linux-kernel - Re: [PATCH V2] perf: x86: Improve accuracy of perf/sched clock

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Message-ID: <alpine.DEB.2.11.1508202130440.3873@nanos>
Date:	Thu, 20 Aug 2015 21:31:34 +0200 (CEST)
From:	Thomas Gleixner <tglx@...utronix.de>
To:	Adrian Hunter <adrian.hunter@...el.com>
cc:	Peter Zijlstra <peterz@...radead.org>,
	Ingo Molnar <mingo@...nel.org>,
	Arnaldo Carvalho de Melo <acme@...nel.org>,
	Andy Lutomirski <luto@...capital.net>,
	linux-kernel@...r.kernel.org,
	Stephane Eranian <eranian@...gle.com>,
	Andi Kleen <ak@...ux.intel.com>
Subject: Re: [PATCH V2] perf: x86: Improve accuracy of perf/sched clock

On Wed, 29 Jul 2015, Adrian Hunter wrote:
> @@ -239,6 +239,8 @@ static void set_cyc2ns_scale(unsigned long cpu_khz, int cpu)
>  	unsigned long long tsc_now, ns_now;
>  	struct cyc2ns_data *data;
>  	unsigned long flags;
> +	u64 mult;
> +	u32 shft = 32;
>  
>  	local_irq_save(flags);
>  	sched_clock_idle_sleep_event();
> @@ -256,12 +258,17 @@ static void set_cyc2ns_scale(unsigned long cpu_khz, int cpu)
>  	 * time function is continuous; see the comment near struct
>  	 * cyc2ns_data.
>  	 */
> -	data->cyc2ns_mul =
> -		DIV_ROUND_CLOSEST(NSEC_PER_MSEC << CYC2NS_SCALE_FACTOR,
> -				  cpu_khz);
> -	data->cyc2ns_shift = CYC2NS_SCALE_FACTOR;
> +	mult = (u64)NSEC_PER_MSEC << 32;
> +	mult += cpu_khz / 2;
> +	do_div(mult, cpu_khz);
> +	while (mult > U32_MAX) {
> +		mult >>= 1;
> +		shft -= 1;
> +	}

This is an open coded variant of clocks_calc_mult_shift(). Can we
please use that one?

Thanks,

	tglx
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