Messages in this thread Patch in this message | | | Date | Mon, 21 Sep 2009 09:08:59 +0800 | From | Wu Fengguang <> | Subject | Re: [PATCH] fs: Fix busyloop in wb_writeback() |
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On Mon, Sep 21, 2009 at 01:43:56AM +0800, Jan Kara wrote: > On Sun 20-09-09 10:35:28, Wu Fengguang wrote: > > 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. > > > > Sorry, I realized that inode_wait_for_writeback() waits for I_SYNC. > > But inodes in b_more_io are not expected to have I_SYNC set. So your > > patch looks like a big no-op? > Hmm, I don't think so. writeback_single_inode() does: > if (inode->i_state & I_SYNC) { > /* > * If this inode is locked for writeback and we are not > * doing > * writeback-for-data-integrity, move it to b_more_io so > * that > * writeback can proceed with the other inodes on s_io. > * > * We'll have another go at writing back this inode when we > * completed a full scan of b_io. > */ > if (!wait) { > requeue_io(inode); > return 0; > } > > So when we see inode under writeback, we put it to b_more_io. So I think > my patch really fixes the issue when two threads are racing on writing the > same inode.
Ah OK. So it busy loops when there are more syncing threads than dirty files. For example, one bdi flush thread plus one process running balance_dirty_pages().
> > The busy loop does exists, when bdi is congested. > > In this case, write_cache_pages() will refuse to write anything, > > we used to be calling congestion_wait() to take a breath, but now > > wb_writeback() purged that call and thus created a busy loop. > I don't think congestion is an issue here. The device needen't be > congested for the busyloop to happen.
bdi congestion is a different case. When there are only one syncing thread, b_more_io inodes won't have I_SYNC, so your patch is a no-op. wb_writeback() or any of its sub-routines must wait/yield for a while to avoid busy looping on the congestion. Where is the wait with Jens' new code?
Another question is, why wbc.more_io can be ignored for kupdate syncs? I guess it would lead to slow writeback of large files.
This patch reflects my concerns on the two problems.
Thanks, Fengguang --- fs/fs-writeback.c | 6 ++++-- 1 file changed, 4 insertions(+), 2 deletions(-)
--- linux.orig/fs/fs-writeback.c 2009-09-20 10:44:25.000000000 +0800 +++ linux/fs/fs-writeback.c 2009-09-21 08:53:09.000000000 +0800 @@ -818,8 +818,10 @@ static long wb_writeback(struct bdi_writ /* * 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.nr_to_write > 0) { + if (wbc.encountered_congestion) + congestion_wait(BLK_RW_ASYNC, HZ); + if (wbc.more_io) continue; break; }
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