Re: [discuss] Re: [PATCH] x86-64: Fix bad assumption that dualcorecpus have synced TSCs

[Scott Lampert]

Langsdorf, Mark wrote:

> > 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.
>
>  

Just to add some end-user input here, I see the same issues regardless 
of whether I'm running with the powernow-k8 or not.  The clock problems 
seem to be unrelated to that, at least on my system.
   -Scott
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