Re: 2.6.14-rc4-rt7

[linux-os (Dick Johnson)]

On Tue, 25 Oct 2005, Ingo Molnar wrote:

>
> John
>
> i found one source of timekeeping bugs on SMP boxes, it's the
> non-monotonicity of the TSC:
>
> ... time warped from 1270809453 to 1270808096.
> ... MTSC warped from 0000000a731a8c3c [0] to 0000000a731a899c [2].
> ... MTSC warped from 0000000a7c93baec [0] to 0000000a7c93b7a8 [3].
> ... MTSC warped from 0000000a881d6afc [0] to 0000000a881d67d0 [2].
> ... MTSC warped from 0000000a924217a0 [0] to 0000000a924216ac [3].
> ... MTSC warped from 0000000a9c592788 [0] to 0000000a9c59232c [2].
> ... MTSC warped from 0000000aa7aa95c8 [0] to 0000000aa7aa9338 [3].
> ... MTSC warped from 0000000b33206d60 [0] to 0000000b33206a48 [3].
> ... time warped from 26699635824 to 26699633144.
> ... MTSC warped from 00000013f379cb88 [0] to 00000013f379c7e0 [3].
> ... MTSC warped from 0000001413df8660 [0] to 0000001413df8200 [3].
> ... MTSC warped from 00000014194f5360 [1] to 00000014194f51b0 [2].
> ... time warped from 60775269225 to 60775266727.
>
> the number in square brackets is the CPU#. I.e. CPUs on this 4-CPU box
> have small TSC differences, which ends up leaking into the generic TOD
> code, causing real time warps, which causes ktimer weirdnesses (timers
> failed to expire, etc.).
>
> (the above output tracks TSC results globally, under a spinlock. It also
> detects time-warps that propagate into the monotonic clock output.)
>
> unfortunately, there's no easy solution for this. We could make
> cycle_last per-CPU, but that again brings up the question of how to set
> up the per-CPU 'TSC offset' values - those would need similar technique
> that the current clear-all-TSCs-on-all-CPUs code does - which as we can
> see failed ...
>
> 	Ingo

Anything that uses the CPU clock is going to fail in the long-run.
Many motherboards are now shipped with "spread-spectrum" clocks
that can't be disabled. This means that the frequency will no
longer be constant. This is particularly horrible when some
boards sweep the clock on only one direction!

FYI, the spread-spectrum method of cheating on the FCC part 15
rules will eventually catch up with manufacturers. There has been
about 10 years of idiocy where the observed interference is simply
smeared by the spectrum analyzer filters to seem like it's 20 dB
or so lower than it really is. The increased interference from
electronic equipment that use such fundamental cheating is only
now beginning to be recognized by th FCC. The DOC (Canada) has
been complaining about this for many years.

Maybe the RTC chip should be used as a RTC instead of a gravitational
clamp used once upon startup and once upon shutdown?

Cheers,
Dick Johnson
Penguin : Linux version 2.6.13.4 on an i686 machine (5589.55 BogoMips).
Warning : 98.36% of all statistics are fiction.
.

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