Re: [PATCH] fs: Fix busyloop in wb_writeback()

Wu Fengguang
2009-09-23T07:56:57Z

On Thu, Sep 17, 2009 at 01:22:48AM +0800, Jan Kara wrote:
> If all inodes are under writeback (e.g. in case when there's only one inode
> with dirty pages), wb_writeback() with WB_SYNC_NONE work basically degrades
> to busylooping until I_SYNC flags of the inode is cleared. Fix the problem by
> waiting on I_SYNC flags of an inode on b_more_io list in case we failed to
> write anything.

Hi Jan,

I reconfirmed that this patch fixed the busy loop in wb_writeback()
path. I also noticed that balance_dirty_pages() looped a lot. Given
that balance_dirty_pages() schedules in between, it cannot be called
busy loop. But the loop interval is about 0.01s, which is shorter than
the expected 0.1s. This is a 2-core CPU, so I guess the cp process got
re-scheduled immediately, long before it timed out.

[ 491.440702] requeue_io() +457: inode=39381
[ 491.442360] mm/page-writeback.c +540 balance_dirty_pages(): comm=cp pid=3302 n=0
[ 491.444257] global dirty=51770 writeback=18201 nfs=0 flags=_M towrite=1536 skipped=0
[ 491.445856] requeue_io() +457: inode=39381
[ 491.446543] mm/page-writeback.c +540 balance_dirty_pages(): comm=cp pid=3302 n=0
[ 491.448346] global dirty=51770 writeback=18201 nfs=0 flags=_M towrite=1536 skipped=0
[ 491.460535] requeue_io() +457: inode=39381
[ 491.462206] mm/page-writeback.c +540 balance_dirty_pages(): comm=cp pid=3302 n=0
[ 491.463574] global dirty=51894 writeback=18077 nfs=0 flags=_M towrite=1536 skipped=0
[ 491.474072] requeue_io() +457: inode=39381
[ 491.475416] mm/page-writeback.c +540 balance_dirty_pages(): comm=cp pid=3302 n=0
[ 491.477220] global dirty=52049 writeback=17922 nfs=0 flags=_M towrite=1536 skipped=0
[ 491.481859] requeue_io() +457: inode=39381
[ 491.482618] mm/page-writeback.c +540 balance_dirty_pages(): comm=cp pid=3302 n=0
[ 491.485026] global dirty=52049 writeback=17798 nfs=0 flags=_M towrite=1536 skipped=0
[ 491.496843] requeue_io() +457: inode=39381
[ 491.499001] mm/page-writeback.c +540 balance_dirty_pages(): comm=cp pid=3302 n=0
[ 491.500658] global dirty=52173 writeback=17674 nfs=0 flags=_M towrite=1536 skipped=0
[ 491.508540] requeue_io() +457: inode=39381
[ 491.509490] mm/page-writeback.c +540 balance_dirty_pages(): comm=cp pid=3302 n=0
[ 491.511300] global dirty=52297 writeback=17674 nfs=0 flags=_M towrite=1536 skipped=0
[ 491.517728] requeue_io() +457: inode=39381
[ 491.518752] mm/page-writeback.c +540 balance_dirty_pages(): comm=cp pid=3302 n=0
[ 491.521498] global dirty=52421 writeback=17550 nfs=0 flags=_M towrite=1536 skipped=0
[ 491.525881] requeue_io() +457: inode=39381
[ 491.527339] mm/page-writeback.c +540 balance_dirty_pages(): comm=cp pid=3302 n=0
[ 491.528730] global dirty=52421 writeback=17550 nfs=0 flags=_M towrite=1536 skipped=0

Thanks,
Fengguang

> CC: Wu Fengguang <fengguang.wu@intel.com>
> Signed-off-by: Jan Kara <jack@suse.cz>
> ---
> fs/fs-writeback.c | 19 ++++++++++++++++++-
> 1 files changed, 18 insertions(+), 1 deletions(-)
>
> diff --git a/fs/fs-writeback.c b/fs/fs-writeback.c
> index 8e1e5e1..c59d673 100644
> --- a/fs/fs-writeback.c
> +++ b/fs/fs-writeback.c
> @@ -706,6 +706,7 @@ static long wb_writeback(struct bdi_writeback *wb,
> };
> unsigned long oldest_jif;
> long wrote = 0;
> + struct inode *inode;
>
> if (wbc.for_kupdate) {
> wbc.older_than_this = &oldest_jif;
> @@ -747,8 +748,24 @@ static long wb_writeback(struct bdi_writeback *wb,
> * If we ran out of stuff to write, bail unless more_io got set
> */
> if (wbc.nr_to_write > 0 || wbc.pages_skipped > 0) {
> - if (wbc.more_io && !wbc.for_kupdate)
> + if (wbc.more_io && !wbc.for_kupdate) {
> + if (wbc.nr_to_write < MAX_WRITEBACK_PAGES)
> + continue;
> + /*
> + * Nothing written. Wait for some inode to
> + * become available for writeback. Otherwise
> + * we'll just busyloop.
> + */
> + spin_lock(&inode_lock);
> + if (!list_empty(&wb->b_more_io)) {
> + inode = list_entry(
> + wb->b_more_io.prev,
> + struct inode, i_list);
> + inode_wait_for_writeback(inode);
> + }
> + spin_unlock(&inode_lock);
> continue;
> + }
> break;
> }
> }
> --
> 1.6.0.2