Re: [PATCH] pktgen: document 32-bit timestamp overflow

[David Miller]

From: Arnd Bergmann <arnd@xxxxxxxx>
Date: Tue,  7 Nov 2017 11:38:32 +0100

> Timestamps in pktgen are currently retrieved using the deprecated
> do_gettimeofday() function that wraps its signed 32-bit seconds in 2038
> (on 32-bit architectures) and requires a division operation to calculate
> microseconds.
> 
> The pktgen header is also defined with the same limitations, hardcoding
> to a 32-bit seconds field that can be interpreted as unsigned to produce
> times that only wrap in 2106. Whatever code reads the timestamps should
> be aware of that problem in general, but probably doesn't care too
> much as we are mostly interested in the time passing between packets,
> and that is correctly represented.
> 
> Using 64-bit nanoseconds would be cheaper and good for 584 years. Using
> monotonic times would also make this unambiguous by avoiding the overflow,
> but would make it harder to correlate to the times with those on remote
> machines. Either approach would require adding a new runtime flag and
> implementing the same thing on the remote side, which we probably don't
> want to do unless someone sees it as a real problem. Also, this should
> be coordinated with other pktgen implementations and might need a new
> magic number.
> 
> For the moment, I'm documenting the overflow in the source code, and
> changing the implementation over to an open-coded ktime_get_real_ts64()
> plus division, so we don't have to look at it again while scanning for
> deprecated time interfaces.
> 
> Signed-off-by: Arnd Bergmann <arnd@xxxxxxxx>

Applied to net-next, thanks Arnd.