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

From: Johannes Berg <johannes@sipsolutions.net>
To: Thomas Gleixner <tglx@linutronix.de>
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 10:39:59 +0200	[thread overview]
Message-ID: <1402562399.8095.5.camel@jlt4.sipsolutions.net> (raw)
In-Reply-To: <1402562114.8095.3.camel@jlt4.sipsolutions.net> (sfid-20140612_103558_525847_209A5C98)

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?
> 
> 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?