Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1753938AbbGQQzI (ORCPT ); Fri, 17 Jul 2015 12:55:08 -0400 Received: from bh-25.webhostbox.net ([208.91.199.152]:48226 "EHLO bh-25.webhostbox.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752250AbbGQQzG (ORCPT ); Fri, 17 Jul 2015 12:55:06 -0400 Message-ID: <55A93365.4000702@roeck-us.net> Date: Fri, 17 Jul 2015 09:55:01 -0700 From: Guenter Roeck User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:31.0) Gecko/20100101 Thunderbird/31.7.0 MIME-Version: 1.0 To: "Odzioba, Lukasz" , Jean Delvare CC: "Yu, Fenghua" , "lm-sensors@lm-sensors.org" , "linux-kernel@vger.kernel.org" Subject: Re: [PATCH] hwmon: coretemp: use list instead of fixed size array for temp data References: <1436976253-4810-1-git-send-email-lukasz.odzioba@intel.com> <20150715230734.76347af2@endymion.delvare> In-Reply-To: Content-Type: text/plain; charset=windows-1252; format=flowed Content-Transfer-Encoding: 7bit X-Authenticated_sender: linux@roeck-us.net X-OutGoing-Spam-Status: No, score=-1.0 X-AntiAbuse: This header was added to track abuse, please include it with any abuse report X-AntiAbuse: Primary Hostname - bh-25.webhostbox.net X-AntiAbuse: Original Domain - vger.kernel.org X-AntiAbuse: Originator/Caller UID/GID - [47 12] / [47 12] X-AntiAbuse: Sender Address Domain - roeck-us.net X-Get-Message-Sender-Via: bh-25.webhostbox.net: authenticated_id: linux@roeck-us.net X-Source: X-Source-Args: X-Source-Dir: Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 2670 Lines: 60 On 07/16/2015 06:17 AM, Odzioba, Lukasz wrote: > On Wednesday, July 15, 2015 11:08 PM Jean Delvare wrote: >> I see the benefit of removing the arbitrary limit, but why use a list >> instead of a dynamically allocated array? This is turning a O(1) >> algorithm into a O(n) algorithm. I know n isn't too large in this case >> but I still consider it bad practice if it can be avoided. > > This patch tries to solve two problems which are present on current hardware: > -cpus with more than 32 cores > -core id is greater than NUM_REAL_CORES > > In both cases it ends up with the following error in dmesg: > coretemp coretemp.0: Adding Core XXX failed > > We could just bump NUM_REAL_CORES to 128 like we did in 2012 and call it > solved, but the problem will come back eventually and it is waste of > memory for cpus with handful of cores. > > If there is way to obtain maximum core id during module initialization, > then we can allocate array and keep O(1) access. If we can't figure out > maximum core id then we can increase size of the array when new cores are > added. The problem with this is that core id enumeration can be sparse so > again we have waste of memory. > >> Do you expect core IDs to become arbitrarily large? >> Significantly larger than the core count? > > The question is what does significantly mean. > According to Documentation/cputopology.txt: > --- > 2) /sys/devices/system/cpu/cpuX/topology/core_id: > > the CPU core ID of cpuX. Typically it is the hardware platform's > identifier (rather than the kernel's). The actual value is > architecture and platform dependent. > --- > > Even now we can have one core present with id like 60 (think of Xeon Phi). > I haven't seen in the wild insane core ids like thousands, but I don't see > a reason why we shouldn't handle it in a proper manner. > > Current patch does not use more memory than it is needed, but the pitfall is > that it increased access complexity from O(1) to O(n). We could slide another > patch on top of this one to reduce access complexity from O(n) to O(logn) > by using i.e. radix tree. I preferred to send functional fix in the first > place, and then work on optimization if there is a concern about it. > Forgive me if it is not appropriate. > You don't really explain why your approach would be better than allocating an array of pointers to struct temp_data and increasing its size using krealloc if needed. Guenter -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/