Ah, I see now what is happening. The userspace code is dealing with
microseconds, while the syscalls all deal with nanoseconds.
So, when using absolute time, we read the clock (in nanoseconds), add
that to our microseconds value after converting (that is, rounding the
value), then set the timer, and read it back.
Take this run, for example:
[pid 27982] timer_create(CLOCK_MONOTONIC, NULL, [0]) = 0
[pid 27982] clock_gettime(CLOCK_MONOTONIC, {tv_sec=53718, tv_nsec=537178810}) = 0
[pid 27982] timer_settime(0, TIMER_ABSTIME, {it_interval={tv_sec=0, tv_nsec=0},
it_value={tv_sec=53718, tv_nsec=587179000}}, {it_interval={tv_sec=0,
tv_nsec=0}, it_value={tv_sec=0, tv_nsec=0}}) = 0
[pid 27982] timer_gettime(0, {it_interval={tv_sec=0, tv_nsec=0},
it_value={tv_sec=0, tv_nsec=49842203}}) = 0
We read the time and set it 587179000 - 537178810 = 50000190 nanoseconds
in the future, which is more than our 50000 microseconds. No surprise
that, if you are fast enough, you are going to read a larger value back.
Not a kernel bug!
Cascardo.
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https://bugs.launchpad.net/bugs/1898633
Title:
timer_settime01 from ubuntu_ltp_syscalls failed on bionic /aws-4.15
on c5.metal
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