Re: [REGRESSION][BISECTED] Long boot time with Xen HVM guests during PV spinlock initialization

[Juergen Gross <jgross@suse.com>]

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Add Thomas Gleixner (author of the patch introducing the regression).


Juergen

On 08.06.26 12:29, Teddy Astie wrote:
> Hello,
> 
> In 6.12.5+ kernels on AMD CPUs, we observe abnormally long boot times where the 
> guest is struggling on PV spinlock initialization.
> 
> This occurs starting with 6.12.5, and also on more recent kernels on Intel 
> platforms, but that hasn't been fully investigated at this time (but I assume 
> it's a variant of the same issue).
> 
> This occurs since a backport of 76031d9 ("clocksource: Make negative motion 
> detection more robust").
> 
> Some (claude-based) analysis made appears to relate that to the lack of proper 
> max_raw_delta in the jiffies clocksource which appears to make the clock fail to 
> progress meaningfully.
> 
> Here is a raw summary of the analysis
>  > We tracked it down to a single stable backport in 6.12.5: commit 1a678f6829a8 
> ("clocksource: Make negative motion detection more robust", upstream 
> 76031d9536a0). It introduces a max_raw_delta field on struct clocksource but 
> never initializes it for the default boot timekeeper (the jiffies clocksource), 
> so clocksource_delta() clamps every delta to 0 and CLOCK_MONOTONIC freezes while 
> that clocksource is active. On this HVM guest, SMP bring-up runs while the 
> jiffies clocksource is still the timekeeper, and the Xen single shot (high 
> resolution) tick then advances jiffies far too slowly, so the secondary CPUs 
> burn seconds in calibrate_delay().
> 
> Teddy


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