Remove silly timespec dance Thomas Gleixner
2014-06-11 23:59   ` Thomas Gleixner
2014-06-12  6:49   ` Johannes Berg
2014-06-12  8:19     ` Thomas Gleixner
2014-06-12  8:35       ` net_timedelta() affected by settimeofday() (was: [patch 12/13] net: mac80211: Remove silly timespec dance) Johannes Berg
2014-06-12  8:39         ` Johannes Berg
2014-06-12  8:57           ` Thomas Gleixner [this message]
2014-06-12  9:21             ` Johannes Berg
2014-06-12 14:09               ` Thomas Gleixner
2014-06-12 14:09                 ` Thomas Gleixner
2014-06-13 17:58                 ` Johannes Berg
2014-06-11 23:59 ` [patch 13/13] tomoyo: Use sensible time interface Thomas Gleixner
2014-06-12  0:08   ` John Stultz
2014-06-12  0:22     ` Thomas Gleixner
2014-06-12  0:28       ` John Stultz
2014-06-12  0:36         ` Thomas Gleixner
2014-06-12 11:53   ` Tetsuo Handa
2014-06-21 20:37   ` [tip:timers/core] " tip-bot for Thomas Gleixner

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