From mboxrd@z Thu Jan 1 00:00:00 1970 From: Shreyas B Prabhu Subject: Re: [PATCH v2] cpuidle: Fix last_residency division Date: Fri, 24 Jun 2016 21:31:35 +0530 Message-ID: <576D595F.9090001@linux.vnet.ibm.com> References: <1466756638-2362-1-git-send-email-shreyas@linux.vnet.ibm.com> <063D6719AE5E284EB5DD2968C1650D6D5F4E3E66@AcuExch.aculab.com> <14940457.SEYBmqcunj@wuerfel> Mime-Version: 1.0 Content-Type: text/plain; charset=windows-1252 Content-Transfer-Encoding: 7bit Return-path: Received: from mx0a-001b2d01.pphosted.com ([148.163.156.1]:23630 "EHLO mx0a-001b2d01.pphosted.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751138AbcFXQBz (ORCPT ); Fri, 24 Jun 2016 12:01:55 -0400 Received: from pps.filterd (m0098409.ppops.net [127.0.0.1]) by mx0a-001b2d01.pphosted.com (8.16.0.11/8.16.0.11) with SMTP id u5OFxChA046530 for ; Fri, 24 Jun 2016 12:01:46 -0400 Received: from e23smtp05.au.ibm.com (e23smtp05.au.ibm.com [202.81.31.147]) by mx0a-001b2d01.pphosted.com with ESMTP id 23rq9m94q7-1 (version=TLSv1.2 cipher=AES256-SHA bits=256 verify=NOT) for ; Fri, 24 Jun 2016 12:01:45 -0400 Received: from localhost by e23smtp05.au.ibm.com with IBM ESMTP SMTP Gateway: Authorized Use Only! Violators will be prosecuted for from ; Sat, 25 Jun 2016 02:01:43 +1000 Received: from d23relay06.au.ibm.com (d23relay06.au.ibm.com [9.185.63.219]) by d23dlp01.au.ibm.com (Postfix) with ESMTP id 0C1182CE805B for ; Sat, 25 Jun 2016 02:01:40 +1000 (EST) Received: from d23av01.au.ibm.com (d23av01.au.ibm.com [9.190.234.96]) by d23relay06.au.ibm.com (8.14.9/8.14.9/NCO v10.0) with ESMTP id u5OG1evW65404988 for ; Sat, 25 Jun 2016 02:01:40 +1000 Received: from d23av01.au.ibm.com (localhost [127.0.0.1]) by d23av01.au.ibm.com (8.14.4/8.14.4/NCO v10.0 AVout) with ESMTP id u5OG1d8V004789 for ; Sat, 25 Jun 2016 02:01:39 +1000 In-Reply-To: <14940457.SEYBmqcunj@wuerfel> Sender: linux-pm-owner@vger.kernel.org List-Id: linux-pm@vger.kernel.org To: Arnd Bergmann , linuxppc-dev@lists.ozlabs.org Cc: David Laight , "rjw@rjwysocki.net" , "daniel.lezcano@linaro.org" , "anton@samba.org" , "linux-pm@vger.kernel.org" On 06/24/2016 03:41 PM, Arnd Bergmann wrote: > On Friday, June 24, 2016 9:00:48 AM CEST David Laight wrote: >> The intent of the >> 10 was probably to avoid an expensive 64bit divide. >> So maybe something like: >> diff = time_end - time_start; >> if (diff >= INT_MAX/2) >> diff_32 = INT_MAX/2/1000; >> else >> diff_32 = diff; >> diff_32 += diff_32 >> 6; >> diff_32 >>= 10; >> } >> >> Adding an extra 1/32 makes the division by be something slightly below 1000. > > Why not change the definition of the time to nanoseconds and update > the users accordingly? > > I see that cpuidle_enter_state() writes to the last_residency value, > and it is read in three places: ladder_select_state(), menu_update() > and show_state_time() (for sysfs). > Updating show_state_time() to convert ns to us gets bit messy since show_state_##_name which is used for disable, usage and time simply returns state_usage->_name. And state_usage->time updation happens in cpuidle_enter_state() itself. > If those functions are called less often than cpuidle_enter_state(), > we could just move the division there. Since the divisor is constant, > do_div() can convert it into a multiply and shift, or we could use > your the code you suggest above, or use a 32-bit division most of > the time: > > if (diff <= UINT_MAX) > diff_32 = (u32)diff / NSECS_PER_USEC; > else > diff_32 = div_u64(diff, NSECS_PER_USEC; > > which gcc itself will turn into a multiplication or series of > shifts on CPUs on which that is faster. > I'm not sure which division method of the three suggested here to use. Does anyone have a strong preference? --Shreyas