hwclock make clock_gettime not accurate

[li feng]

HI guys.

I'm doing some tests about clock_gettime.
And I found that clock_gettime will be affected by hwclock.
It makes clock_gettime slip advance some milliseconds.

Actually, each line prints out every 1ms.


 $ ./a.out -r CLOCK_MONOTONIC
                           130 â
Using delay=1 ms between loop.
Using clock=CLOCK_MONOTONIC.
Clock resolution sec=0 nsec=1
Initial time sec=1621884 nsec=285113956

[delay=1ms] Slip time: 0 s 32 ms <---------hwclock
[delay=1ms] Slip time: 0 s 16 ms <---------hwclock

>From perf:

$ perf record -F 999 hwclock

# To display the perf.data header info, please use
--header/--header-only options.

#

# Samples: 22  of event 'cpu-clock'

# Event count (approx.): 22022022

#

# Overhead  Command  Shared Object      Symbol

# ........  .......  .................  ...............................

#

   77.27%  hwclock  [kernel.kallsyms]  [k] native_read_tsc

   13.64%  hwclock  [kernel.kallsyms]  [k] delay_tsc

    4.55%  hwclock  [kernel.kallsyms]  [k] _raw_spin_unlock_irqrestore

    4.55%  hwclock  libc-2.17.so       [.] __strftime_l




$perf record -F 999 ./a.out

# To display the perf.data header info, please use
--header/--header-only options.

#

# Samples: 7K of event 'cpu-clock'

# Event count (approx.): 79010100220

#

# Overhead  Command  Shared Object      Symbol

# ........  .......  .................  ...............................

#

   28.18%  a.out    [kernel.kallsyms]  [k] _raw_spin_unlock_irqrestore

   20.64%  a.out    [vdso]             [.] __vdso_clock_gettime

   18.46%  a.out    [kernel.kallsyms]  [k] native_read_tsc

    9.81%  a.out    a.out              [.] busy_loop

    4.62%  a.out    a.out              [.] calc_1ms

    2.15%  a.out    libc-2.17.so       [.] clock_gettime

    2.02%  a.out    a.out              [.] overhead_clock

I thought there is a lock contention.
However, when I ran two a.out, the output is correct, not like hwclock.

Anyone knows why?

Thanks.