From patchwork Mon Apr 3 01:18:51 2017 Content-Type: text/plain; charset="utf-8" MIME-Version: 1.0 Content-Transfer-Encoding: 7bit X-Patchwork-Submitter: NeilBrown X-Patchwork-Id: 9658715 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 2D95B60237 for ; Mon, 3 Apr 2017 01:19:35 +0000 (UTC) Received: from mail.wl.linuxfoundation.org (localhost [127.0.0.1]) by mail.wl.linuxfoundation.org (Postfix) with ESMTP id 1C0F028403 for ; Mon, 3 Apr 2017 01:19:35 +0000 (UTC) Received: by mail.wl.linuxfoundation.org (Postfix, from userid 486) id 0F05828415; Mon, 3 Apr 2017 01:19:35 +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 7CA9E28403 for ; Mon, 3 Apr 2017 01:19:34 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1751245AbdDCBTd (ORCPT ); Sun, 2 Apr 2017 21:19:33 -0400 Received: from mx2.suse.de ([195.135.220.15]:57725 "EHLO mx2.suse.de" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1750992AbdDCBTd (ORCPT ); Sun, 2 Apr 2017 21:19:33 -0400 X-Virus-Scanned: by amavisd-new at test-mx.suse.de Received: from relay1.suse.de (charybdis-ext.suse.de [195.135.220.254]) by mx2.suse.de (Postfix) with ESMTP id 92231AAB2; Mon, 3 Apr 2017 01:19:31 +0000 (UTC) From: NeilBrown To: Jens Axboe Date: Mon, 03 Apr 2017 11:18:51 +1000 Cc: linux-block@vger.kernel.org, linux-mm@kvack.org, LKML Subject: [PATCH] loop: Add PF_LESS_THROTTLE to block/loop device thread. Message-ID: <871staffus.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. Signed-off-by: NeilBrown Reviewed-by: Christoph Hellwig Acked-by: Michal Hocko --- drivers/block/loop.c | 3 +++ 1 file changed, 3 insertions(+) diff --git a/drivers/block/loop.c b/drivers/block/loop.c index 0ecb6461ed81..a7e1dd215fc2 100644 --- a/drivers/block/loop.c +++ b/drivers/block/loop.c @@ -1694,8 +1694,11 @@ static void loop_queue_work(struct kthread_work *work) { struct loop_cmd *cmd = container_of(work, struct loop_cmd, work); + int oldflags = current->flags & PF_LESS_THROTTLE; + current->flags |= PF_LESS_THROTTLE; loop_handle_cmd(cmd); + current->flags = (current->flags & ~PF_LESS_THROTTLE) | oldflags; } static int loop_init_request(void *data, struct request *rq,