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=-7.0 required=3.0 tests=HEADER_FROM_DIFFERENT_DOMAINS, INCLUDES_PATCH,MAILING_LIST_MULTI,SIGNED_OFF_BY,SPF_PASS,URIBL_BLOCKED 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 76077C04EB9 for ; Thu, 29 Nov 2018 23:33:30 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 3A75D2086B for ; Thu, 29 Nov 2018 23:33:30 +0000 (UTC) DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org 3A75D2086B Authentication-Results: mail.kernel.org; dmarc=none (p=none dis=none) header.from=suse.com 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 S1726950AbeK3Kkl (ORCPT ); Fri, 30 Nov 2018 05:40:41 -0500 Received: from mx2.suse.de ([195.135.220.15]:41250 "EHLO mx1.suse.de" rhost-flags-OK-OK-OK-FAIL) by vger.kernel.org with ESMTP id S1726393AbeK3Kkl (ORCPT ); Fri, 30 Nov 2018 05:40:41 -0500 X-Virus-Scanned: by amavisd-new at test-mx.suse.de Received: from relay2.suse.de (unknown [195.135.220.254]) by mx1.suse.de (Postfix) with ESMTP id 6A431AF4A; Thu, 29 Nov 2018 23:33:26 +0000 (UTC) From: NeilBrown To: Al Viro , Andrew Morton , Linus Torvalds Date: Fri, 30 Nov 2018 10:33:18 +1100 Cc: 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() In-Reply-To: <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> <20181005014002.GS32577@ZenIV.linux.org.uk> Message-ID: <87mupro3wh.fsf@notabene.neil.brown.name> MIME-Version: 1.0 Content-Type: multipart/signed; boundary="=-=-="; micalg=pgp-sha256; protocol="application/pgp-signature" Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org --=-=-= Content-Type: text/plain Content-Transfer-Encoding: quoted-printable 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 p= erspective) Signed-off-by: NeilBrown =2D-- Al Viro says "I can live with this one" but this still hasn't landed. Maybe someone else could apply it? Thanks, NeilBrown Full quote from Al on 5th Oct: > Umm... IIRC, the last one got sidetracked on the other thing in the seri= es... > that was s_anon stuff. I can live with this one; FWIW, what kind > of load would trigger the impact of the change? Paul? which Paul replied to. fs/namespace.c | 2 +- 1 file changed, 1 insertion(+), 1 deletion(-) diff --git a/fs/namespace.c b/fs/namespace.c index a7f91265ea67..43a0d2c7449d 100644 =2D-- a/fs/namespace.c +++ b/fs/namespace.c @@ -1360,7 +1360,7 @@ static void namespace_unlock(void) if (likely(hlist_empty(&head))) return; =20 =2D synchronize_rcu(); + synchronize_rcu_expedited(); =20 group_pin_kill(&head); } =2D-=20 2.14.0.rc0.dirty --=-=-= Content-Type: application/pgp-signature; name="signature.asc" -----BEGIN PGP SIGNATURE----- iQIzBAEBCAAdFiEEG8Yp69OQ2HB7X0l6Oeye3VZigbkFAlwAdz4ACgkQOeye3VZi gblqLg/+PwPLRxOkRa8exmADt69qsclBqzWP3tAL5qtTl0a3JuH0Tlv04YxeVbct FUY/sqt6lG6pWf52eTrRWteCxlnQv22kJy+EO+rKmntuRzaF96R8g5Ri4ijQ66l3 xZcaQ8gG4313vG9lW2dj+4cGo4TuvJt/SZUmUI4nkthXfB0crChuZe/4f9G9s4Xx hcBLnv+xbKS5Uf/ItQVXFx22aviLOCQqGnxEPY1U5jjX/Fkg8oX6lHHJEqdmdr/Q S8qoMFbWQQgr2Sky4oN8FN3nMmwYoojukUFeA5ddc46M5hxbWqdTAWh+qk2HL4vB 5vCLTlzSf7eH6DP0augawbDr7cijzCjXVnQOU0G6nc6hlm7klVqJ3NUXwARtBpZt FkMjeBzMRQDq4pXtTNhDueKpXDp9fCn4cKwh7nZIQGp7tyFEC4Dw5Xid7AATuszu FpKXY9UY4ihm71Cgs4ZRV4lI8zK4LvAzJFLYBNtsgXzMi8ohIX2KV2JCrXKf9vXs FtSsyKvCKxEjT2F7Q1+U8/TSvjxLIRUqC8WbMFqYNcRqcBw/KOQY+Dy6QTSjvH7d rGb9gEguAsUlTQtCjFlt6+h+lmxBMPTCd7PGJv96LcP7NvZKK1gOf405m0d68s/c KYv1PdmKX8e2Pnj6+prAq1f0lVjJOdJY6Oakw3MtQ203vrBEX/o= =M18W -----END PGP SIGNATURE----- --=-=-=--