[PATCH] writeback: Fix occasional slow sync(1)

[PATCH] writeback: Fix occasional slow sync(1)

[Jan Kara]

In case when system contains no dirty pages, wakeup_flusher_threads()
will submit WB_SYNC_NONE writeback for 0 pages so wb_writeback() exits
immediately without doing anything. Thus sync(1) will write all the
dirty inodes from a WB_SYNC_ALL writeback pass which is slow.

Fix the problem by using get_nr_dirty_pages() in
wakeup_flusher_threads() instead of calculating number of dirty pages
manually. That function also takes number of dirty inodes into account.

CC: stable@vger.kernel.org
Reported-by: Paul Taysom <taysom@chromium.org>
Signed-off-by: Jan Kara <jack@suse.cz>
---
 fs/fs-writeback.c | 6 ++----
 1 file changed, 2 insertions(+), 4 deletions(-)

diff --git a/fs/fs-writeback.c b/fs/fs-writeback.c
index a85ac4e..d0d70a8 100644
--- a/fs/fs-writeback.c
+++ b/fs/fs-writeback.c
@@ -1055,10 +1055,8 @@ void wakeup_flusher_threads(long nr_pages, enum wb_reason reason)
 {
 	struct backing_dev_info *bdi;
 
-	if (!nr_pages) {
-		nr_pages = global_page_state(NR_FILE_DIRTY) +
-				global_page_state(NR_UNSTABLE_NFS);
-	}
+	if (!nr_pages)
+		nr_pages = get_nr_dirty_pages();
 
 	rcu_read_lock();
 	list_for_each_entry_rcu(bdi, &bdi_list, bdi_list) {
-- 
1.8.1.4

Re: [PATCH] writeback: Fix occasional slow sync(1)

[Jan Kara]

On Mon 15-07-13 11:45:14, Jan Kara wrote:
> In case when system contains no dirty pages, wakeup_flusher_threads()
> will submit WB_SYNC_NONE writeback for 0 pages so wb_writeback() exits
> immediately without doing anything. Thus sync(1) will write all the
> dirty inodes from a WB_SYNC_ALL writeback pass which is slow.
> 
> Fix the problem by using get_nr_dirty_pages() in
> wakeup_flusher_threads() instead of calculating number of dirty pages
> manually. That function also takes number of dirty inodes into account.
> 
> CC: stable@vger.kernel.org
> Reported-by: Paul Taysom <taysom@chromium.org>
> Signed-off-by: Jan Kara <jack@suse.cz>
  Ping Al?

							Honza

> ---
>  fs/fs-writeback.c | 6 ++----
>  1 file changed, 2 insertions(+), 4 deletions(-)
> 
> diff --git a/fs/fs-writeback.c b/fs/fs-writeback.c
> index a85ac4e..d0d70a8 100644
> --- a/fs/fs-writeback.c
> +++ b/fs/fs-writeback.c
> @@ -1055,10 +1055,8 @@ void wakeup_flusher_threads(long nr_pages, enum wb_reason reason)
>  {
>  	struct backing_dev_info *bdi;
>  
> -	if (!nr_pages) {
> -		nr_pages = global_page_state(NR_FILE_DIRTY) +
> -				global_page_state(NR_UNSTABLE_NFS);
> -	}
> +	if (!nr_pages)
> +		nr_pages = get_nr_dirty_pages();
>  
>  	rcu_read_lock();
>  	list_for_each_entry_rcu(bdi, &bdi_list, bdi_list) {
> -- 
> 1.8.1.4
> 
-- 
Jan Kara <jack@suse.cz>
SUSE Labs, CR

Re: [PATCH] writeback: Fix occasional slow sync(1)

[Andrew Morton]

On Mon, 15 Jul 2013 11:45:14 +0200 Jan Kara <jack@suse.cz> wrote:

> In case when system contains no dirty pages, wakeup_flusher_threads()
> will submit WB_SYNC_NONE writeback for 0 pages so wb_writeback() exits
> immediately without doing anything. Thus sync(1) will write all the
> dirty inodes from a WB_SYNC_ALL writeback pass which is slow.
> 
> Fix the problem by using get_nr_dirty_pages() in
> wakeup_flusher_threads() instead of calculating number of dirty pages
> manually. That function also takes number of dirty inodes into account.

Fair enough.

> CC: stable@vger.kernel.org

Unexpected.  What was your thinking here?

Re: [PATCH] writeback: Fix occasional slow sync(1)

[Jan Kara]

On Thu 15-08-13 15:15:04, Andrew Morton wrote:
> On Mon, 15 Jul 2013 11:45:14 +0200 Jan Kara <jack@suse.cz> wrote:
> 
> > In case when system contains no dirty pages, wakeup_flusher_threads()
> > will submit WB_SYNC_NONE writeback for 0 pages so wb_writeback() exits
> > immediately without doing anything. Thus sync(1) will write all the
> > dirty inodes from a WB_SYNC_ALL writeback pass which is slow.
> > 
> > Fix the problem by using get_nr_dirty_pages() in
> > wakeup_flusher_threads() instead of calculating number of dirty pages
> > manually. That function also takes number of dirty inodes into account.
> 
> Fair enough.
> 
> > CC: stable@vger.kernel.org
> 
> Unexpected.  What was your thinking here?
  Right, probably the problem isn't serious enough. Just discard the CC
please. Thanks!

								Honza
-- 
Jan Kara <jack@suse.cz>
SUSE Labs, CR