Re: [PATCH] x86: TSC resync

Ingo Molnar
2008-09-26T07:02:41Z

* Michael Davidson <md@google.com> wrote:

> This patch is a slightly cleaned up version of one which has been in
> use at for some time now at Google.
>
> It uses an HPET based time source to resynchronize the TSC on systems
> where it would otherwise be unsynchronized - eg early AMD Opteron
> based systems where the TSC rate drifts when going in and out of the
> C1E halt state.
>
> While the approach is quite crude it has been effective for systems
> where user space code relies on the TSC advancing at a constant rate
> and being reasonably well synchronized between CPUs. The skew between
> TSC's on different processors as seen from user space is typically
> less than +/- 1000 clock cycles which has proved to be sufficient for
> the applications that we care about.

hm, this patch syncs the TSCs every 20 seconds. That is enough to sync
up AMD CPUs where the TSC slows down _slightly_ (at 10 ppm per second or
so) when it's in HLT.

How does it behave in face of 'TSC stops' systems - systems with C2/C3
sleeps? Basically all modern CPUs that save power are affected by that:
the TSC get brutally cut when idle - almost all modern power saving
laptop, desktop and server CPUs.

Also, what does it do in face of cpufreq-affected TSCs? That too is a
large category of systems. (but most currently shipping CPUs fortunately
have a cpufreq-invariant TSC already.)

> I don't expect this patch to be of much general interest, but if you
> happen to be unlucky enough to have a system where the TSC is not
> synchronized across CPUs and user space code which relies on the
> assumption that it is, then this patch may be useful.
>
> Signed-off-by: Michael Davidson <md@google.com>

This actually looks pretty interesting IMO, and the code looks simple,
clean and straightforward enough - but it might not be enough to be a
generic solution.

Ingo