Messages in this thread | | | Date | Tue, 12 May 2015 21:23:04 -0400 | From | Ed Cashin <> | Subject | Re: [Y2038] [PATCH] aoe: Use 64-bit timestamp in frame |
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On 05/12/2015 07:14 AM, Arnd Bergmann wrote: > On Tuesday 12 May 2015 11:44:21 Arnd Bergmann wrote: >> There are of course multiple ways to do this. One way would be to >> change the code to work on 32-bit nanoseconds instead of 32-bit >> microseconds. This requires proving that the we cannot exceed >> 4.29 seconds of round-trip time in calc_rttavg(). >> Is that a valid assumption or not? >> >> If not, we could replace do_gettimeofday() with ktime_get_ts64(). >> This will ensure we don't need a 64-bit division when converting >> the ts64 to a 32-bit microsecond value, and combined with the >> conversion is still no slower than do_gettimeofday(), and it >> still avoids the double bookkeeping because it uses a monotonic >> timebase that is robust against settimeofday. > Two other approaches that occurred to me later: > > - introduce common ktime_get_ms(), ktime_get_us(), ktime_get_real_ms() > and ktime_get_real_is() interfaces, to match the other interfaces > we already provide. These could be done as efficiently or better > than what aoe does manually today. > > - change the timebase that is used for the computations in aoe to use > scaled nanoseconds instead of microseconds. Using > > u32 time = ktime_get_ns() >> 10; > > would give you a similar range and precision as microseconds, but > completely avoid integer division. You could also use a different > shift value to either extend the range beyond 71 minutes, or the > extend the precision to something below a microsecond. This would > be the most efficient implementation, but also require significant > changes to the driver. >
That is an interesting idea. People do care about aoe_deadsecs being pretty accurate, so there would need to be a way to make that remain accurate. The driver will fail outstanding I/O to the target and mark it as "down" after unsuccessfully retransmitting commands to the target for a number of seconds equal to aoe_deadsecs.
As to the efficient ktime_get_us idea, that sounds appealing since you mention that they would be efficient.
Thanks for the analysis.
-- Ed
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