Re: [Y2038] [PATCH] aoe: Use 64-bit timestamp in frame

From: Ed Cashin
Date: Tue May 12 2015 - 21:30:00 EST

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.

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