On Tue, 2005-10-25 at 17:44 +0200, 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 ...
I presume there would be no workarounds either in terms of kernel
configuration options, right? I have a bunch of SMP machines waiting for
the right kernel :-)
BTW: perhaps this is what is actually hitting me, I've been running a UP
kernel since yesterday as a test with no problems so far.
-- Fernando
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