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

[Thomas Gleixner <tglx@kernel.org>]

On Mon, Jun 08 2026 at 12:29, Teddy Astie wrote:
> 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.

Bah. jiffies clocksource is registered way _after_ timekeeping started to
use it.

The untested below should fix that.

Thanks,

        tglx
---
--- a/kernel/time/jiffies.c
+++ b/kernel/time/jiffies.c
@@ -60,15 +60,9 @@ EXPORT_SYMBOL(get_jiffies_64);
 
 EXPORT_SYMBOL(jiffies);
 
-static int __init init_jiffies_clocksource(void)
-{
-	return __clocksource_register(&clocksource_jiffies);
-}
-
-core_initcall(init_jiffies_clocksource);
-
 struct clocksource * __init __weak clocksource_default_clock(void)
 {
+	clocksource_register(&clocksource_jiffies);
 	return &clocksource_jiffies;
 }