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=-5.2 required=3.0 tests=HEADER_FROM_DIFFERENT_DOMAINS, MAILING_LIST_MULTI,SIGNED_OFF_BY,SPF_PASS,URIBL_BLOCKED,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 833C2C64EB8 for ; Fri, 5 Oct 2018 01:40:11 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 41B7420834 for ; Fri, 5 Oct 2018 01:40:11 +0000 (UTC) DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org 41B7420834 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 S1727198AbeJEIgc (ORCPT ); Fri, 5 Oct 2018 04:36:32 -0400 Received: from zeniv.linux.org.uk ([195.92.253.2]:34510 "EHLO ZenIV.linux.org.uk" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1726844AbeJEIgc (ORCPT ); Fri, 5 Oct 2018 04:36:32 -0400 Received: from viro by ZenIV.linux.org.uk with local (Exim 4.90_1 #2 (Red Hat Linux)) id 1g8F62-00069i-D4; Fri, 05 Oct 2018 01:40:02 +0000 Date: Fri, 5 Oct 2018 02:40:02 +0100 From: Al Viro To: NeilBrown Cc: Andrew Morton , paulmck@linux.vnet.ibm.com, Florian Weimer , linux-fsdevel@vger.kernel.org, linux-kernel@vger.kernel.org, Josh Triplett Subject: Re: [PATCH - resend] VFS: use synchronize_rcu_expedited() in namespace_unlock() Message-ID: <20181005014002.GS32577@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> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <87tvm1rxme.fsf@notabene.neil.brown.name> User-Agent: Mutt/1.9.1 (2017-09-22) Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Fri, Oct 05, 2018 at 11:27:37AM +1000, 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 > --- > > I posted this last October, then again last November (cc:ing Linus) > Paul is happy enough with it, but no other response. > I'm hoping it can get applied this time.... Umm... IIRC, the last one got sidetracked on the other thing in the series... that was s_anon stuff. I can live with this one; FWIW, what kind of load would trigger the impact of the change? Paul?