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SubjectRe: [PATCH 17/17] Add timekeeping documentation
On 06/16/2010 10:55 PM, Andi Kleen wrote:
> Zachary Amsden<> writes:
> I think listing all the obscure bits in the PIT was an attempt to
> weed out the weak and weary readers early, right?

Very perceptive of you ;)

>> +this as well. Several hardware limitations make the problem worse - if it is
>> +not possible to write the full 32-bits of the TSC, it may be impossible to
>> +match the TSC in newly arriving CPUs to that of the rest of the system,
>> +resulting in unsynchronized TSCs. This may be done by BIOS or system software,
>> +but in practice, getting a perfectly synchronized TSC will not be possible
>> +unless all values are read from the same clock, which generally only is
>> +possible on single socket systems or those with special hardware
>> +support.
> That's not true, single crystal for all sockets is very common
> as long as you only have a single motherboard.
> Of course there might be other reasons why the TSC is unsynchronized
> (e.g. stop count in C-states), but the single clock is not the problem.

The point is about hotplug CPUs. Any hotplugged CPU will not have a
perfectly synchronized TSC, ever, even on a single socket, single
crystal board.

>> +3.4) TSC and C-states
>> +
>> +C-states, or idling states of the processor, especially C1E and deeper sleep
>> +states may be problematic for TSC as well. The TSC may stop advancing in such
>> +a state, resulting in a TSC which is behind that of other CPUs when execution
>> +is resumed. Such CPUs must be detected and flagged by the operating system
>> +based on CPU and chipset identifications.
>> +
>> +The TSC in such a case may be corrected by catching it up to a known external
>> +clocksource.
> ... This is fixed in recent CPUs ...

And has a CPU flag associated with it (NONSTOP_TSC). But whether it
remains fixed across all models and vendors remains to be seen.

>> +
>> +3.5) TSC frequency change / P-states
>> +
>> +To make things slightly more interesting, some CPUs may change requency. They
>> +may or may not run the TSC at the same rate, and because the frequency change
>> +may be staggered or slewed, at some points in time, the TSC rate may not be
>> +known other than falling within a range of values. In this case, the TSC will
>> +not be a stable time source, and must be calibrated against a known, stable,
>> +external clock to be a usable source of time.
>> +
>> +Whether the TSC runs at a constant rate or scales with the P-state is model
>> +dependent and must be determined by inspecting CPUID, chipset or various MSR
>> +fields.
> ... In general newer CPUs should not have problems with this anymore

But that's not the point. Old CPUs will, and I'm detailing all of the
existing issues, relevant to new CPUs or not. A lot of these "old" CPUs
are still in service and will be for quite some time.

>> +
>> +4) Virtualization Problems
>> +
>> +Timekeeping is especially problematic for virtualization because a number of
>> +challenges arise. The most obvious problem is that time is now shared between
>> +the host and, potentially, a number of virtual machines. This happens
>> +naturally on X86 systems when SMM mode is used by the BIOS, but not to such a
>> +degree nor with such frequency. However, the fact that SMM mode may cause
> The SMM reference here seems at best odd.

SMIs are notorious for frustrating writers of careful timing loops, and
several pieces of kernel code take time measurements multiple times to
rule out outliers from it.

Seems a perfectly reasonable reference to me, perhaps I should explain
it better.

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