From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by smtp.lore.kernel.org (Postfix) with ESMTP id 601D6C433FE for ; Wed, 26 Oct 2022 06:45:03 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S233009AbiJZGpB (ORCPT ); Wed, 26 Oct 2022 02:45:01 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:50268 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S232906AbiJZGox (ORCPT ); Wed, 26 Oct 2022 02:44:53 -0400 Received: from szxga03-in.huawei.com (szxga03-in.huawei.com [45.249.212.189]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id 942291A047; Tue, 25 Oct 2022 23:44:50 -0700 (PDT) Received: from dggpemm500023.china.huawei.com (unknown [172.30.72.56]) by szxga03-in.huawei.com (SkyGuard) with ESMTP id 4MxzkL1NrmzJnGN; Wed, 26 Oct 2022 14:42:02 +0800 (CST) Received: from dggpemm500006.china.huawei.com (7.185.36.236) by dggpemm500023.china.huawei.com (7.185.36.83) with Microsoft SMTP Server (version=TLS1_2, cipher=TLS_ECDHE_RSA_WITH_AES_128_GCM_SHA256) id 15.1.2375.31; Wed, 26 Oct 2022 14:44:48 +0800 Received: from [10.174.178.55] (10.174.178.55) by dggpemm500006.china.huawei.com (7.185.36.236) with Microsoft SMTP Server (version=TLS1_2, cipher=TLS_ECDHE_RSA_WITH_AES_128_GCM_SHA256) id 15.1.2375.31; Wed, 26 Oct 2022 14:44:47 +0800 Subject: Re: [PATCH v7 00/11] kallsyms: Optimizes the performance of lookup symbols To: Luis Chamberlain CC: Josh Poimboeuf , Jiri Kosina , Miroslav Benes , Petr Mladek , Joe Lawrence , , , Masahiro Yamada , Alexei Starovoitov , Jiri Olsa , Kees Cook , Andrew Morton , , Steven Rostedt , "Ingo Molnar" References: <20221017064950.2038-1-thunder.leizhen@huawei.com> From: "Leizhen (ThunderTown)" Message-ID: <77f1c8f0-5e67-0e57-9285-15ba613044fb@huawei.com> Date: Wed, 26 Oct 2022 14:44:36 +0800 User-Agent: Mozilla/5.0 (Windows NT 10.0; WOW64; rv:60.0) Gecko/20100101 Thunderbird/60.7.0 MIME-Version: 1.0 In-Reply-To: Content-Type: text/plain; charset="utf-8" Content-Language: en-US Content-Transfer-Encoding: 7bit X-Originating-IP: [10.174.178.55] X-ClientProxiedBy: dggems705-chm.china.huawei.com (10.3.19.182) To dggpemm500006.china.huawei.com (7.185.36.236) X-CFilter-Loop: Reflected Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 2022/10/26 1:53, Luis Chamberlain wrote: > On Wed, Oct 19, 2022 at 10:11:58PM +0800, Leizhen (ThunderTown) wrote: >> >> >> On 2022/10/19 20:01, Luis Chamberlain wrote: >>> On Mon, Oct 17, 2022 at 02:49:39PM +0800, Zhen Lei wrote: >>>> Currently, to search for a symbol, we need to expand the symbols in >>>> 'kallsyms_names' one by one, and then use the expanded string for >>>> comparison. This is very slow. >>>> >>>> In fact, we can first compress the name being looked up and then use >>>> it for comparison when traversing 'kallsyms_names'. >>>> >>>> This patch series optimizes the performance of function kallsyms_lookup_name(), >>>> and function klp_find_object_symbol() in the livepatch module. Based on the >>>> test results, the performance overhead is reduced to 5%. That is, the >>>> performance of these functions is improved by 20 times. >>> >>> Stupid question, is a hash table in order? >> >> No hash table. >> >> All symbols are arranged in ascending order of address. For example: cat /proc/kallsyms >> >> The addresses of all symbols are stored in kallsyms_addresses[], and names of all symbols >> are stored in kallsyms_names[]. The elements in these two arrays are in a one-to-one >> relationship. For any symbol, it has the same index in both arrays. >> >> Therefore, when we look up a symbolic name based on an address, we use a binary lookup. >> However, when we look up an address based on a symbol name, we can only traverse array >> kallsyms_names[] in sequence. I think the reason why hash is not used is to save memory. > > This answers how we don't use a hash table, the question was *should* we > use one? I'm not the original author, and I can only answer now based on my understanding. Maybe the original author didn't think of the hash method, or he has weighed it out. Hash is a good solution if only performance is required and memory overhead is not considered. Using hash will increase the memory size by up to "4 * kallsyms_num_syms + 4 * ARRAY_SIZE(hashtable)" bytes, kallsyms_num_syms is about 1-2 million. Because I don't know what hash algorithm will be used, the cost of generating the hash value corresponding to the symbol name is unknown now. But I think it's gonna be small. But it definitely needs a simpler algorithm, the tool needs to implement the same hash algorithm. If the hash is not very uniform or ARRAY_SIZE(hashtable) is small, then my current approach still makes sense. So maybe hash can be deferred to the next phase of improvement. > > Luis > . > -- Regards, Zhen Lei