Re: [PATCH 04/15] powerpc/time: Prepare to stop elapsing in dynticks-idle

Christophe Leroy (CS GROUP)
2026-02-25T17:59:27Z

Le 25/02/2026 à 18:47, Shrikanth Hegde a écrit :
> Hi Christophe.
>
>> I think I'm starting to understand now.
>>
>> I think the problem is that acct->starttime has an invalid value the
>> very first time it is used.
>>
>> We are probably lacking an initial value in paca->accounting.starttime.
>> This should likely be initialised from mftb in head_64.S in
>> start_here_common for main CPU and __secondary_start for other CPUs or
>> maybe at higher level in C in setup_arch() and start_secondary()
>>
>> Christophe
>
> How about below? this works too.

Fine it is works, it means we found the real problem.

What about using the newly added vtime_reset() ? See below (untested)

diff --git a/arch/powerpc/kernel/time.c b/arch/powerpc/kernel/time.c
index 9b3167274653..f4aef85106ac 100644
--- a/arch/powerpc/kernel/time.c
+++ b/arch/powerpc/kernel/time.c
@@ -377,7 +377,6 @@ void vtime_task_switch(struct task_struct *prev)
}
}

-#ifdef CONFIG_NO_HZ_COMMON
/**
* vtime_reset - Fast forward vtime entry clocks
*
@@ -394,6 +393,7 @@ void vtime_reset(void)
#endif
}

+#ifdef CONFIG_NO_HZ_COMMON
/**
* vtime_dyntick_start - Inform vtime about entry to idle-dynticks
*
@@ -931,6 +931,7 @@ static void __init set_decrementer_max(void)
static void __init init_decrementer_clockevent(void)
{
register_decrementer_clockevent(smp_processor_id());
+ vtime_reset();
}

void secondary_cpu_time_init(void)
@@ -946,6 +947,7 @@ void secondary_cpu_time_init(void)
/* FIME: Should make unrelated change to move snapshot_timebase
* call here ! */
register_decrementer_clockevent(smp_processor_id());
+ vtime_reset();
}

/*

>
> ---
>
> diff --git a/arch/powerpc/kernel/time.c b/arch/powerpc/kernel/time.c
> index 18506740f4a4..af129645b7f7 100644
> --- a/arch/powerpc/kernel/time.c
> +++ b/arch/powerpc/kernel/time.c
> @@ -928,9 +928,24 @@ static void __init set_decrementer_max(void)
>                 bits, decrementer_max);
>  }
>
> +#ifdef CONFIG_VIRT_CPU_ACCOUNTING_NATIVE
> +/*
> + * This is done to initialize the starttime correctly. with this
> + * /proc/stat show correct values similar to
> CONFIG_VIRT_CPU_ACCOUNTING_GEN
> + */
> +static void init_cpu_accounting_startime(void)
> +{
> +       struct cpu_accounting_data *acct = get_accounting(current);
> +       acct->starttime = mftb();
> +}
> +#else
> +static void init_cpu_accounting_startime(void) { };
> +#endif
> +
>  static void __init init_decrementer_clockevent(void)
>  {
>         register_decrementer_clockevent(smp_processor_id());
> +       init_cpu_accounting_startime();
>  }
>
>  void secondary_cpu_time_init(void)
> @@ -946,6 +961,8 @@ void secondary_cpu_time_init(void)
>         /* FIME: Should make unrelated change to move snapshot_timebase
>          * call here ! */
>         register_decrementer_clockevent(smp_processor_id());
> +
> +       init_cpu_accounting_startime();
>  }
>
>  /*
>