RE: [discuss] Re: [PATCH] x86-64: Fix bad assumption thatdualcore cpus have synced TSCs

[Langsdorf, Mark]

> On Mon, 2005-09-19 at 21:49 +0200, Andi Kleen wrote:
> > On Mon, Sep 19, 2005 at 12:42:16PM -0700, john stultz wrote:
> > > On Mon, 2005-09-19 at 21:31 +0200, Andi Kleen wrote:
> > > > On Mon, Sep 19, 2005 at 12:16:43PM -0700, john stultz wrote:
> > > > > 	This patch should resolve the issue seen in 
> bugme bug #5105, 
> > > > > where it is assumed that dualcore x86_64 systems have synced 
> > > > > TSCs. This is not the case, and alternate timesources 
> should be 
> > > > > used instead.
> > > > 
> > > > 
> > > > I asked AMD some time ago and they told me it was synchronized. 
> > > > The TSC on K8 is C state invariant, but not P state 
> invariant, but 
> > > > P states always happen synchronized on dual cores.
> > > > 
> > > > So I'm not quite convinced of your explanation yet.
> > > 
> > > Would a litter userspace test checking the TSC 
> synchronization maybe 
> > > shed additional light on the issue?
> > 
> > Sure you can try it.
> 
> So, bugzilla.kernel.org has (temporarily at least) lost the 
> reports from yesterday, but from the email i got, folks using 
> my TSC consistency check that I posted were seeing what 
> appears to be unsynched TSCs on dualcore AMD systems.

My understanding was that each TSC on a dual-core processor
will advance individually and atomically.  They will not 
always be in synchronization.

> Personally I suspect that the powernow driver is putting the 
> cores independently into low power sleep and the TSCs are 
> being independently halted, causing them to become unsynchronized.

The powernow-k8 driver doesn't know what a low power sleep state
is, so I strongly doubt it is involved here.  It only handles
pstates.
 
-Mark Langsdorf
K8 PowerNow! Maintainer
AMD, Inc.

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