From patchwork Fri Jun 16 05:02:09 2017 Content-Type: text/plain; charset="utf-8" MIME-Version: 1.0 Content-Transfer-Encoding: 7bit X-Patchwork-Submitter: NeilBrown X-Patchwork-Id: 9790543 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 DDF4360231 for ; Fri, 16 Jun 2017 05:04:17 +0000 (UTC) Received: from mail.wl.linuxfoundation.org (localhost [127.0.0.1]) by mail.wl.linuxfoundation.org (Postfix) with ESMTP id D84C928638 for ; Fri, 16 Jun 2017 05:04:17 +0000 (UTC) Received: by mail.wl.linuxfoundation.org (Postfix, from userid 486) id CD5352863A; Fri, 16 Jun 2017 05:04:17 +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=unavailable 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 A3C7B2863B for ; Fri, 16 Jun 2017 05:04:16 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1752094AbdFPFEK (ORCPT ); Fri, 16 Jun 2017 01:04:10 -0400 Received: from mx2.suse.de ([195.135.220.15]:34082 "EHLO mx1.suse.de" rhost-flags-OK-OK-OK-FAIL) by vger.kernel.org with ESMTP id S1751815AbdFPFEI (ORCPT ); Fri, 16 Jun 2017 01:04:08 -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 mx1.suse.de (Postfix) with ESMTP id 5676EAB09; Fri, 16 Jun 2017 05:04:07 +0000 (UTC) From: NeilBrown To: Jens Axboe Date: Fri, 16 Jun 2017 15:02:09 +1000 Subject: [PATCH 2/2] loop: Add PF_LESS_THROTTLE to block/loop device thread. Cc: linux-block@vger.kernel.org, linux-kernel@vger.kernel.org Message-ID: <149758932925.10006.1411305319375332201.stgit@noble> In-Reply-To: <149758925866.10006.12779875832895865043.stgit@noble> References: <149758925866.10006.12779875832895865043.stgit@noble> User-Agent: StGit/0.17.1-dirty 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 Reviewed-by: Ming Lei Suggested-by: Michal Hocko Acked-by: Michal Hocko Signed-off-by: NeilBrown --- drivers/block/loop.c | 8 +++++++- 1 file changed, 7 insertions(+), 1 deletion(-) diff --git a/drivers/block/loop.c b/drivers/block/loop.c index 9c457ca6c55e..6ed7c4506951 100644 --- a/drivers/block/loop.c +++ b/drivers/block/loop.c @@ -843,10 +843,16 @@ static void loop_unprepare_queue(struct loop_device *lo) kthread_stop(lo->worker_task); } +static int loop_kthread_worker_fn(void *worker_ptr) +{ + current->flags |= PF_LESS_THROTTLE; + return kthread_worker_fn(worker_ptr); +} + static int loop_prepare_queue(struct loop_device *lo) { kthread_init_worker(&lo->worker); - lo->worker_task = kthread_run(kthread_worker_fn, + lo->worker_task = kthread_run(loop_kthread_worker_fn, &lo->worker, "loop%d", lo->lo_number); if (IS_ERR(lo->worker_task)) return -ENOMEM;