linux-kernel - Re: [PATCH v14 24/25] sched: early boot clock

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Date:   Thu, 19 Jul 2018 12:39:03 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Pavel Tatashin <pasha.tatashin@...cle.com>
Cc:     steven.sistare@...cle.com, daniel.m.jordan@...cle.com,
        linux@...linux.org.uk, schwidefsky@...ibm.com,
        heiko.carstens@...ibm.com, john.stultz@...aro.org,
        sboyd@...eaurora.org, x86@...nel.org, linux-kernel@...r.kernel.org,
        mingo@...hat.com, tglx@...utronix.de, hpa@...or.com,
        douly.fnst@...fujitsu.com, prarit@...hat.com, feng.tang@...el.com,
        pmladek@...e.com, gnomes@...rguk.ukuu.org.uk,
        linux-s390@...r.kernel.org, boris.ostrovsky@...cle.com,
        jgross@...e.com, pbonzini@...hat.com
Subject: Re: [PATCH v14 24/25] sched: early boot clock

On Tue, Jul 17, 2018 at 10:22:10PM -0400, Pavel Tatashin wrote:

> diff --git a/kernel/sched/clock.c b/kernel/sched/clock.c
> index 0e9dbb2d9aea..7a8a63b940ee 100644
> --- a/kernel/sched/clock.c
> +++ b/kernel/sched/clock.c
> @@ -202,7 +202,15 @@ static void __sched_clock_gtod_offset(void)
>  
>  void __init sched_clock_init(void)
>  {
> +	unsigned long flags;
> +
>  	sched_clock_running = 1;
> +
> +	/* Adjust __gtod_offset for contigious transition from early clock */
> +	local_irq_save(flags);
> +	sched_clock_tick();
> +	local_irq_restore(flags);
> +	__sched_clock_gtod_offset();

I think we want to keep __sched_clock_gtod_offset() inside the IRQ
disabled region.

And I just looked at my patch:

  https://lkml.kernel.org/r/20180626090003.GA2458@hirez.programming.kicks-ass.net

and that had a comment about how we wanted to set the gtod offset
_before_ setting sched_clock_running, yet here you do it the other way
around. Hmm?

>  }
>  /*
>   * We run this as late_initcall() such that it runs after all built-in drivers,
> @@ -356,7 +364,7 @@ u64 sched_clock_cpu(int cpu)
>  		return sched_clock() + __sched_clock_offset;
>  
>  	if (unlikely(!sched_clock_running))
> -		return 0ull;
> +		return sched_clock();
>  
>  	preempt_disable_notrace();
>  	scd = cpu_sdc(cpu);