linux-kernel - Re: TSC Problems (warp between CPUs)

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Message-ID: <e44ff95b0f7962a11cfcdb83da6c5595@xor.bz>
Date:	Sun, 29 Dec 2013 11:21:19 +1100
From:	Alex <xor@....bz>
To:	<linux-kernel@...r.kernel.org>
Subject: Re: TSC Problems (warp between CPUs)

On 2013-12-29 01:35, One Thousand Gnomes wrote:
>> not guaranteed to be precise. For example a SMI (System Management
>> Interrupt) could interrupt the software flow that is attempting to 
>> write
>> the time-stamp counter immediately prior to the WRMSR. This could 
>> mean
>> the value written to the TSC could vary by thousands to millions of
>> clocks.
>
> Yes SMI is a disaster area for any real time activity (and many other
> things ;) ), but many systems actually make little use of it, 
> especially
> once the USB is owned by the OS.
>
> For synchronization you can retry the sync if it isn't within an
> acceptable range. The odds of getting an SMI mid sync setup should be
> very very low, so the odds of repeating the failure several times 
> should
> be negligible and after a few tries you could give up and assume the
> hardware is buggered then fall back to HPET.

Hi Alan,

The sync retry sounds like an interesting strategy but is outside my 
knowledge
scope as a regular end user (I do have a reasonable understanding of C 
and can patch+
recompile) but I lack the architecture knowledge to implement a patch 
to do this.

Any suggestions?

-- 
Kind Regards,
Alex.

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