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=-2.5 required=3.0 tests=HEADER_FROM_DIFFERENT_DOMAINS, MAILING_LIST_MULTI,SPF_PASS,USER_AGENT_MUTT 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 2AC20C46460 for ; Tue, 14 Aug 2018 18:41:50 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id E2CB921715 for ; Tue, 14 Aug 2018 18:41:49 +0000 (UTC) DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org E2CB921715 Authentication-Results: mail.kernel.org; dmarc=none (p=none dis=none) header.from=fieldses.org Authentication-Results: mail.kernel.org; spf=none smtp.mailfrom=linux-kernel-owner@vger.kernel.org Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1728305AbeHNVaQ (ORCPT ); Tue, 14 Aug 2018 17:30:16 -0400 Received: from fieldses.org ([173.255.197.46]:58848 "EHLO fieldses.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1728146AbeHNVaQ (ORCPT ); Tue, 14 Aug 2018 17:30:16 -0400 Received: by fieldses.org (Postfix, from userid 2815) id C01C21F67; Tue, 14 Aug 2018 14:41:46 -0400 (EDT) Date: Tue, 14 Aug 2018 14:41:46 -0400 From: "J. Bruce Fields" To: NeilBrown Cc: Jeff Layton , Alexander Viro , Martin Wilck , linux-fsdevel@vger.kernel.org, Frank Filz , linux-kernel@vger.kernel.org Subject: Re: [PATCH 0/5 v2] locks: avoid thundering-herd wake-ups Message-ID: <20180814184146.GM7906@fieldses.org> References: <153421852728.24426.2111161640156686201.stgit@noble> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <153421852728.24426.2111161640156686201.stgit@noble> User-Agent: Mutt/1.5.21 (2010-09-15) Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org This version looks correct to me, and simpler. I'll be curious to hear whatever you learn from testing! --b. On Tue, Aug 14, 2018 at 01:56:51PM +1000, NeilBrown wrote: > > V2, which added wake_non_conflicts() was more broken than V1 - as > Bruce explained there is no transitivity in the blocking relation > between locks. > So this series takes a simpler approach. > It still attached waiters between other waiters as necessary to ensure > that: > - a waiter is blocked by it's parent (fl->blocker) and all further > ancestors, and > - the list of waiters on fl_blocked are mutually non-conflicting. > > When a lock (the root of a tree of requests) is released, only its > immediate children (fl_blocked) are woken. > When any lock is woken (either because its fl_blocker was released > to due to a signal or similar) it with either: > - be granted > - be aborted > - be re-queued beneath some other lock. > > In the first case tree of blocked locks is moved across to the newly > created lock, and the invariants still hold. > In the order two cases, the tree or blocked waiters are all detached > and woken. > > Note that this series has not received much testing yet. > > Original description: > If you have a many-core machine, and have many threads all wanting to > briefly lock a give file (udev is known to do this), you can get quite > poor performance. > > When one thread releases a lock, it wakes up all other threads that > are waiting (classic thundering-herd) - one will get the lock and the > others go to sleep. > When you have few cores, this is not very noticeable: by the time the > 4th or 5th thread gets enough CPU time to try to claim the lock, the > earlier threads have claimed it, done what was needed, and released. > With 50+ cores, the contention can easily be measured. > > This patchset creates a tree of pending lock request in which siblings > don't conflict and each lock request does conflict with its parent. > When a lock is released, only requests which don't conflict with each > other a woken. > > Testing shows that lock-acquisitions-per-second is now fairly stable even > as number of contending process goes to 1000. Without this patch, > locks-per-second drops off steeply after a few 10s of processes. > > There is a small cost to this extra complexity. > At 20 processes running a particular test on 72 cores, the lock > acquisitions per second drops from 1.8 million to 1.4 million with > this patch. For 100 processes, this patch still provides 1.4 million > while without this patch there are about 700,000. > > NeilBrown > > --- > > NeilBrown (5): > fs/locks: rename some lists and pointers. > fs/locks: split out __locks_wake_up_blocks(). > fs/locks: allow a lock request to block other requests. > fs/locks: change all *_conflict() functions to return bool. > fs/locks: create a tree of dependent requests. > > > fs/cifs/file.c | 2 - > fs/locks.c | 156 ++++++++++++++++++++++++++------------- > include/linux/fs.h | 7 +- > include/trace/events/filelock.h | 16 ++-- > 4 files changed, 119 insertions(+), 62 deletions(-) > > -- > Signature