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=-15.3 required=3.0 tests=BAYES_00, HEADER_FROM_DIFFERENT_DOMAINS,INCLUDES_CR_TRAILER,INCLUDES_PATCH, MAILING_LIST_MULTI,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 80874C433E0 for ; Fri, 8 Jan 2021 16:03:58 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 42D0023884 for ; Fri, 8 Jan 2021 16:03:58 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1727459AbhAHQDm (ORCPT ); Fri, 8 Jan 2021 11:03:42 -0500 Received: from mx2.suse.de ([195.135.220.15]:40218 "EHLO mx2.suse.de" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1726386AbhAHQDl (ORCPT ); Fri, 8 Jan 2021 11:03:41 -0500 X-Virus-Scanned: by amavisd-new at test-mx.suse.de Received: from relay2.suse.de (unknown [195.135.221.27]) by mx2.suse.de (Postfix) with ESMTP id 3EC15ACC6; Fri, 8 Jan 2021 16:03:00 +0000 (UTC) Received: by ds.suse.cz (Postfix, from userid 10065) id C89EADA6E9; Fri, 8 Jan 2021 17:01:09 +0100 (CET) Date: Fri, 8 Jan 2021 17:01:09 +0100 From: David Sterba To: Josef Bacik Cc: linux-btrfs@vger.kernel.org, kernel-team@fb.com, Nikolay Borisov Subject: Re: [PATCH v5 2/8] btrfs: only let one thread pre-flush delayed refs in commit Message-ID: <20210108160109.GB6430@twin.jikos.cz> Reply-To: dsterba@suse.cz Mail-Followup-To: dsterba@suse.cz, Josef Bacik , linux-btrfs@vger.kernel.org, kernel-team@fb.com, Nikolay Borisov References: <9e47b11bdfe5b4905fdaa81e952de2e2466c6335.1608319304.git.josef@toxicpanda.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <9e47b11bdfe5b4905fdaa81e952de2e2466c6335.1608319304.git.josef@toxicpanda.com> User-Agent: Mutt/1.5.23.1-rc1 (2014-03-12) Precedence: bulk List-ID: X-Mailing-List: linux-btrfs@vger.kernel.org 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).