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authorNeilBrown <neilb@suse.com>2017-06-16 08:02:09 +0300
committerJens Axboe <axboe@fb.com>2017-06-18 18:07:42 +0300
commitb2ee7d46befc43e355ffaf7bfabb00e7a901b3a0 (patch)
treee2c05bc23d7c4de20b441f13781bd01686e94bee /drivers/block
parentc27b2d634f81d1472a8107fc857481b67555d9bd (diff)
downloadlinux-b2ee7d46befc43e355ffaf7bfabb00e7a901b3a0.tar.xz
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. Reviewed-by: Christoph Hellwig <hch@lst.de> Reviewed-by: Ming Lei <tom.leiming@gmail.com> Suggested-by: Michal Hocko <mhocko@suse.com> Acked-by: Michal Hocko <mhocko@suse.com> Signed-off-by: NeilBrown <neilb@suse.com> Signed-off-by: Jens Axboe <axboe@fb.com>
Diffstat (limited to 'drivers/block')
-rw-r--r--drivers/block/loop.c8
1 files changed, 7 insertions, 1 deletions
diff --git a/drivers/block/loop.c b/drivers/block/loop.c
index 9cdf771b66ed..0de11444e317 100644
--- a/drivers/block/loop.c
+++ b/drivers/block/loop.c
@@ -854,10 +854,16 @@ 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;