linux-kernel - Re: [PATCH 0/4] x86/platform/UV: Update TSC support

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Date:   Fri, 29 Sep 2017 10:46:32 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     mike.travis@....com
Cc:     Ingo Molnar <mingo@...hat.com>, "H. Peter Anvin" <hpa@...or.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Bin Gao <bin.gao@...ux.intel.com>,
        Prarit Bhargava <prarit@...hat.com>,
        Dimitri Sivanich <dimitri.sivanich@....com>,
        Andrew Banman <andrew.banman@....com>,
        Russ Anderson <russ.anderson@....com>,
        linux-kernel@...r.kernel.org, x86@...nel.org
Subject: Re: [PATCH 0/4] x86/platform/UV: Update TSC support

On Thu, Sep 28, 2017 at 01:03:39PM -0500, mike.travis@....com wrote:
> 
> The UV BIOS goes to considerable effort to get the TSC synchronization
> accurate across the entire system.  Included in that are multiple chassis
> that can have 32+ sockets.  The architecture does support an external
> high resolution clock to aid in maintaining this synchronization.
> 
> The resulting TSC accuracy set by the UV system BIOS is much better
> than the generic kernel TSC ADJUST functions.  This is important for
> applications that read the TSC values directly for accessing data bases.
> 
> *   These patches disable an assumption made by the kernel tsc sync
>     functions that Socket 0 in the system should have a TSC ADJUST
>     value of zero.  This is not correct when the chassis are reset
>     asynchronously to each other so which TSC's should be zero is
>     not predictable.
> 
> *   When the system BIOS determines that the TSC is not stable, it then
>     sets a flag so the UV kernel setup can set the "tsc is unstable"
>     flag.  A patch now prevents the kernel from attempting to fix the
>     TSC causing a slew of warning messages.
> 
> *   It also eliminates another avalanche of warning messages from older
>     BIOS that did not have the TSC ADJUST MSR (ex. >3000 msgs in a 32
>     socket Skylake system).  It now notes this with a single warning
>     message and then moves on with fixing them.

So I would still like to get clarification on how ART works (or likely
doesn't) on your systems. I think for now its fairly prudent to kill
detect_art() on UV.

Also, while indeed not strictly required, that TSC_ADJUST==0 test on
bootcpu is nice for consumer systems, BIOS did something 'weird' if that
is not true. Is something like is_uv_system() available early enough?

Other than that, the patches look good to me.