From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mx2.suse.de ([195.135.220.15]:49173 "EHLO mx2.suse.de" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752615AbdDFXsO (ORCPT ); Thu, 6 Apr 2017 19:48:14 -0400 From: NeilBrown To: Jens Axboe Date: Fri, 07 Apr 2017 09:47:32 +1000 Cc: Michal Hocko Cc: linux-block@vger.kernel.org, linux-mm@kvack.org, LKML , Ming Lei Subject: [PATCH v3] loop: Add PF_LESS_THROTTLE to block/loop device thread. In-Reply-To: <20170406065326.GB5497@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> <878tnegtoo.fsf@notabene.neil.brown.name> <20170406065326.GB5497@dhcp22.suse.cz> Message-ID: <87o9w9yu7f.fsf@notabene.neil.brown.name> MIME-Version: 1.0 Content-Type: multipart/signed; boundary="=-=-="; micalg=pgp-sha256; protocol="application/pgp-signature" Sender: linux-block-owner@vger.kernel.org List-Id: linux-block@vger.kernel.org --=-=-= Content-Type: text/plain Content-Transfer-Encoding: quoted-printable 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 =2D-- Hi Jens, I think this version meets with everyone's approval. Thanks, 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 0ecb6461ed81..035b8651b8bf 100644 =2D-- 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 +static 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); =2D 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; =2D-=20 2.12.2 --=-=-= Content-Type: application/pgp-signature; name="signature.asc" -----BEGIN PGP SIGNATURE----- iQIzBAEBCAAdFiEEG8Yp69OQ2HB7X0l6Oeye3VZigbkFAljm05QACgkQOeye3VZi gbn4cw/+NsFUKuHxkgtl1GFr33VMY74tjDH1pU44pMj0k75+ocjbgfH/86cNrJxs otqw0AXjILPI4oJo3LanwBCKbx6J7DRCml654AgckyztaksSftmzfvsY9Vg5gvqS M5kX7Wi++HXluhZGqfN8tAZ7unQDqgaI2Io9FjZIsphgH+4qKB8MbWi+QoqziUnk koZylFmXTxZcVa0NVe1P6fmMO9UJZsY2WFuaRosWFBn6fr8p2EYf1Q9FydFOng3n ovM/nASMNlfS4mUrqkGQu0wpzU3ckYG0EgFBFUG4PC0zaLjl6hkf0N3RKg6e9LW5 fo3auYMkqcQ8Yv8BHDNrRxlaPVajfVmXt2ssluNu0LgwqRoS7hLZUwBA1px+OwTC kLp3IrXLiHk945uUNaAZMUuqeV4sFIWghRF2IlBmMPBUgvjHAYg+X06OXfepCYSF o9XSEufaB3EUejrurxAbukx1V6rEZtBZv16PMQGkjJAF9s3OuTLizL+kdspKsnZu yN64nwNhBZC3hEtijyiFrcHvKFZAmV3O3PKa64SEwMHmZzIigrBcfzjlyjUZCbrW CXzDvkUMccgIQhlHewuJ+tdI8MFIrayLihNJyC0aBCjy4NXT3D0UP2wN5BIltl0C d14giCYX+29xhryYSVBvx0BuQuMKVf99svnwhZ/ocqM+CBGAxS8= =kdw3 -----END PGP SIGNATURE----- --=-=-=--