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SubjectRe: [PATCH 0/4] x86/platform/UV: Update TSC support

On 9/29/2017 1:46 AM, Peter Zijlstra wrote:
> On Thu, Sep 28, 2017 at 01:03:39PM -0500, 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.

I tested with both detect_art enabled and disabled and didn't notice a
difference though I wasn't sure what test to run to verify whether it
was being used or not. (I'd be glad to run some specific test if one
can be suggested?) The num/denom setting for a 2100Mhz CPU was 168/2 if
that information helps?

> 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?

My previous version of the patches had me setting a flag that could be
checked by the tsc_sanitize_first_cpu() function and disable the
requirement of "TSC == 0 on socket 0" for any arch that specified it.
(And UV did set that flag.)

But Thomas said it was "hackery" and that TSC being 0 on socket 0 was no
longer a requirement. So I took it out for this version and made the
"TSC == 0 on socket 0" no longer the default for any arch.
> Other than that, the patches look good to me.

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