From patchwork Thu Apr 6 02:23:51 2017 Content-Type: text/plain; charset="utf-8" MIME-Version: 1.0 Content-Transfer-Encoding: 7bit X-Patchwork-Submitter: NeilBrown X-Patchwork-Id: 9665963 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 D2666602B5 for ; Thu, 6 Apr 2017 02:24:55 +0000 (UTC) Received: from mail.wl.linuxfoundation.org (localhost [127.0.0.1]) by mail.wl.linuxfoundation.org (Postfix) with ESMTP id AFD502817F for ; Thu, 6 Apr 2017 02:24:55 +0000 (UTC) Received: by mail.wl.linuxfoundation.org (Postfix, from userid 486) id 9210A2823D; Thu, 6 Apr 2017 02:24:55 +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 853002817F for ; Thu, 6 Apr 2017 02:24:54 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1752207AbdDFCYx (ORCPT ); Wed, 5 Apr 2017 22:24:53 -0400 Received: from mx2.suse.de ([195.135.220.15]:60866 "EHLO mx2.suse.de" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752052AbdDFCYw (ORCPT ); Wed, 5 Apr 2017 22:24:52 -0400 X-Virus-Scanned: by amavisd-new at test-mx.suse.de Received: from relay2.suse.de (charybdis-ext.suse.de [195.135.220.254]) by mx2.suse.de (Postfix) with ESMTP id 811CFAAB4; Thu, 6 Apr 2017 02:24:50 +0000 (UTC) From: NeilBrown To: Michal Hocko Date: Thu, 06 Apr 2017 12:23:51 +1000 Cc: Jens Axboe , linux-block@vger.kernel.org, linux-mm@kvack.org, LKML , Ming Lei Subject: Re: [PATCH v2] loop: Add PF_LESS_THROTTLE to block/loop device thread. In-Reply-To: <20170405073233.GD6035@dhcp22.suse.cz> References: <871staffus.fsf@notabene.neil.brown.name> <87wpazh3rl.fsf@notabene.neil.brown.name> <20170405071927.GA7258@dhcp22.suse.cz> <20170405073233.GD6035@dhcp22.suse.cz> Message-ID: <878tnegtoo.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 On Wed, Apr 05 2017, Michal Hocko wrote: > On Wed 05-04-17 09:19:27, Michal Hocko wrote: >> On Wed 05-04-17 14:33:50, NeilBrown wrote: > [...] >> > 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; >> >> As mentioned elsewhere, PF flags should be updated only on the current >> task otherwise there is potential rmw race. Is this safe? The code runs >> concurrently with the worker thread. > > I believe you need something like this instead > --- > diff --git a/drivers/block/loop.c b/drivers/block/loop.c > index f347285c67ec..07b2a909e4fb 100644 > --- a/drivers/block/loop.c > +++ b/drivers/block/loop.c > @@ -844,10 +844,16 @@ static void loop_unprepare_queue(struct loop_device *lo) > kthread_stop(lo->worker_task); > } > > +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; Arg - of course. How about we just split the kthread_create from the wake_up? Thanks, NeilBrown From: NeilBrown Subject: [PATCH] loop: Add PF_LESS_THROTTLE to block/loop device thread. 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 --- drivers/block/loop.c | 4 +++- 1 file changed, 3 insertions(+), 1 deletion(-) diff --git a/drivers/block/loop.c b/drivers/block/loop.c index 0ecb6461ed81..95679d988725 100644 --- a/drivers/block/loop.c +++ b/drivers/block/loop.c @@ -847,10 +847,12 @@ static void loop_unprepare_queue(struct loop_device *lo) 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_create(kthread_worker_fn, &lo->worker, "loop%d", lo->lo_number); if (IS_ERR(lo->worker_task)) return -ENOMEM; + lo->worker_task->flags |= PF_LESS_THROTTLE; + wake_up_process(lo->worker_task); set_user_nice(lo->worker_task, MIN_NICE); return 0; }