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 7191DC10F14 for ; Tue, 23 Apr 2019 04:07:10 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 4807520843 for ; Tue, 23 Apr 2019 04:07:10 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1726594AbfDWEHG (ORCPT ); Tue, 23 Apr 2019 00:07:06 -0400 Received: from mx1.redhat.com ([209.132.183.28]:48734 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1726204AbfDWEHG (ORCPT ); Tue, 23 Apr 2019 00:07:06 -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 39ADB85A04; Tue, 23 Apr 2019 04:07:05 +0000 (UTC) Received: from colo-mx.corp.redhat.com (colo-mx02.intmail.prod.int.phx2.redhat.com [10.5.11.21]) by smtp.corp.redhat.com (Postfix) with ESMTPS id 8FBE41001DDC; Tue, 23 Apr 2019 04:07:04 +0000 (UTC) Received: from zmail21.collab.prod.int.phx2.redhat.com (zmail21.collab.prod.int.phx2.redhat.com [10.5.83.24]) by colo-mx.corp.redhat.com (Postfix) with ESMTP id 902C241F3C; Tue, 23 Apr 2019 04:07:03 +0000 (UTC) Date: Tue, 23 Apr 2019 00:07:03 -0400 (EDT) From: Pankaj Gupta To: Jeff Moyer Cc: Dan Williams , Christoph Hellwig , Jan Kara , 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 Message-ID: <1744824090.22901808.1555992423081.JavaMail.zimbra@redhat.com> In-Reply-To: References: <20190410040826.24371-1-pagupta@redhat.com> <20190418161833.GA22970@infradead.org> Subject: Re: [PATCH v5 1/6] libnvdimm: nd_region flush callback support MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Transfer-Encoding: 7bit X-Originating-IP: [10.65.16.97, 10.4.195.5] Thread-Topic: libnvdimm: nd_region flush callback support Thread-Index: Nrs/K0iSToe0tO+hWxMTOjkrnUwU2A== 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 [10.5.110.26]); Tue, 23 Apr 2019 04:07:05 +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 Mon, Apr 22, 2019 at 8:59 AM Jeff Moyer wrote: > >> > >> 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. > > > > I'd rather say that the if "if (!op) do_generic()" pattern is more > > readable in the general case, saves grepping for who set the op in the > > common case. The fact that it has the potential to be faster is gravy > > at that point. > > If the primary motivation is performance, then I'd expect performance > numbers to back it up. If that isn't the primary motivation, then > choose whichever way you feel is appropriate. Agree. This change enhances the code readability. Will add this change in v6 with other changes. Thank you! Pankaj > > Cheers, > Jeff >