From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from eggs.gnu.org ([209.51.188.92]:49830) by lists.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1hHBZ6-0005Yq-N2 for qemu-devel@nongnu.org; Thu, 18 Apr 2019 14:15:17 -0400 Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.71) (envelope-from ) id 1hHBZ5-000131-Hm for qemu-devel@nongnu.org; Thu, 18 Apr 2019 14:15:16 -0400 Received: from mail-ot1-x344.google.com ([2607:f8b0:4864:20::344]:39366) by eggs.gnu.org with esmtps (TLS1.0:RSA_AES_128_CBC_SHA1:16) (Exim 4.71) (envelope-from ) id 1hHBZ5-00011w-3u for qemu-devel@nongnu.org; Thu, 18 Apr 2019 14:15:15 -0400 Received: by mail-ot1-x344.google.com with SMTP id f10so2474782otb.6 for ; Thu, 18 Apr 2019 11:15:13 -0700 (PDT) MIME-Version: 1.0 References: <20190410040826.24371-1-pagupta@redhat.com> <20190410040826.24371-2-pagupta@redhat.com> <20190412083230.GA29850@quack2.suse.cz> <20190418161833.GA22970@infradead.org> In-Reply-To: <20190418161833.GA22970@infradead.org> From: Dan Williams Date: Thu, 18 Apr 2019 11:14:59 -0700 Message-ID: Content-Type: text/plain; charset="UTF-8" Subject: Re: [Qemu-devel] [PATCH v5 1/6] libnvdimm: nd_region flush callback support List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , To: Christoph Hellwig Cc: Jeff Moyer , Jan Kara , Pankaj Gupta , linux-nvdimm , Linux Kernel Mailing List , virtualization@lists.linux-foundation.org, KVM list , linux-fsdevel , Linux ACPI , Qemu Developers , linux-ext4 , linux-xfs , Ross Zwisler , Vishal L Verma , Dave Jiang , "Michael S. Tsirkin" , Jason Wang , Matthew Wilcox , "Rafael J. Wysocki" , Len Brown , Theodore Ts'o , Andreas Dilger , "Darrick J. Wong" , lcapitulino@redhat.com, Kevin Wolf , Igor Mammedov , Nitesh Narayan Lal , Rik van Riel , Stefan Hajnoczi , Andrea Arcangeli , David Hildenbrand , david , cohuck@redhat.com, Xiao Guangrong , Paolo Bonzini , kilobyte@angband.pl, yuval shaia On Thu, Apr 18, 2019 at 9:18 AM Christoph Hellwig wrote: > > On Thu, Apr 18, 2019 at 09:05:05AM -0700, Dan Williams wrote: > > > > I'd either add a comment about avoiding retpoline overhead here or just > > > > make ->flush == NULL mean generic_nvdimm_flush(). Just so that people don't > > > > get confused by the code. > > > > > > Isn't this premature optimization? I really don't like adding things > > > like this without some numbers to show it's worth it. > > > > I don't think it's premature given this optimization technique is > > already being deployed elsewhere, see: > > > > https://lwn.net/Articles/774347/ > > For one this one was backed by numbers, and second after feedback > from Linux we switched to the NULL pointer check instead. Ok I should have noticed the switch to NULL pointer check. However, the question still stands do we want everyone to run numbers to justify this optimization, or make it a new common kernel coding practice to do: if (!object->op) generic_op(object); else object->op(object); ...in hot paths? I agree with not doing premature optimization in principle, but this hack is minimally intrusive from a readability perspective similar to likely()/unlikely() usage which also don't come with numbers on a per patch basis. 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=-0.7 required=3.0 tests=DKIM_INVALID,DKIM_SIGNED, HEADER_FROM_DIFFERENT_DOMAINS,MAILING_LIST_MULTI,SPF_PASS 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 3402EC282DD for ; Thu, 18 Apr 2019 18:34:00 +0000 (UTC) Received: from lists.gnu.org (lists.gnu.org [209.51.188.17]) (using TLSv1 with cipher AES256-SHA (256/256 bits)) (No client certificate requested) by mail.kernel.org (Postfix) with ESMTPS id 0104E2064A for ; Thu, 18 Apr 2019 18:34:00 +0000 (UTC) Authentication-Results: mail.kernel.org; dkim=fail reason="signature verification failed" (2048-bit key) header.d=intel-com.20150623.gappssmtp.com header.i=@intel-com.20150623.gappssmtp.com header.b="ktsdaFbH" DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org 0104E2064A Authentication-Results: mail.kernel.org; dmarc=fail (p=none dis=none) header.from=intel.com Authentication-Results: mail.kernel.org; spf=pass smtp.mailfrom=qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Received: from localhost ([127.0.0.1]:45612 helo=lists.gnu.org) by lists.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1hHBrD-0003Pn-A0 for qemu-devel@archiver.kernel.org; Thu, 18 Apr 2019 14:33:59 -0400 Received: from eggs.gnu.org ([209.51.188.92]:49830) by lists.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1hHBZ6-0005Yq-N2 for qemu-devel@nongnu.org; Thu, 18 Apr 2019 14:15:17 -0400 Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.71) (envelope-from ) id 1hHBZ5-000131-Hm for qemu-devel@nongnu.org; Thu, 18 Apr 2019 14:15:16 -0400 Received: from mail-ot1-x344.google.com ([2607:f8b0:4864:20::344]:39366) by eggs.gnu.org with esmtps (TLS1.0:RSA_AES_128_CBC_SHA1:16) (Exim 4.71) (envelope-from ) id 1hHBZ5-00011w-3u for qemu-devel@nongnu.org; Thu, 18 Apr 2019 14:15:15 -0400 Received: by mail-ot1-x344.google.com with SMTP id f10so2474782otb.6 for ; Thu, 18 Apr 2019 11:15:13 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=intel-com.20150623.gappssmtp.com; s=20150623; h=mime-version:references:in-reply-to:from:date:message-id:subject:to :cc; bh=cNvcYTnY3+vvHUTxTkoLoLivaNOP88tmLELkxGdG9NU=; b=ktsdaFbHHTTd3R9uOez0tgfH91piu1Xl5DlUQg9AdDFLkPBDJI5BaCJt8wdik0yC9L dH1VUAIEKpMOG3eMgmYm1jbHjjxODglk6p70vY9qXowThleT7Q0gQUDAJS9MA3j7Y9lF v8ZxpajT9Rk0mPmn47Onvt24QMYIumY0g4+EJAIXZlXQcKxDlrzTO/b2iH+eG5vkZQVH ouopY1EtaMEeFiq04GySGtlZKge2N6IsV7o1pM7Je/rREB1BqCRPzbORKPO8w/0MElm/ Opf6d01RSEY9uEf/J9C9GZzgTb2wZrkTq4GMtT+Dl+3EhN2pMqMJwWjbjUxgrKq+DhMm CmXw== X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20161025; h=x-gm-message-state:mime-version:references:in-reply-to:from:date :message-id:subject:to:cc; bh=cNvcYTnY3+vvHUTxTkoLoLivaNOP88tmLELkxGdG9NU=; b=B5v6R6sHOEOjuSD6lxKlXLsKjKyguxfcJFANJbAq0EB5sD3YHFBt4//rt7rJHAqSl5 0ghE5DlAROpUFjpf7RPLVMnEG69yX5D7qY5se+ItqQFZXaYO7moE2WdrEAlKz8GYq2eK Zn3s9cX04U/9vSpoqMQsvfleWfRDVUrJ7YRgRL1VKgOZOU8ssATiTe5COPW1JGRQ6ejo PZuEB4+cybRbBXjB4rtxUTcsVOgn6AZveTW1D9viMGLcanLlPhdTrSwyw2pBo79iM0JP YfZtRXrCI0/rVzSj/lkODtzUbPubweDFjt1sHgoLAi4ZdQWKD8OIvxmiuYGbVzpyPlo0 rBmw== X-Gm-Message-State: APjAAAVVK9UbtzGr6dh0wuJQTiVdCiyENFvSo9C/bc8wDEQks6fCcga/ 1kQ1IkmdYZ34bzl5gaIrQBfYNOjh6VPXjKThIlk2fg== X-Google-Smtp-Source: APXvYqw+Y0/Df9bqsKP+vBWmJnbCdRcBlSHVgj/brP9uo3QhMkPjrQhjEhP/44UfrxTt+scRqmIZz03SqgCzRGRlQNI= X-Received: by 2002:a05:6830:15d4:: with SMTP id j20mr4679415otr.367.1555611312059; Thu, 18 Apr 2019 11:15:12 -0700 (PDT) MIME-Version: 1.0 References: <20190410040826.24371-1-pagupta@redhat.com> <20190410040826.24371-2-pagupta@redhat.com> <20190412083230.GA29850@quack2.suse.cz> <20190418161833.GA22970@infradead.org> In-Reply-To: <20190418161833.GA22970@infradead.org> From: Dan Williams Date: Thu, 18 Apr 2019 11:14:59 -0700 Message-ID: To: Christoph Hellwig Content-Type: text/plain; charset="UTF-8" X-detected-operating-system: by eggs.gnu.org: Genre and OS details not recognized. X-Received-From: 2607:f8b0:4864:20::344 Subject: Re: [Qemu-devel] [PATCH v5 1/6] libnvdimm: nd_region flush callback support X-BeenThere: qemu-devel@nongnu.org X-Mailman-Version: 2.1.21 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Cc: Pankaj Gupta , cohuck@redhat.com, Jan Kara , KVM list , "Michael S. Tsirkin" , Jason Wang , david , Qemu Developers , virtualization@lists.linux-foundation.org, Andreas Dilger , Ross Zwisler , Andrea Arcangeli , Dave Jiang , linux-nvdimm , Vishal L Verma , David Hildenbrand , Matthew Wilcox , Linux ACPI , Jeff Moyer , linux-ext4 , Len Brown , kilobyte@angband.pl, Rik van Riel , yuval shaia , Stefan Hajnoczi , Paolo Bonzini , lcapitulino@redhat.com, Kevin Wolf , Nitesh Narayan Lal , Theodore Ts'o , Xiao Guangrong , "Darrick J. Wong" , "Rafael J. Wysocki" , Linux Kernel Mailing List , linux-xfs , linux-fsdevel , Igor Mammedov Errors-To: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Sender: "Qemu-devel" Message-ID: <20190418181459.5JezRXLo3LIcKXj6SWKlsaxR0Qtt_jIXHbAruXl69Fc@z> On Thu, Apr 18, 2019 at 9:18 AM Christoph Hellwig wrote: > > On Thu, Apr 18, 2019 at 09:05:05AM -0700, Dan Williams wrote: > > > > I'd either add a comment about avoiding retpoline overhead here or just > > > > make ->flush == NULL mean generic_nvdimm_flush(). Just so that people don't > > > > get confused by the code. > > > > > > Isn't this premature optimization? I really don't like adding things > > > like this without some numbers to show it's worth it. > > > > I don't think it's premature given this optimization technique is > > already being deployed elsewhere, see: > > > > https://lwn.net/Articles/774347/ > > For one this one was backed by numbers, and second after feedback > from Linux we switched to the NULL pointer check instead. Ok I should have noticed the switch to NULL pointer check. However, the question still stands do we want everyone to run numbers to justify this optimization, or make it a new common kernel coding practice to do: if (!object->op) generic_op(object); else object->op(object); ...in hot paths? I agree with not doing premature optimization in principle, but this hack is minimally intrusive from a readability perspective similar to likely()/unlikely() usage which also don't come with numbers on a per patch basis.