Re: IA64 implementation of timesource for new time of day subsystem

[Christoph Lameter <clameter@engr.sgi.com>]

On Mon, 16 May 2005, john stultz wrote:

> Actually that shouldn't be necessary. Look at my arch-x86-64 patch or
> vsyscall-i386 patch for how the arch_vsyscall_gtod_update() function is
> used. It provides an arch specific hook called by the timeofday core to
> provide the information you desire. 
> 
> Please let me know if it is not sufficient for some reason. 

Obviously this wont work since you cannot execute C code nor functions in 
an ia64 fastcall. I need the variables exported.

> > I would recommend to add jitter compensation to the time sources. Otherwise
> > each ITC/TSC like timesource will have to implement that on its own.
> 
> Just to clarify for others, this is the same unsynced cpu cycle counter
> problem that affects the TSC on i386 and x86-64. ia64 gets around the
> problem by checking on every call to gettimeofday() if the ITC value is
> less then the ITC value used on the previous call to gettimeofday(). If
> the value is less (ie: would result in time going backwards) it just
> uses the last value to calculate time. It then uses cmpxchg to
> atomically update the last ITC value. 

Nope. You are way off here. Unsynched cpu cycle counters lead to the ITC 
timesource not being registered.

> The problem I have with this is it that if the ITCs are not synced, they
> really are not good timesources. If one cpu's ITC is behind another, the
> net result of the above algorithm is cpu 2 will always just use cpu 1's
> last calculated time. This could cause jumps in time when a process
> moves from cpu2 to cpu1.

Note again that the use of cmpxchg is NOT covering the case of ITCs not 
being synced. If the ITCs are not synced then no timesource will be 
established for ITC!

This is the case of ITC's running synchronous but at a tiny offset. The 
startup on IA64 syncs the ITCs but cannot guarantee a complete sync. There 
may be a small offset of a few clock ticks. The cmpxchg is 
needed to compensate for that small offset. I imagine that other 
architectures have similar issues.

> Since it only affects the TSC and ITC, I think keeping the decision to
> use cmpxchg in the timesource code, as you've implemented with the ITC
> is the best way to go. If you really want to you can special case the
> arch specific fsyscall code by switching on the time source .name, and
> that would allow you to use a similar cmpxchg algorithm there as well. 

Again this will not work on IA64 since it does the fast system calls in a 
different way.

Clock jitter can affect multiple clock sources that may fluctuate
in a minor way due to a variety of influences. Jitter compensation may 
help in these situations.

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