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=-5.5 required=3.0 tests=HEADER_FROM_DIFFERENT_DOMAINS, MAILING_LIST_MULTI,SIGNED_OFF_BY,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 A7D13C04EB9 for ; Thu, 29 Nov 2018 23:52:32 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 7386F21019 for ; Thu, 29 Nov 2018 23:52:32 +0000 (UTC) DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org 7386F21019 Authentication-Results: mail.kernel.org; dmarc=none (p=none dis=none) header.from=zeniv.linux.org.uk 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 S1727281AbeK3K7r (ORCPT ); Fri, 30 Nov 2018 05:59:47 -0500 Received: from zeniv.linux.org.uk ([195.92.253.2]:36166 "EHLO ZenIV.linux.org.uk" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1726425AbeK3K7r (ORCPT ); Fri, 30 Nov 2018 05:59:47 -0500 Received: from viro by ZenIV.linux.org.uk with local (Exim 4.91 #2 (Red Hat Linux)) id 1gSW6Y-0005Je-KL; Thu, 29 Nov 2018 23:52:22 +0000 Date: Thu, 29 Nov 2018 23:52:22 +0000 From: Al Viro To: NeilBrown Cc: Andrew Morton , Linus Torvalds , paulmck@linux.vnet.ibm.com, Florian Weimer , linux-fsdevel@vger.kernel.org, linux-kernel@vger.kernel.org, Josh Triplett Subject: Re: [PATCH - resend*2] VFS: use synchronize_rcu_expedited() in namespace_unlock() Message-ID: <20181129235222.GJ2217@ZenIV.linux.org.uk> References: <87y3nyd4pu.fsf@notabene.neil.brown.name> <20171026122743.GX3659@linux.vnet.ibm.com> <20171127144125.GF3624@linux.vnet.ibm.com> <87induxd3u.fsf@notabene.neil.brown.name> <87tvm1rxme.fsf@notabene.neil.brown.name> <20181005014002.GS32577@ZenIV.linux.org.uk> <87mupro3wh.fsf@notabene.neil.brown.name> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <87mupro3wh.fsf@notabene.neil.brown.name> User-Agent: Mutt/1.10.1 (2018-07-13) Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Fri, Nov 30, 2018 at 10:33:18AM +1100, NeilBrown wrote: > > The synchronize_rcu() in namespace_unlock() is called every time > a filesystem is unmounted. If a great many filesystems are mounted, > this can cause a noticable slow-down in, for example, system shutdown. > > The sequence: > mkdir -p /tmp/Mtest/{0..5000} > time for i in /tmp/Mtest/*; do mount -t tmpfs tmpfs $i ; done > time umount /tmp/Mtest/* > > on a 4-cpu VM can report 8 seconds to mount the tmpfs filesystems, and > 100 seconds to unmount them. > > Boot the same VM with 1 CPU and it takes 18 seconds to mount the > tmpfs filesystems, but only 36 to unmount. > > If we change the synchronize_rcu() to synchronize_rcu_expedited() > the umount time on a 4-cpu VM drop to 0.6 seconds > > I think this 200-fold speed up is worth the slightly high system > impact of using synchronize_rcu_expedited(). > > Acked-by: Paul E. McKenney (from general rcu perspective) > Signed-off-by: NeilBrown > --- > > Al Viro says "I can live with this one" but this still hasn't landed. > Maybe someone else could apply it? Applied (in work.misc, once I push it out)