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 X-Spam-Level: X-Spam-Status: No, score=-10.2 required=3.0 tests=BAYES_00, HEADER_FROM_DIFFERENT_DOMAINS,INCLUDES_CR_TRAILER,MAILING_LIST_MULTI, SPF_HELO_NONE,SPF_PASS,USER_AGENT_SANE_1 autolearn=unavailable autolearn_force=no version=3.4.0 Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id 71299C49EA7 for ; Fri, 25 Jun 2021 10:31:03 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 52E5D6141F for ; Fri, 25 Jun 2021 10:31:03 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S231422AbhFYKdW (ORCPT ); Fri, 25 Jun 2021 06:33:22 -0400 Received: from foss.arm.com ([217.140.110.172]:52532 "EHLO foss.arm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S229956AbhFYKdV (ORCPT ); Fri, 25 Jun 2021 06:33:21 -0400 Received: from usa-sjc-imap-foss1.foss.arm.com (unknown [10.121.207.14]) by usa-sjc-mx-foss1.foss.arm.com (Postfix) with ESMTP id 9E888ED1; Fri, 25 Jun 2021 03:31:00 -0700 (PDT) Received: from localhost (unknown [10.1.195.40]) by usa-sjc-imap-foss1.foss.arm.com (Postfix) with ESMTPSA id 3E8BB3F719; Fri, 25 Jun 2021 03:31:00 -0700 (PDT) Date: Fri, 25 Jun 2021 11:30:58 +0100 From: Ionela Voinescu To: Viresh Kumar Cc: Rafael Wysocki , linux-pm@vger.kernel.org, Vincent Guittot , Qian Cai , linux-kernel@vger.kernel.org Subject: Re: [PATCH V3 2/4] cpufreq: cppc: Pass structure instance by reference Message-ID: <20210625103058.GC15540@arm.com> References: <20210623134533.GB12411@arm.com> <20210624022252.zrxsftrvcd43eqra@vireshk-i7> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20210624022252.zrxsftrvcd43eqra@vireshk-i7> User-Agent: Mutt/1.9.4 (2018-02-28) Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Hey, On Thursday 24 Jun 2021 at 07:52:52 (+0530), Viresh Kumar wrote: > On 23-06-21, 14:45, Ionela Voinescu wrote: > > On Monday 21 Jun 2021 at 14:49:35 (+0530), Viresh Kumar wrote: > > > Don't pass structure instance by value, pass it by reference instead. > > > > > > > Might be best to justify the change a bit :) > > I had it and removed later as I thought it would be obvious :) > > > For me this is a judgement call, and I don't really see the reasons for > > changing it: we don't care if the structure is modified or not, as we're > > not reusing the data after the call to cppc_get_rate_from_fbctrs(). > > More so, in this scenario we might not even want for the called function > > to modify the counter values. Also there is no further call to a function > > in cppc_get_rate_from_fbctrs(), that might require references to the > > fb_ctrs. > > > > So what is the reason behind this change? > > How about this commit log then: > > Theoretically speaking, call by reference is cheaper/faster than call by value > for structures as the later requires the compiler to make a new copy of the > whole structure (which has four u64 values here), to be used by the called > function, while with call by reference we just need to pass a single pointer > (u64 on 64-bit architectures) to the existing structure. > > Yes, on modern architectures, the compilers will likely end up using the > processor registers for passing this structure as it isn't doesn't have lot of > fields and it shouldn't be bad eventually, but nevertheless the code should do > the right thing without assuming about the compiler's or architecture's > optimizations. > Yes, that's why "judgement call", which I'll let you make. The code is sane and I like the longer commit message. Reviewed-by: Ionela Voinescu > Don't pass structure instance by value, pass it by reference instead. > > -- > viresh