linux-kernel - Re: [PATCH 13/15] sched: Use a static_key for sched_clock

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Date:	Wed, 22 Jan 2014 18:53:22 -0500
From:	Josh Boyer <jwboyer@...oraproject.org>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	Markus Trippelsdorf <markus@...ppelsdorf.de>,
	Sasha Levin <sasha.levin@...cle.com>,
	Arjan van de Ven <arjan@...ux.intel.com>,
	Len Brown <lenb@...nel.org>,
	"Rafael J. Wysocki" <rjw@...ysocki.net>,
	Eliezer Tamir <eliezer.tamir@...ux.intel.com>,
	Zhang Rui <rui.zhang@...el.com>, jacob.jun.pan@...ux.intel.com,
	Mike Galbraith <bitbucket@...ine.de>,
	Ingo Molnar <mingo@...nel.org>,
	"H. Peter Anvin" <hpa@...or.com>,
	Paul McKenney <paulmck@...ux.vnet.ibm.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	John Stultz <john.stultz@...aro.org>,
	Andy Lutomirski <luto@...capital.net>,
	"Linux-Kernel@...r. Kernel. Org" <linux-kernel@...r.kernel.org>,
	Dave Young <dyoung@...hat.com>
Subject: Re: [PATCH 13/15] sched: Use a static_key for sched_clock_stable

On Wed, Jan 22, 2014 at 4:08 PM, Peter Zijlstra <peterz@...radead.org> wrote:
>>
>> I think its the right region to look through. My current suspect is the
>> linear continuity fit with the initial 'random' multiplier.
>>
>> That initial 'random' multiplier can get us quite high, and we'll fit
>> the function to match that but continue at a sane rate.
>>
>> I'll try and prod a little more later this evening as time permits.
>
> Does this cure things?

This seems to correct the time jump I was seeing on my i7 box as well.

josh

>
> ---
>  arch/x86/kernel/tsc.c | 11 +++++++----
>  1 file changed, 7 insertions(+), 4 deletions(-)
>
> diff --git a/arch/x86/kernel/tsc.c b/arch/x86/kernel/tsc.c
> index a3acbac2ee72..bb04148c5fe0 100644
> --- a/arch/x86/kernel/tsc.c
> +++ b/arch/x86/kernel/tsc.c
> @@ -237,7 +237,7 @@ static inline unsigned long long cycles_2_ns(unsigned long long cyc)
>  /* XXX surely we already have this someplace in the kernel?! */
>  #define DIV_ROUND(n, d) (((n) + ((d) / 2)) / (d))
>
> -static void set_cyc2ns_scale(unsigned long cpu_khz, int cpu)
> +static void set_cyc2ns_scale(unsigned long cpu_khz, int cpu, bool origin)
>  {
>         unsigned long long tsc_now, ns_now;
>         struct cyc2ns_data *data;
> @@ -252,7 +252,10 @@ static void set_cyc2ns_scale(unsigned long cpu_khz, int cpu)
>         data = cyc2ns_write_begin(cpu);
>
>         rdtscll(tsc_now);
> -       ns_now = cycles_2_ns(tsc_now);
> +       if (origin)
> +               ns_now = 0;
> +       else
> +               ns_now = cycles_2_ns(tsc_now);
>
>         /*
>          * Compute a new multiplier as per the above comment and ensure our
> @@ -926,7 +929,7 @@ static int time_cpufreq_notifier(struct notifier_block *nb, unsigned long val,
>                         mark_tsc_unstable("cpufreq changes");
>         }
>
> -       set_cyc2ns_scale(tsc_khz, freq->cpu);
> +       set_cyc2ns_scale(tsc_khz, freq->cpu, false);
>
>         return 0;
>  }
> @@ -1199,7 +1202,7 @@ void __init tsc_init(void)
>          */
>         for_each_possible_cpu(cpu) {
>                 cyc2ns_init(cpu);
> -               set_cyc2ns_scale(cpu_khz, cpu);
> +               set_cyc2ns_scale(cpu_khz, cpu, true);
>         }
>
>         if (tsc_disabled > 0)
> --
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