Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1753170AbdDFCY4 (ORCPT ); Wed, 5 Apr 2017 22:24:56 -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 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 Content-Type: multipart/signed; boundary="=-=-="; micalg=pgp-sha256; protocol="application/pgp-signature" Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 4991 Lines: 140 --=-=-= Content-Type: text/plain Content-Transfer-Encoding: quoted-printable 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 |=3D PF_LESS_THROTTLE; >> > return 0; >>=20 >> 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); > } >=20=20 > +int loop_kthread_worker_fn(void *worker_ptr) > +{ > + current->flags |=3D 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 =3D kthread_run(kthread_worker_fn, > + lo->worker_task =3D 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 =2D-- 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 =2D-- 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); =2D lo->worker_task =3D kthread_run(kthread_worker_fn, + lo->worker_task =3D kthread_create(kthread_worker_fn, &lo->worker, "loop%d", lo->lo_number); if (IS_ERR(lo->worker_task)) return -ENOMEM; + lo->worker_task->flags |=3D PF_LESS_THROTTLE; + wake_up_process(lo->worker_task); set_user_nice(lo->worker_task, MIN_NICE); return 0; } =2D-=20 2.12.2 --=-=-= Content-Type: application/pgp-signature; name="signature.asc" -----BEGIN PGP SIGNATURE----- iQIzBAEBCAAdFiEEG8Yp69OQ2HB7X0l6Oeye3VZigbkFAljlprcACgkQOeye3VZi gblZRQ/7BeHOJGPoRjeKz48keHq5kDiA9HwF/AAbwiOJy0Lp3EBQxBnhsT8X5H6z h8Rv+ifQs44ypQaV5zNyxHeX5qbIyKqjWsljnlG8Jt/NN5NGIUiGbnHsFBJTsphy WKaAgr5RcdBAqb6PoXPlcBbqEGIAVdVlfpDT2PsOI8EZSGVb5JnCYn9hBj4KUKhZ k7lwgPIFoYgJ/7R7nsheC6t6rJd+vNQ5igPRJ//DLZR82ktmvKiPbxY2JAFH/n+5 3y9CsFE+Edw46rHPvCFPAaTBfqbQh/jtd17R/f/ZWqbnCY2yKlC3jDsywvdYnLtM Qu+gYvBJ9F/GERQEVWdtEBgnB6HOJYwMlgW0fQMQYfLmToMwJn9nRx1544B4CpGo JxOhk5sJ9k/A2oWd6T+rLCMCYjbSS9yutg5MFT15GFDlE6PWWOd8mXyhykRyeJe0 H+EzuWpod6nV986WbHzreAjrPxBEOQd36NQAhdj5Y7wuDWQ9Sstfb+81n7qnW+dd fMuG+Gv03j+S22X2uq4JR5TjEkNQtFbZ/bnPG8Bd3/3V371xka2egmAAsndIasYW T2geORbTyedzJ9Wqk6BUvOBR12svHQ502ztbc56P/oeYtcTKPov7WAQiGHysEHuv VAEGSiDQw/oY6cw9PdlU27AzwHG2a9HprgAYd1xEy/ugcSpglKI= =DFxu -----END PGP SIGNATURE----- --=-=-=--