threads runtime value is incorrect (tc_cpu_ticks() problem)

[Svatopluk Kraus]

Hi,

  I've tested FreeBSD-current from June 16 2011 on x86 (AMD Elan
SC400). I found out that a sum of runtimes of all threads is about 120
minutes after 180 minutes of system uptime and the difference is
getting worse with time. The problem is in tc_cpu_ticks()
implementation which takes into acount just one timecounter overflow,
but in tested BSP (16-bit hardware counter) very often more than one
overflow occured between two tc_cpu_ticks() calls.

  I understand that 16-bit timecounter is a real relict nowadays, but
I would like to solve the problem somehow reasonably. I have a few
questions.

  According to description in definition of timecounter structure
(sys/timetc.h), tc_get_timecount() should read the counter and
tc_counter_mask should mask off any unimplemented bits. In
tc_cpu_ticks(), if ticks count returned from tc_get_timecount()
overflows then (tc_counter_mask + 1) is added to result.

  However, timecounter hardware can be initialized to value from
interval (0, tc_counter_mask>, so if the description of
tc_get_timecount() doesn't lie then adding (tc_counter_mask + 1) value
at all times is not correct. Better description which satisfies
tc_cpu_ticks() implementation is that tc_get_timecount() should count
the ticks in interval <0, tc_counter_mask>. That's what
i8254_get_timecount() (in sys/x86/isa/clock.c) does really. However,
if tc_get_timecount() should count the ticks (and doesn't read the
counter) then it can count the ticks in full uint64_t range? And
tc_cpu_ticks() implementation could be very simple (not masking, not
overflow checking). In i8254_get_timecount(), it is enough to change
global variable 'i8254_offset' and local variable 'count' from
uint16_t to uint64_t type.

  Now, cpu_ticks() (whichs point to tc_cpu_ticks() by default) is
called from mi_switch() which must be called often enough to satisfy
tc_cpu_ticks() implementation (recognize just one timecounter
overflow). That limits some of system parameters (at least hz
selection).

  It looks that tc_counter_mask is a little bit misused?

  Maybe, tc_cpu_ticks() is only used for back compatibility and new
system should use set_cputicker() to change this default?

  Thanks for some help to better understand that.

      Svata