Date: Wed, 21 Sep 2005 17:04:04 +0200
From: Andi Kleen <ak@suse.de>
To: Ray Bryant <raybry@mpdtxmail.amd.com>
Cc: Daniel Jacobowitz <dan@debian.org>, john stultz <johnstul@us.ibm.com>,
       Andi Kleen <ak@suse.de>, Andrew Morton <akpm@osdl.org>,
       lkml <linux-kernel@vger.kernel.org>, discuss@x86-64.org
Subject: Re: [discuss] Re: [PATCH] x86-64: Fix bad assumption that dualcore cpus have synced TSCs
Message-ID: <20050921150404.GD12810@verdi.suse.de>
References: <1127157404.3455.209.camel@cog.beaverton.ibm.com> <1127242785.11080.20.camel@cog.beaverton.ibm.com> <20050921040342.GA7175@nevyn.them.org> <200509211015.09356.raybry@mpdtxmail.amd.com>
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On Wed, Sep 21, 2005 at 10:15:08AM -0500, Ray Bryant wrote:
> On Tuesday 20 September 2005 23:03, Daniel Jacobowitz wrote:
> 
> >
> > FYI, at least I have reproduced this without powernow loaded.
> 
> There are cases that we are aware of where the TSC will count slower while the 
> processor is halted.    This can make TSC's get out of sync on dual cores.

Ok thanks for the confirmation. I guess John's patch is ok then.
Drawback is much slower to extremly slow gettimeofday  (depending
if the chipset/BIOS has usable HPET, most seem not to) 

> 
> I wonder if you can reproduce this problem while also running a pair of cpu 
> bound tasks on your dual core box.   If you can't, then this is the culprit.
> 
> In general, however, on multisocket systems, you can't depend on TSC's being 
> synchronized between sockets, so all of this is moot.   We just have to deal 
> with it. 

We handle this, but single socket dual core was special cased because
I was told previously it should be ok.

-Andi
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