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=-1.0 required=3.0 tests=HEADER_FROM_DIFFERENT_DOMAINS, MAILING_LIST_MULTI,SPF_PASS,URIBL_BLOCKED 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 28BF9C10F11 for ; Mon, 22 Apr 2019 15:52:01 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 03E512077C for ; Mon, 22 Apr 2019 15:52:01 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1727486AbfDVPvu (ORCPT ); Mon, 22 Apr 2019 11:51:50 -0400 Received: from mx1.redhat.com ([209.132.183.28]:60968 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1726945AbfDVPvt (ORCPT ); Mon, 22 Apr 2019 11:51:49 -0400 Received: from smtp.corp.redhat.com (int-mx07.intmail.prod.int.phx2.redhat.com [10.5.11.22]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by mx1.redhat.com (Postfix) with ESMTPS id A5B52C049E20; Mon, 22 Apr 2019 15:51:48 +0000 (UTC) Received: from segfault.boston.devel.redhat.com (segfault.boston.devel.redhat.com [10.19.60.26]) by smtp.corp.redhat.com (Postfix) with ESMTPS id 389081018A0D; Mon, 22 Apr 2019 15:51:36 +0000 (UTC) From: Jeff Moyer To: Dan Williams Cc: Christoph Hellwig , 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 Subject: Re: [PATCH v5 1/6] libnvdimm: nd_region flush callback support References: <20190410040826.24371-1-pagupta@redhat.com> <20190410040826.24371-2-pagupta@redhat.com> <20190412083230.GA29850@quack2.suse.cz> <20190418161833.GA22970@infradead.org> X-PGP-KeyID: 1F78E1B4 X-PGP-CertKey: F6FE 280D 8293 F72C 65FD 5A58 1FF8 A7CA 1F78 E1B4 Date: Mon, 22 Apr 2019 11:51:35 -0400 In-Reply-To: (Dan Williams's message of "Thu, 18 Apr 2019 11:14:59 -0700") Message-ID: User-Agent: Gnus/5.13 (Gnus v5.13) Emacs/26.1 (gnu/linux) MIME-Version: 1.0 Content-Type: text/plain X-Scanned-By: MIMEDefang 2.84 on 10.5.11.22 X-Greylist: Sender IP whitelisted, not delayed by milter-greylist-4.5.16 (mx1.redhat.com [10.5.110.31]); Mon, 22 Apr 2019 15:51:49 +0000 (UTC) Sender: linux-ext4-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-ext4@vger.kernel.org Dan Williams writes: > 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 don't think nvdimm_flush is a hot path. Numbers of some representative workload would prove one of us right. -Jeff