linux-kernel - Re: [PATCH v8 1/6] x86/tsc: remove tsc

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Date:   Mon, 13 Nov 2017 11:52:14 +0800
From:   Dou Liyang <douly.fnst@...fujitsu.com>
To:     Pavel Tatashin <pasha.tatashin@...cle.com>,
        <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>
Subject: Re: [PATCH v8 1/6] x86/tsc: remove tsc_disabled flag

Hi Pavel,

At 11/09/2017 11:01 AM, Pavel Tatashin wrote:
> tsc_disabled is set when notsc is passed as kernel parameter. The reason we
> have notsc is to avoid timing problems on multi-socket systems.  We already
> have a mechanism, however, to detect and resolve these issues by invoking
> tsc unstable path. Thus, make notsc to behave the same as tsc=unstable.
>
> Signed-off-by: Pavel Tatashin <pasha.tatashin@...cle.com>

I am not sure if I could add the signature.

Anyway, it looks good to me.

Reviewed-by: Dou Liyang <douly.fnst@...fujitsu.com>

> ---
>  arch/x86/kernel/tsc.c | 19 +++----------------
>  1 file changed, 3 insertions(+), 16 deletions(-)
>
> diff --git a/arch/x86/kernel/tsc.c b/arch/x86/kernel/tsc.c
> index 97907e152356..dbce6fa32aa9 100644
> --- a/arch/x86/kernel/tsc.c
> +++ b/arch/x86/kernel/tsc.c
> @@ -37,11 +37,6 @@ EXPORT_SYMBOL(tsc_khz);
>   */
>  static int __read_mostly tsc_unstable;
>
> -/* native_sched_clock() is called before tsc_init(), so
> -   we must start with the TSC soft disabled to prevent
> -   erroneous rdtsc usage on !boot_cpu_has(X86_FEATURE_TSC) processors */
> -static int __read_mostly tsc_disabled = -1;
> -
>  static DEFINE_STATIC_KEY_FALSE(__use_tsc);
>
>  int tsc_clocksource_reliable;
> @@ -247,8 +242,7 @@ EXPORT_SYMBOL_GPL(check_tsc_unstable);
>  #ifdef CONFIG_X86_TSC
>  int __init notsc_setup(char *str)
>  {
> -	pr_warn("Kernel compiled with CONFIG_X86_TSC, cannot disable TSC completely\n");
> -	tsc_disabled = 1;
> +	mark_tsc_unstable("boot parameter notsc");
>  	return 1;
>  }
>  #else
> @@ -1229,7 +1223,7 @@ static void tsc_refine_calibration_work(struct work_struct *work)
>
>  static int __init init_tsc_clocksource(void)
>  {
> -	if (!boot_cpu_has(X86_FEATURE_TSC) || tsc_disabled > 0 || !tsc_khz)
> +	if (!boot_cpu_has(X86_FEATURE_TSC) || !tsc_khz)
>  		return 0;
>
>  	if (tsc_clocksource_reliable)
> @@ -1330,12 +1324,6 @@ void __init tsc_init(void)
>  		set_cyc2ns_scale(tsc_khz, cpu, cyc);
>  	}
>
> -	if (tsc_disabled > 0)
> -		return;
> -
> -	/* now allow native_sched_clock() to use rdtsc */
> -
> -	tsc_disabled = 0;
>  	static_branch_enable(&__use_tsc);
>
>  	if (!no_sched_irq_time)
> @@ -1365,10 +1353,9 @@ void __init tsc_init(void)
>  unsigned long calibrate_delay_is_known(void)
>  {
>  	int sibling, cpu = smp_processor_id();
> -	int constant_tsc = cpu_has(&cpu_data(cpu), X86_FEATURE_CONSTANT_TSC);
>  	const struct cpumask *mask = topology_core_cpumask(cpu);
>
> -	if (tsc_disabled || !constant_tsc || !mask)
> +	if (!cpu_has(&cpu_data(cpu), X86_FEATURE_CONSTANT_TSC) || !mask)
>  		return 0;
>
>  	sibling = cpumask_any_but(mask, cpu);
>