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=-17.3 required=3.0 tests=BAYES_00,DKIM_SIGNED, DKIM_VALID,DKIM_VALID_AU,HEADER_FROM_DIFFERENT_DOMAINS,INCLUDES_CR_TRAILER, INCLUDES_PATCH,MAILING_LIST_MULTI,NICE_REPLY_A,SPF_HELO_NONE,SPF_PASS, URIBL_BLOCKED,USER_AGENT_SANE_1 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 DA213C433DB for ; Mon, 11 Jan 2021 08:34:47 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 9044F2251F for ; Mon, 11 Jan 2021 08:34:47 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1727633AbhAKIeb (ORCPT ); Mon, 11 Jan 2021 03:34:31 -0500 Received: from mx2.suse.de ([195.135.220.15]:46930 "EHLO mx2.suse.de" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1727240AbhAKIeb (ORCPT ); Mon, 11 Jan 2021 03:34:31 -0500 X-Virus-Scanned: by amavisd-new at test-mx.suse.de DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=suse.com; s=susede1; t=1610354023; h=from:from:reply-to:date:date:message-id:message-id:to:to:cc: mime-version:mime-version:content-type:content-type: content-transfer-encoding:content-transfer-encoding: in-reply-to:in-reply-to:references:references:autocrypt:autocrypt; bh=UXLPNKsBD5GYGpszvZzUTmSUT/besY4xtdUTVo+/xX0=; b=Rb1Z+larmx4oNe8HaYMzSqv8vGauzsbkYNIdHfzvxJlia21orhn57DAb4uab9afP/Yadyk LHlFL0Zuf3tLyDkNJUGe4MjRQDFwAGmZUus3RNbKxmkZwBtJYKXXYr+p5CC0OkxG1zPl2H x2XlO/jllM2XFgyukFQ0lIzj8ljBUMo= Received: from relay2.suse.de (unknown [195.135.221.27]) by mx2.suse.de (Postfix) with ESMTP id 685D7AD1E; Mon, 11 Jan 2021 08:33:43 +0000 (UTC) Subject: Re: [PATCH v5 2/8] btrfs: only let one thread pre-flush delayed refs in commit To: dsterba@suse.cz, Josef Bacik , linux-btrfs@vger.kernel.org, kernel-team@fb.com References: <9e47b11bdfe5b4905fdaa81e952de2e2466c6335.1608319304.git.josef@toxicpanda.com> <20210108160109.GB6430@twin.jikos.cz> From: Nikolay Borisov Autocrypt: addr=nborisov@suse.com; prefer-encrypt=mutual; keydata= mQINBFiKBz4BEADNHZmqwhuN6EAzXj9SpPpH/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/ogKPk4ZxOXQEZetkEyTFghbRH2BIwARAQABtCNOaWtvbGF5IEJv cmlzb3YgPG5ib3Jpc292QHN1c2UuY29tPokCOAQTAQIAIgUCWIo48QIbAwYLCQgHAwIGFQgC CQoLBBYCAwECHgECF4AACgkQcb6CRuU/KFc0eg/9GLD3wTQz9iZHMFbjiqTCitD7B6dTLV1C ddZVlC8Hm/TophPts1bWZORAmYIihHHI1EIF19+bfIr46pvfTu0yFrJDLOADMDH+Ufzsfy2v HSqqWV/nOSWGXzh8bgg/ncLwrIdEwBQBN9SDS6aqsglagvwFD91UCg/TshLlRxD5BOnuzfzI Leyx2c6YmH7Oa1R4MX9Jo79SaKwdHt2yRN3SochVtxCyafDlZsE/efp21pMiaK1HoCOZTBp5 VzrIP85GATh18pN7YR9CuPxxN0V6IzT7IlhS4Jgj0NXh6vi1DlmKspr+FOevu4RVXqqcNTSS E2rycB2v6cttH21UUdu/0FtMBKh+rv8+yD49FxMYnTi1jwVzr208vDdRU2v7Ij/TxYt/v4O8 V+jNRKy5Fevca/1xroQBICXsNoFLr10X5IjmhAhqIH8Atpz/89ItS3+HWuE4BHB6RRLM0gy8 T7rN6ja+KegOGikp/VTwBlszhvfLhyoyjXI44Tf3oLSFM+8+qG3B7MNBHOt60CQlMkq0fGXd mm4xENl/SSeHsiomdveeq7cNGpHi6i6ntZK33XJLwvyf00PD7tip/GUj0Dic/ZUsoPSTF/mG EpuQiUZs8X2xjK/AS/l3wa4Kz2tlcOKSKpIpna7V1+CMNkNzaCOlbv7QwprAerKYywPCoOSC 7P25Ag0EWIoHPgEQAMiUqvRBZNvPvki34O/dcTodvLSyOmK/MMBDrzN8Cnk302XfnGlW/YAQ csMWISKKSpStc6tmD+2Y0z9WjyRqFr3EGfH1RXSv9Z1vmfPzU42jsdZn667UxrRcVQXUgoKg QYx055Q2FdUeaZSaivoIBD9WtJq/66UPXRRr4H/+Y5FaUZx+gWNGmBT6a0S/GQnHb9g3nonD jmDKGw+YO4P6aEMxyy3k9PstaoiyBXnzQASzdOi39BgWQuZfIQjN0aW+Dm8kOAfT5i/yk59h VV6v3NLHBjHVw9kHli3jwvsizIX9X2W8tb1SefaVxqvqO1132AO8V9CbE1DcVT8fzICvGi42 FoV/k0QOGwq+LmLf0t04Q0csEl+h69ZcqeBSQcIMm/Ir+NorfCr6HjrB6lW7giBkQl6hhomn l1mtDP6MTdbyYzEiBFcwQD4terc7S/8ELRRybWQHQp7sxQM/Lnuhs77MgY/e6c5AVWnMKd/z MKm4ru7A8+8gdHeydrRQSWDaVbfy3Hup0Ia76J9FaolnjB8YLUOJPdhI2vbvNCQ2ipxw3Y3c KhVIpGYqwdvFIiz0Fej7wnJICIrpJs/+XLQHyqcmERn3s/iWwBpeogrx2Lf8AGezqnv9woq7 OSoWlwXDJiUdaqPEB/HmGfqoRRN20jx+OOvuaBMPAPb+aKJyle8zABEBAAGJAh8EGAECAAkF AliKBz4CGwwACgkQcb6CRuU/KFdacg/+M3V3Ti9JYZEiIyVhqs+yHb6NMI1R0kkAmzsGQ1jU zSQUz9AVMR6T7v2fIETTT/f5Oout0+Hi9cY8uLpk8CWno9V9eR/B7Ifs2pAA8lh2nW43FFwp IDiSuDbH6oTLmiGCB206IvSuaQCp1fed8U6yuqGFcnf0ZpJm/sILG2ECdFK9RYnMIaeqlNQm iZicBY2lmlYFBEaMXHoy+K7nbOuizPWdUKoKHq+tmZ3iA+qL5s6Qlm4trH28/fPpFuOmgP8P K+7LpYLNSl1oQUr+WlqilPAuLcCo5Vdl7M7VFLMq4xxY/dY99aZx0ZJQYFx0w/6UkbDdFLzN upT7NIN68lZRucImffiWyN7CjH23X3Tni8bS9ubo7OON68NbPz1YIaYaHmnVQCjDyDXkQoKC R82Vf9mf5slj0Vlpf+/Wpsv/TH8X32ajva37oEQTkWNMsDxyw3aPSps6MaMafcN7k60y2Wk/ TCiLsRHFfMHFY6/lq/c0ZdOsGjgpIK0G0z6et9YU6MaPuKwNY4kBdjPNBwHreucrQVUdqRRm RcxmGC6ohvpqVGfhT48ZPZKZEWM+tZky0mO7bhZYxMXyVjBn4EoNTsXy1et9Y1dU3HVJ8fod 5UqrNrzIQFbdeM0/JqSLrtlTcXKJ7cYFa9ZM2AP7UIN9n1UWxq+OPY9YMOewVfYtL8M= Message-ID: <52aef9a6-efc7-0820-7056-067e69c2a856@suse.com> Date: Mon, 11 Jan 2021 10:33:42 +0200 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:68.0) Gecko/20100101 Thunderbird/68.10.0 MIME-Version: 1.0 In-Reply-To: <20210108160109.GB6430@twin.jikos.cz> Content-Type: text/plain; charset=utf-8 Content-Language: en-US Content-Transfer-Encoding: 8bit Precedence: bulk List-ID: X-Mailing-List: linux-btrfs@vger.kernel.org On 8.01.21 г. 18:01 ч., David Sterba wrote: > On Fri, Dec 18, 2020 at 02:24:20PM -0500, Josef Bacik wrote: >> I've been running a stress test that runs 20 workers in their own >> subvolume, which are running an fsstress instance with 4 threads per >> worker, which is 80 total fsstress threads. In addition to this I'm >> running balance in the background as well as creating and deleting >> snapshots. This test takes around 12 hours to run normally, going >> slower and slower as the test goes on. >> >> The reason for this is because fsstress is running fsync sometimes, and >> because we're messing with block groups we often fall through to >> btrfs_commit_transaction, so will often have 20-30 threads all calling >> btrfs_commit_transaction at the same time. >> >> These all get stuck contending on the extent tree while they try to run >> delayed refs during the initial part of the commit. >> >> This is suboptimal, really because the extent tree is a single point of >> failure we only want one thread acting on that tree at once to reduce >> lock contention. Fix this by making the flushing mechanism a bit >> operation, to make it easy to use test_and_set_bit() in order to make >> sure only one task does this initial flush. >> >> Once we're into the transaction commit we only have one thread doing >> delayed ref running, it's just this initial pre-flush that is >> problematic. With this patch my stress test takes around 90 minutes to >> run, instead of 12 hours. >> >> Reviewed-by: Nikolay Borisov >> Signed-off-by: Josef Bacik >> --- >> fs/btrfs/delayed-ref.h | 12 ++++++------ >> fs/btrfs/transaction.c | 33 ++++++++++++++++----------------- >> 2 files changed, 22 insertions(+), 23 deletions(-) >> >> diff --git a/fs/btrfs/delayed-ref.h b/fs/btrfs/delayed-ref.h >> index 1c977e6d45dc..6e414785b56f 100644 >> --- a/fs/btrfs/delayed-ref.h >> +++ b/fs/btrfs/delayed-ref.h >> @@ -135,6 +135,11 @@ struct btrfs_delayed_data_ref { >> u64 offset; >> }; >> >> +enum btrfs_delayed_ref_flags { >> + /* Used to indicate that we are flushing delayed refs for the commit. */ >> + BTRFS_DELAYED_REFS_FLUSHING, >> +}; >> + >> struct btrfs_delayed_ref_root { >> /* head ref rbtree */ >> struct rb_root_cached href_root; >> @@ -158,12 +163,7 @@ struct btrfs_delayed_ref_root { >> >> u64 pending_csums; >> >> - /* >> - * set when the tree is flushing before a transaction commit, >> - * used by the throttling code to decide if new updates need >> - * to be run right away >> - */ >> - int flushing; >> + unsigned long flags; >> >> u64 run_delayed_start; >> >> diff --git a/fs/btrfs/transaction.c b/fs/btrfs/transaction.c >> index f51f9e39bcee..ccd37fbe5db1 100644 >> --- a/fs/btrfs/transaction.c >> +++ b/fs/btrfs/transaction.c >> @@ -909,9 +909,9 @@ bool btrfs_should_end_transaction(struct btrfs_trans_handle *trans) >> { >> struct btrfs_transaction *cur_trans = trans->transaction; >> >> - smp_mb(); >> if (cur_trans->state >= TRANS_STATE_COMMIT_START || >> - cur_trans->delayed_refs.flushing) >> + test_bit(BTRFS_DELAYED_REFS_FLUSHING, >> + &cur_trans->delayed_refs.flags)) >> return true; >> >> return should_end_transaction(trans); >> @@ -2043,23 +2043,22 @@ int btrfs_commit_transaction(struct btrfs_trans_handle *trans) >> btrfs_trans_release_metadata(trans); >> trans->block_rsv = NULL; >> >> - /* make a pass through all the delayed refs we have so far >> - * any runnings procs may add more while we are here >> - */ >> - ret = btrfs_run_delayed_refs(trans, 0); >> - if (ret) { >> - btrfs_end_transaction(trans); >> - return ret; >> - } >> - >> - cur_trans = trans->transaction; >> - >> /* >> - * set the flushing flag so procs in this transaction have to >> - * start sending their work down. >> + * We only want one transaction commit doing the flushing so we do not >> + * waste a bunch of time on lock contention on the extent root node. >> */ >> - cur_trans->delayed_refs.flushing = 1; >> - smp_wmb(); > > This barrier obviously separates the flushing = 1 and the rest of the > code, now implemented as test_and_set_bit, which implies full barrier. > > However, hunk in btrfs_should_end_transaction removes the barrier and > I'm not sure whether this is correct: > > - smp_mb(); > if (cur_trans->state >= TRANS_STATE_COMMIT_START || > - cur_trans->delayed_refs.flushing) > + test_bit(BTRFS_DELAYED_REFS_FLUSHING, > + &cur_trans->delayed_refs.flags)) > return true; > > This is never called under locks so we don't have complete > synchronization of neither the transaction state nor the flushing bit. > btrfs_should_end_transaction is merely a hint and not called in critical > places so we could probably afford to keep it without a barrier, or keep > it with comment(s). I think the point is moot in this case, because the test_bit either sees the flag or it doesn't. It's not possible for the flag to be set AND should_end_transaction return false that would be gross violation of program correctness. >