From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from eggs.gnu.org ([209.51.188.92]:52703) by lists.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1hH9ct-0000K1-Mq for qemu-devel@nongnu.org; Thu, 18 Apr 2019 12:11:04 -0400 Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.71) (envelope-from ) id 1hH9cs-0007vn-Mq for qemu-devel@nongnu.org; Thu, 18 Apr 2019 12:11:03 -0400 Received: from mx1.redhat.com ([209.132.183.28]:42759) by eggs.gnu.org with esmtps (TLS1.0:DHE_RSA_AES_256_CBC_SHA1:32) (Exim 4.71) (envelope-from ) id 1hH9cs-0007uq-Ey for qemu-devel@nongnu.org; Thu, 18 Apr 2019 12:11:02 -0400 From: Jeff Moyer References: <20190410040826.24371-1-pagupta@redhat.com> <20190410040826.24371-2-pagupta@redhat.com> <20190412083230.GA29850@quack2.suse.cz> Date: Thu, 18 Apr 2019 12:10:44 -0400 In-Reply-To: (Dan Williams's message of "Thu, 18 Apr 2019 09:05:05 -0700") Message-ID: MIME-Version: 1.0 Content-Type: text/plain 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: Dan Williams Cc: 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" , Christoph Hellwig , 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 Dan Williams writes: > On Fri, Apr 12, 2019 at 6:12 AM Jeff Moyer wrote: >> >> Jan Kara writes: >> >> > On Thu 11-04-19 07:51:48, Dan Williams wrote: >> >> On Tue, Apr 9, 2019 at 9:09 PM Pankaj Gupta wrote: >> >> > + } else { >> >> > + if (nd_region->flush(nd_region)) >> >> > + rc = -EIO; >> >> >> >> Given the common case wants to be fast and synchronous I think we >> >> should try to avoid retpoline overhead by default. So something like >> >> this: >> >> >> >> if (nd_region->flush == generic_nvdimm_flush) >> >> rc = generic_nvdimm_flush(...); >> > >> > 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/ The technique is fine, but that doesn't mean it should be applied everywhere. Is *this* code path really going to benefit from the optimization? -Jeff 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=-0.9 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 03ADAC10F0E for ; Thu, 18 Apr 2019 16:12:57 +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 C5A8820835 for ; Thu, 18 Apr 2019 16:12:56 +0000 (UTC) DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org C5A8820835 Authentication-Results: mail.kernel.org; dmarc=fail (p=none dis=none) header.from=redhat.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]:43840 helo=lists.gnu.org) by lists.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1hH9eh-0001QM-Dw for qemu-devel@archiver.kernel.org; Thu, 18 Apr 2019 12:12:55 -0400 Received: from eggs.gnu.org ([209.51.188.92]:52703) by lists.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1hH9ct-0000K1-Mq for qemu-devel@nongnu.org; Thu, 18 Apr 2019 12:11:04 -0400 Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.71) (envelope-from ) id 1hH9cs-0007vn-Mq for qemu-devel@nongnu.org; Thu, 18 Apr 2019 12:11:03 -0400 Received: from mx1.redhat.com ([209.132.183.28]:42759) by eggs.gnu.org with esmtps (TLS1.0:DHE_RSA_AES_256_CBC_SHA1:32) (Exim 4.71) (envelope-from ) id 1hH9cs-0007uq-Ey for qemu-devel@nongnu.org; Thu, 18 Apr 2019 12:11:02 -0400 Received: from smtp.corp.redhat.com (int-mx06.intmail.prod.int.phx2.redhat.com [10.5.11.16]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by mx1.redhat.com (Postfix) with ESMTPS id 042CB306C3EC; Thu, 18 Apr 2019 16:11:00 +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 024235C206; Thu, 18 Apr 2019 16:10:45 +0000 (UTC) From: Jeff Moyer To: Dan Williams References: <20190410040826.24371-1-pagupta@redhat.com> <20190410040826.24371-2-pagupta@redhat.com> <20190412083230.GA29850@quack2.suse.cz> X-PGP-KeyID: 1F78E1B4 X-PGP-CertKey: F6FE 280D 8293 F72C 65FD 5A58 1FF8 A7CA 1F78 E1B4 Date: Thu, 18 Apr 2019 12:10:44 -0400 In-Reply-To: (Dan Williams's message of "Thu, 18 Apr 2019 09:05:05 -0700") Message-ID: User-Agent: Gnus/5.13 (Gnus v5.13) Emacs/26.1 (gnu/linux) MIME-Version: 1.0 Content-Type: text/plain; charset="UTF-8" X-Scanned-By: MIMEDefang 2.79 on 10.5.11.16 X-Greylist: Sender IP whitelisted, not delayed by milter-greylist-4.5.16 (mx1.redhat.com [10.5.110.45]); Thu, 18 Apr 2019 16:11:00 +0000 (UTC) X-detected-operating-system: by eggs.gnu.org: GNU/Linux 2.2.x-3.x [generic] X-Received-From: 209.132.183.28 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 , Christoph Hellwig , Linux ACPI , 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: <20190418161044.esjDyxXdOQiIV5PdwB2X_2itSThnjSRMpOqvJsj9O8A@z> Dan Williams writes: > On Fri, Apr 12, 2019 at 6:12 AM Jeff Moyer wrote: >> >> Jan Kara writes: >> >> > On Thu 11-04-19 07:51:48, Dan Williams wrote: >> >> On Tue, Apr 9, 2019 at 9:09 PM Pankaj Gupta wrote: >> >> > + } else { >> >> > + if (nd_region->flush(nd_region)) >> >> > + rc = -EIO; >> >> >> >> Given the common case wants to be fast and synchronous I think we >> >> should try to avoid retpoline overhead by default. So something like >> >> this: >> >> >> >> if (nd_region->flush == generic_nvdimm_flush) >> >> rc = generic_nvdimm_flush(...); >> > >> > 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/ The technique is fine, but that doesn't mean it should be applied everywhere. Is *this* code path really going to benefit from the optimization? -Jeff