linux-kernel - Re: x86/tsc: Fix ART for TSC_KNOWN

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Date:   Mon, 13 Mar 2017 20:04:14 +0100 (CET)
From:   Thomas Gleixner <tglx@...utronix.de>
To:     Peter Zijlstra <peterz@...radead.org>
cc:     linux-kernel@...r.kernel.org, subhransu.s.prusty@...el.com,
        kevin.b.stanton@...el.com, akataria@...are.com,
        john.stultz@...aro.org, christopher.s.hall@...el.com,
        Ingo Molnar <mingo@...nel.org>
Subject: Re: x86/tsc: Fix ART for TSC_KNOWN_FREQ

On Mon, 13 Mar 2017, Peter Zijlstra wrote:

> 
> Subhransu reported that convert_art_to_tsc() isn't working for him.
> 
> It turns out that because of commit:
> 
>   57779dc2b3b7 ("x86, tsc: Skip refined tsc calibration on systems with reliable TSC")

That's the wrong culprit. The problem was introduced with the ART support
patches. I fixed the changelog up.

> Systems with TSC_KNOWN_FREQ (due to having CPUID.15h) will not run the
> refined calibration thing, and then forget to set the ART clocksource
> relation.
> 
> We cannot set the ART relation prior to running the refined calibration;
> therefore duplicate the lines.
> 
> XXX: should we clear this in mark_tsc_unstable() ?

It's already handled. get_device_system_timestamp() which convert ART to
clock monotonic checks whether the timekeeper clocksource is the same as
the correlated ART clocksource, i.e. TSC. So when timekeeping switches away
from TSC get_device_system_timestamp() returns -ENODEV.

Thanks,

	tglx