From patchwork Wed Apr 5 04:33:50 2017 Content-Type: text/plain; charset="utf-8" MIME-Version: 1.0 Content-Transfer-Encoding: 7bit X-Patchwork-Submitter: NeilBrown X-Patchwork-Id: 9662913 Return-Path: Received: from mail.wl.linuxfoundation.org (pdx-wl-mail.web.codeaurora.org [172.30.200.125]) by pdx-korg-patchwork.web.codeaurora.org (Postfix) with ESMTP id B1F2360353 for ; Wed, 5 Apr 2017 04:34:00 +0000 (UTC) Received: from mail.wl.linuxfoundation.org (localhost [127.0.0.1]) by mail.wl.linuxfoundation.org (Postfix) with ESMTP id A3C01283BA for ; Wed, 5 Apr 2017 04:34:00 +0000 (UTC) Received: by mail.wl.linuxfoundation.org (Postfix, from userid 486) id 988EA28494; Wed, 5 Apr 2017 04:34:00 +0000 (UTC) X-Spam-Checker-Version: SpamAssassin 3.3.1 (2010-03-16) on pdx-wl-mail.web.codeaurora.org X-Spam-Level: X-Spam-Status: No, score=-6.9 required=2.0 tests=BAYES_00,RCVD_IN_DNSWL_HI autolearn=ham version=3.3.1 Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.wl.linuxfoundation.org (Postfix) with ESMTP id 20F70283BA for ; Wed, 5 Apr 2017 04:34:00 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1751127AbdDEEd7 (ORCPT ); Wed, 5 Apr 2017 00:33:59 -0400 Received: from mx2.suse.de ([195.135.220.15]:60414 "EHLO mx2.suse.de" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1750871AbdDEEd6 (ORCPT ); Wed, 5 Apr 2017 00:33:58 -0400 X-Virus-Scanned: by amavisd-new at test-mx.suse.de X-Amavis-Alert: BAD HEADER SECTION, Duplicate header field: "Cc" Received: from relay2.suse.de (charybdis-ext.suse.de [195.135.220.254]) by mx2.suse.de (Postfix) with ESMTP id 32439AAB4; Wed, 5 Apr 2017 04:33:57 +0000 (UTC) From: NeilBrown To: Jens Axboe Date: Wed, 05 Apr 2017 14:33:50 +1000 Cc: linux-block@vger.kernel.org, linux-mm@kvack.org, LKML Subject: [PATCH v2] loop: Add PF_LESS_THROTTLE to block/loop device thread. In-Reply-To: <871staffus.fsf@notabene.neil.brown.name> References: <871staffus.fsf@notabene.neil.brown.name> cc: Ming Lei Message-ID: <87wpazh3rl.fsf@notabene.neil.brown.name> MIME-Version: 1.0 Sender: linux-block-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-block@vger.kernel.org X-Virus-Scanned: ClamAV using ClamSMTP When a filesystem is mounted from a loop device, writes are throttled by balance_dirty_pages() twice: once when writing to the filesystem and once when the loop_handle_cmd() writes to the backing file. This double-throttling can trigger positive feedback loops that create significant delays. The throttling at the lower level is seen by the upper level as a slow device, so it throttles extra hard. The PF_LESS_THROTTLE flag was created to handle exactly this circumstance, though with an NFS filesystem mounted from a local NFS server. It reduces the throttling on the lower layer so that it can proceed largely unthrottled. To demonstrate this, create a filesystem on a loop device and write (e.g. with dd) several large files which combine to consume significantly more than the limit set by /proc/sys/vm/dirty_ratio or dirty_bytes. Measure the total time taken. When I do this directly on a device (no loop device) the total time for several runs (mkfs, mount, write 200 files, umount) is fairly stable: 28-35 seconds. When I do this over a loop device the times are much worse and less stable. 52-460 seconds. Half below 100seconds, half above. When I apply this patch, the times become stable again, though not as fast as the no-loop-back case: 53-72 seconds. There may be room for further improvement as the total overhead still seems too high, but this is a big improvement. Reviewed-by: Christoph Hellwig Acked-by: Michal Hocko Signed-off-by: NeilBrown Reviewed-by: Ming Lei --- I moved where the flag is set, thanks to suggestion from Ming Lei. I've preserved the *-by: tags I was offered despite the code being different, as the concept is identical. Thanks, NeilBrown drivers/block/loop.c | 1 + 1 file changed, 1 insertion(+) diff --git a/drivers/block/loop.c b/drivers/block/loop.c index 0ecb6461ed81..44b3506fd086 100644 --- a/drivers/block/loop.c +++ b/drivers/block/loop.c @@ -852,6 +852,7 @@ static int loop_prepare_queue(struct loop_device *lo) if (IS_ERR(lo->worker_task)) return -ENOMEM; set_user_nice(lo->worker_task, MIN_NICE); + lo->worker_task->flags |= PF_LESS_THROTTLE; return 0; }