On Thursday 25 March 2010, Benjamin Herrenschmidt wrote:

On Thu, 2010-03-25 at 10:41 +0800, Csdncannon wrote:

quoted

         In my program, the value of the 64-bit time base register is
read out, and you will find the later value is even smaller than the
earlier value from the log “log_timebase”. While the kernel depends on
the accuracy of the timebase for the compensation of the lost PIT
interrupt, the negative value between two continual timebase reading
will bring to the jump of the jiffies. And this timebase problem will
bring to the instability of the gettimeofday system call.

         Do you have any idea about this problem, thanks for your any
advice. Attached is the code and log.

This is a concern, it should definitely not happen. What machine is
that ? is the code compiled 32-bit or 64-bit ? What kernel version ?

Arnd, any chance that could relate to the bug you've been chasing on
Cell ?

We're still busy with the problem analysis on Cell, waiting for a time
slot to run the next test kernel. So far it seems like the timebase
is actually synchronized at a significant accuracy on QS22 to never
cause this problem with correct code, however it is possible to
observe incorrect timebase values on Cell whenever the mftb instruction
is not serialized with memory accesses, e.g. by using an isync in front
of the mftb. On Power6 and other CPUs, that problem will not happen.

	Arnd