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=-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 D309AC10F14 for ; Thu, 18 Apr 2019 16:11:05 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id AAA1621479 for ; Thu, 18 Apr 2019 16:11:05 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S2389264AbfDRQLB (ORCPT ); Thu, 18 Apr 2019 12:11:01 -0400 Received: from mx1.redhat.com ([209.132.183.28]:41390 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1725888AbfDRQLA (ORCPT ); Thu, 18 Apr 2019 12:11:00 -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 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 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> 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 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) Sender: linux-fsdevel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-fsdevel@vger.kernel.org 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