From mboxrd@z Thu Jan  1 00:00:00 1970
Return-Path: <linux-kernel-owner@vger.kernel.org>
Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand
	id S1753369AbbHUGtn (ORCPT <rfc822;w@1wt.eu>);
	Fri, 21 Aug 2015 02:49:43 -0400
Received: from mga11.intel.com ([192.55.52.93]:49460 "EHLO mga11.intel.com"
	rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP
	id S1751900AbbHUGtm (ORCPT <rfc822;linux-kernel@vger.kernel.org>);
	Fri, 21 Aug 2015 02:49:42 -0400
X-ExtLoop1: 1
X-IronPort-AV: E=Sophos;i="5.15,720,1432623600"; 
   d="scan'208";a="545880468"
Message-ID: <55D6C961.9040303@intel.com>
Date: Fri, 21 Aug 2015 09:46:57 +0300
From: Adrian Hunter <adrian.hunter@intel.com>
Organization: Intel Finland Oy, Registered Address: PL 281, 00181 Helsinki,
 Business Identity Code: 0357606 - 4, Domiciled in Helsinki 
User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:31.0) Gecko/20100101 Thunderbird/31.8.0
MIME-Version: 1.0
To: Thomas Gleixner <tglx@linutronix.de>
CC: Peter Zijlstra <peterz@infradead.org>, Ingo Molnar <mingo@kernel.org>,
        Arnaldo Carvalho de Melo <acme@kernel.org>,
        Andy Lutomirski <luto@amacapital.net>, linux-kernel@vger.kernel.org,
        Stephane Eranian <eranian@google.com>, Andi Kleen <ak@linux.intel.com>
Subject: Re: [PATCH V2] perf: x86: Improve accuracy of perf/sched clock
References: <1438118085-3641-1-git-send-email-adrian.hunter@intel.com> <alpine.DEB.2.11.1508202130440.3873@nanos>
In-Reply-To: <alpine.DEB.2.11.1508202130440.3873@nanos>
Content-Type: text/plain; charset=windows-1252
Content-Transfer-Encoding: 7bit
Sender: linux-kernel-owner@vger.kernel.org
List-ID: <linux-kernel.vger.kernel.org>
X-Mailing-List: linux-kernel@vger.kernel.org

On 20/08/15 22:31, Thomas Gleixner wrote:
> 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?

Sure.  clocks_calc_mult_shift() does a division on each shift which is a bit
slower (avoids a 1-bit rounding error but that will never be more than 1 in
2^32 in this case), and the 'maxsec' functionality is not needed because
mul_u64_u32_shr() is used to avoid 64-bit overflow.

I will send V3.