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Date: Wed, 08 Nov 2017 15:56:47 +0900 (KST) From: David Miller <davem@...emloft.net> To: arnd@...db.de Cc: stephen@...workplumber.org, fan.du@...driver.com, edumazet@...gle.com, john.fastabend@...il.com, oleg@...hat.com, ast@...mgrid.com, minipli@...glemail.com, tgraf@...g.ch, johannes.berg@...el.com, netdev@...r.kernel.org, linux-kernel@...r.kernel.org Subject: Re: [PATCH] pktgen: document 32-bit timestamp overflow From: Arnd Bergmann <arnd@...db.de> 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@...db.de> Applied to net-next, thanks Arnd.
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