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=-6.8 required=3.0 tests=HEADER_FROM_DIFFERENT_DOMAINS, INCLUDES_PATCH,MAILING_LIST_MULTI,SIGNED_OFF_BY,SPF_PASS,URIBL_BLOCKED autolearn=ham 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 1C41DC4161D for ; Thu, 11 Oct 2018 12:28:20 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 977DA2077C for ; Thu, 11 Oct 2018 12:28:19 +0000 (UTC) DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org 977DA2077C Authentication-Results: mail.kernel.org; dmarc=none (p=none dis=none) header.from=suse.com Authentication-Results: mail.kernel.org; spf=none smtp.mailfrom=linux-btrfs-owner@vger.kernel.org Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1727329AbeJKTzR (ORCPT ); Thu, 11 Oct 2018 15:55:17 -0400 Received: from mx2.suse.de ([195.135.220.15]:48834 "EHLO mx1.suse.de" rhost-flags-OK-OK-OK-FAIL) by vger.kernel.org with ESMTP id S1726212AbeJKTzR (ORCPT ); Thu, 11 Oct 2018 15:55:17 -0400 X-Virus-Scanned: by amavisd-new at test-mx.suse.de Received: from relay1.suse.de (unknown [195.135.220.254]) by mx1.suse.de (Postfix) with ESMTP id 6DB18ADD7; Thu, 11 Oct 2018 12:28:16 +0000 (UTC) Subject: Re: [PATCH 5/6] btrfs: simplify btrfs_select_ref_head and cleanup some local variables To: Lu Fengqi Cc: linux-btrfs@vger.kernel.org References: <20181011054038.5428-1-lufq.fnst@cn.fujitsu.com> <20181011054038.5428-6-lufq.fnst@cn.fujitsu.com> <20181011121558.GF5899@fnst.localdomain> From: Nikolay Borisov Openpgp: preference=signencrypt Autocrypt: addr=nborisov@suse.com; prefer-encrypt=mutual; keydata= xsFNBFiKBz4BEADNHZmqwhuN6EAzXj9SpPpH/nSSP8YgfwoOqwrP+JR4pIqRK0AWWeWCSwmZ T7g+RbfPFlmQp+EwFWOtABXlKC54zgSf+uulGwx5JAUFVUIRBmnHOYi/lUiE0yhpnb1KCA7f u/W+DkwGerXqhhe9TvQoGwgCKNfzFPZoM+gZrm+kWv03QLUCr210n4cwaCPJ0Nr9Z3c582xc bCUVbsjt7BN0CFa2BByulrx5xD9sDAYIqfLCcZetAqsTRGxM7LD0kh5WlKzOeAXj5r8DOrU2 GdZS33uKZI/kZJZVytSmZpswDsKhnGzRN1BANGP8sC+WD4eRXajOmNh2HL4P+meO1TlM3GLl EQd2shHFY0qjEo7wxKZI1RyZZ5AgJnSmehrPCyuIyVY210CbMaIKHUIsTqRgY5GaNME24w7h TyyVCy2qAM8fLJ4Vw5bycM/u5xfWm7gyTb9V1TkZ3o1MTrEsrcqFiRrBY94Rs0oQkZvunqia c+NprYSaOG1Cta14o94eMH271Kka/reEwSZkC7T+o9hZ4zi2CcLcY0DXj0qdId7vUKSJjEep c++s8ncFekh1MPhkOgNj8pk17OAESanmDwksmzh1j12lgA5lTFPrJeRNu6/isC2zyZhTwMWs k3LkcTa8ZXxh0RfWAqgx/ogKPk4ZxOXQEZetkEyTFghbRH2BIwARAQABzSJOaWtvbGF5IEJv cmlzb3YgPG5ib3Jpc292QHN1c2UuZGU+wsF4BBMBAgAiBQJYijkSAhsDBgsJCAcDAgYVCAIJ CgsEFgIDAQIeAQIXgAAKCRBxvoJG5T8oV/B6D/9a8EcRPdHg8uLEPywuJR8URwXzkofT5bZE IfGF0Z+Lt2ADe+nLOXrwKsamhweUFAvwEUxxnndovRLPOpWerTOAl47lxad08080jXnGfYFS Dc+ew7C3SFI4tFFHln8Y22Q9075saZ2yQS1ywJy+TFPADIprAZXnPbbbNbGtJLoq0LTiESnD w/SUC6sfikYwGRS94Dc9qO4nWyEvBK3Ql8NkoY0Sjky3B0vL572Gq0ytILDDGYuZVo4alUs8 LeXS5ukoZIw1QYXVstDJQnYjFxYgoQ5uGVi4t7FsFM/6ykYDzbIPNOx49Rbh9W4uKsLVhTzG BDTzdvX4ARl9La2kCQIjjWRg+XGuBM5rxT/NaTS78PXjhqWNYlGc5OhO0l8e5DIS2tXwYMDY LuHYNkkpMFksBslldvNttSNei7xr5VwjVqW4vASk2Aak5AleXZS+xIq2FADPS/XSgIaepyTV tkfnyreep1pk09cjfXY4A7qpEFwazCRZg9LLvYVc2M2eFQHDMtXsH59nOMstXx2OtNMcx5p8 0a5FHXE/HoXz3p9bD0uIUq6p04VYOHsMasHqHPbsMAq9V2OCytJQPWwe46bBjYZCOwG0+x58 fBFreP/NiJNeTQPOa6FoxLOLXMuVtpbcXIqKQDoEte9aMpoj9L24f60G4q+pL/54ql2VRscK d87BTQRYigc+ARAAyJSq9EFk28++SLfg791xOh28tLI6Yr8wwEOvM3wKeTfTZd+caVb9gBBy wxYhIopKlK1zq2YP7ZjTP1aPJGoWvcQZ8fVFdK/1nW+Z8/NTjaOx1mfrrtTGtFxVBdSCgqBB jHTnlDYV1R5plJqK+ggEP1a0mr/rpQ9dFGvgf/5jkVpRnH6BY0aYFPprRL8ZCcdv2DeeicOO YMobD5g7g/poQzHLLeT0+y1qiLIFefNABLN06Lf0GBZC5l8hCM3Rpb4ObyQ4B9PmL/KTn2FV Xq/c0scGMdXD2QeWLePC+yLMhf1fZby1vVJ59pXGq+o7XXfYA7xX0JsTUNxVPx/MgK8aLjYW hX+TRA4bCr4uYt/S3ThDRywSX6Hr1lyp4FJBwgyb8iv42it8KvoeOsHqVbuCIGRCXqGGiaeX Wa0M/oxN1vJjMSIEVzBAPi16tztL/wQtFHJtZAdCnuzFAz8ue6GzvsyBj97pzkBVacwp3/Mw qbiu7sDz7yB0d7J2tFBJYNpVt/Lce6nQhrvon0VqiWeMHxgtQ4k92Eja9u80JDaKnHDdjdwq FUikZirB28UiLPQV6PvCckgIiukmz/5ctAfKpyYRGfez+JbAGl6iCvHYt/wAZ7Oqe/3Cirs5 KhaXBcMmJR1qo8QH8eYZ+qhFE3bSPH446+5oEw8A9v5oonKV7zMAEQEAAcLBXwQYAQIACQUC WIoHPgIbDAAKCRBxvoJG5T8oV1pyD/4zdXdOL0lhkSIjJWGqz7Idvo0wjVHSSQCbOwZDWNTN JBTP0BUxHpPu/Z8gRNNP9/k6i63T4eL1xjy4umTwJaej1X15H8Hsh+zakADyWHadbjcUXCkg OJK4NsfqhMuaIYIHbToi9K5pAKnV953xTrK6oYVyd/Rmkmb+wgsbYQJ0Ur1Ficwhp6qU1CaJ mJwFjaWaVgUERoxcejL4ruds66LM9Z1Qqgoer62ZneID6ovmzpCWbi2sfbz98+kW46aA/w8r 7sulgs1KXWhBSv5aWqKU8C4twKjlV2XsztUUsyrjHFj91j31pnHRklBgXHTD/pSRsN0UvM26 lPs0g3ryVlG5wiZ9+JbI3sKMfbdfdOeLxtL25ujs443rw1s/PVghphoeadVAKMPINeRCgoJH zZV/2Z/myWPRWWl/79amy/9MfxffZqO9rfugRBORY0ywPHLDdo9Kmzoxoxp9w3uTrTLZaT9M KIuxEcV8wcVjr+Wr9zRl06waOCkgrQbTPp631hToxo+4rA1jiQF2M80HAet65ytBVR2pFGZF zGYYLqiG+mpUZ+FPjxk9kpkRYz61mTLSY7tuFljExfJWMGfgSg1OxfLV631jV1TcdUnx+h3l Sqs2vMhAVt14zT8mpIuu2VNxcontxgVr1kzYA/tQg32fVRbGr449j1gw57BV9i0vww== Message-ID: <0b78a3dc-bf22-fb99-afce-96b74d936173@suse.com> Date: Thu, 11 Oct 2018 15:28:15 +0300 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:52.0) Gecko/20100101 Thunderbird/52.9.1 MIME-Version: 1.0 In-Reply-To: <20181011121558.GF5899@fnst.localdomain> Content-Type: text/plain; charset=utf-8 Content-Language: en-US Content-Transfer-Encoding: 7bit Sender: linux-btrfs-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-btrfs@vger.kernel.org On 11.10.2018 15:15, Lu Fengqi wrote: > On Thu, Oct 11, 2018 at 09:40:52AM +0300, Nikolay Borisov wrote: >> >> >> On 11.10.2018 08:40, Lu Fengqi wrote: >>> If the return value of find_ref_head() is NULL, the only possibility is >>> that delayed_refs' head ref rbtree is empty. Hence, the second >>> find_ref_head() is pointless. >>>> Besides, the local variables loop and start are unnecessary, just remove >>> them. >> >> So the objective of that function is to get a reference to the first >> delayed head which is not processed. This is done by essentially keeping >> track of the last range that was processed in >> delayed_refs->run_delayed_start >>> >>> Signed-off-by: Lu Fengqi >>> --- >>> fs/btrfs/delayed-ref.c | 17 +++-------------- >>> 1 file changed, 3 insertions(+), 14 deletions(-) >>> >>> diff --git a/fs/btrfs/delayed-ref.c b/fs/btrfs/delayed-ref.c >>> index 885581852bea..2726d2fb4bbe 100644 >>> --- a/fs/btrfs/delayed-ref.c >>> +++ b/fs/btrfs/delayed-ref.c >>> @@ -354,20 +354,11 @@ struct btrfs_delayed_ref_head * >>> btrfs_select_ref_head(struct btrfs_delayed_ref_root *delayed_refs) >>> { >>> struct btrfs_delayed_ref_head *head; >>> - u64 start; >>> - bool loop = false; >>> >>> again: >>> - start = delayed_refs->run_delayed_start; >>> - head = find_ref_head(delayed_refs, start, 1); >>> - if (!head && !loop) { >>> + head = find_ref_head(delayed_refs, delayed_refs->run_delayed_start, 1); >>> + if (!head) { >>> delayed_refs->run_delayed_start = 0; >>> - start = 0; >>> - loop = true; >>> - head = find_ref_head(delayed_refs, start, 1); >>> - if (!head) >>> - return NULL; >>> - } else if (!head && loop) { >> >> I believe this will have a negative impact since it actually will >> prevent finding a head which was added BEFORE the last processed head. >> So when a ref head is selected in btrfs_obtain_ref_head then the >> delayed_refs->lock is dropped and the given head is locked and >> delayed_refs->run_delayed_start points to the end of the selected range >> that the head represents. At this point it's possible that another >> thread modifies a different range which is before the one we have >> selected so graphically it will be something like: >> >> >> ---[HEAD2]----->[HEAD1]------ >> 0 N >> >> Where HEAD1 is the head returned from first invocation of >> btrfs_obtain_ref_head. Once btrfs_obtain_ref_head is called the 2nd >> time it will not find HEAD2 so will just reset run_delayed_start to 0 >> and return. So it will be up to another run of the delayed refs to >> actually find head2. Essentially you made btrfs_obtain_ref_head less > > Not exactly. In fact, find_ref_head hides such a logic. When > return_bigger is set, if there is no larger entry to return, the first > entry will be returned. Please see the comment I add in the PATCH 6. > > Hence, the 2nd invocation of btrfs_obtain_ref_head still will return > HEAD2. There is no functional change here. > > However, your question makes me consider whether such hidden logic > should be extracted from find_ref_head to btrfs_select_ref_head. Right I agree with your. As it stands I will expect that if return_bigger is true to specifically return a bigger entry or if nothing is found to return null. IMO this behavior is higher level and belongs to btrfs_delayed_ref_head. > >> greedy. Have you characterized what kind of performance impact this have? > > I noticed that there is a macro called SCRAMBLE_DELAYED_REFS in the > extent-tree.c. I am a bit curious whether it has been forgotten by > everyone, I have not found any test results about its performance impact. I guess it was used during testing but nothing currently sets it. I.e it might make sense to enable it if BTRFS_DEBUG is set.