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

Teddy Astie <[email protected]> Tue, 9 Jun 2026 11:27:41 +0200

Newsgroups	dev.linux.lists.regressions,org.kernel.vger.linux-kernel,org.xenproject.lists.xen-devel
Message-ID	<1780997262.8631fc262581453bbf619ec5b2062170.19eabb58e51000701b@vates.tech>

Le 08/06/2026 à 17:15, Thomas Gleixner a écrit :
> 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;
>   }
>   
> 
> 

Hmm, there's a case where clocksource_mutex gets taken twice (both in 
__clocksource_register_scale() (through clocksource_register()) and 
clocksource_done_booting()).

What about making clocksource_jiffies gain ->enable() to setup what ever 
needs to be (like calling __clocksource_update_freq_scale()) ?

Teddy

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