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 Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by smtp.lore.kernel.org (Postfix) with ESMTP id 48EF2C433EF for ; Thu, 17 Feb 2022 18:55:51 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S233691AbiBQS4E (ORCPT ); Thu, 17 Feb 2022 13:56:04 -0500 Received: from mxb-00190b01.gslb.pphosted.com ([23.128.96.19]:34474 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S237519AbiBQS4A (ORCPT ); Thu, 17 Feb 2022 13:56:00 -0500 Received: from dfw.source.kernel.org (dfw.source.kernel.org [139.178.84.217]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id A080758E6F; Thu, 17 Feb 2022 10:55:45 -0800 (PST) Received: from smtp.kernel.org (relay.kernel.org [52.25.139.140]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by dfw.source.kernel.org (Postfix) with ESMTPS id 3C5B361B8A; Thu, 17 Feb 2022 18:55:45 +0000 (UTC) Received: by smtp.kernel.org (Postfix) with ESMTPSA id 02E7CC340E8; Thu, 17 Feb 2022 18:55:43 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=linuxfoundation.org; s=korg; t=1645124144; bh=Rh0qZRyhLF/JeJYDSRhXWN0tLJc2VU+e0gkrEgt5rFo=; h=Date:From:To:Cc:Subject:References:In-Reply-To:From; b=LwvbAGf9J25gZA48/mDNJ6DibV8IANCyxD6WtW/RFemLIbgE+LJaLy+EvhGcPjEtr am4y3jvh3KGJSTMf1HHFIkrJqKq3qIlyxIM0EIVidMjVV6kJqK+9XaNbK+ciyyKSNz 0/lNivsSwhVFSMMidaIdtSJS4o5en5LR8Tsd7YVM= Date: Thu, 17 Feb 2022 19:55:41 +0100 From: Greg KH To: Baokun Li Cc: linux-kernel@vger.kernel.org, stable@vger.kernel.org, oliver.sang@intel.com, beibei.si@intel.com, jannh@google.com, mszeredi@redhat.com, torvalds@linux-foundation.org, yukuai3@huawei.com Subject: Re: [PATCH 5.10] fget: clarify and improve __fget_files() implementation Message-ID: References: <20220215065107.3045023-1-libaokun1@huawei.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20220215065107.3045023-1-libaokun1@huawei.com> Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Tue, Feb 15, 2022 at 02:51:07PM +0800, Baokun Li wrote: > From: Linus Torvalds > > commit e386dfc56f837da66d00a078e5314bc8382fab83 upstream. > > Commit 054aa8d439b9 ("fget: check that the fd still exists after getting > a ref to it") fixed a race with getting a reference to a file just as it > was being closed. It was a fairly minimal patch, and I didn't think > re-checking the file pointer lookup would be a measurable overhead, > since it was all right there and cached. > > But I was wrong, as pointed out by the kernel test robot. > > The 'poll2' case of the will-it-scale.per_thread_ops benchmark regressed > quite noticeably. Admittedly it seems to be a very artificial test: > doing "poll()" system calls on regular files in a very tight loop in > multiple threads. > > That means that basically all the time is spent just looking up file > descriptors without ever doing anything useful with them (not that doing > 'poll()' on a regular file is useful to begin with). And as a result it > shows the extra "re-check fd" cost as a sore thumb. > > Happily, the regression is fixable by just writing the code to loook up > the fd to be better and clearer. There's still a cost to verify the > file pointer, but now it's basically in the noise even for that > benchmark that does nothing else - and the code is more understandable > and has better comments too. > > [ Side note: this patch is also a classic case of one that looks very > messy with the default greedy Myers diff - it's much more legible with > either the patience of histogram diff algorithm ] > > Link: https://lore.kernel.org/lkml/20211210053743.GA36420@xsang-OptiPlex-9020/ > Link: https://lore.kernel.org/lkml/20211213083154.GA20853@linux.intel.com/ > Reported-by: kernel test robot > Tested-by: Carel Si > Cc: Jann Horn > Cc: Miklos Szeredi > Signed-off-by: Linus Torvalds > Signed-off-by: Baokun Li > --- > fs/file.c | 72 ++++++++++++++++++++++++++++++++++++++++++------------- > 1 file changed, 56 insertions(+), 16 deletions(-) Now queued up, thanks. Any chance you can do this for 5.4 and older kernels too? greg k-h