Messages in this thread | | | Date | Mon, 21 Sep 2009 15:45:11 +0200 | From | Jan Kara <> | Subject | Re: [PATCH] fs: Fix busyloop in wb_writeback() |
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On Mon 21-09-09 09:08:59, Wu Fengguang wrote: > 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(). Yes.
> > > 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? I agree someone must wait when we bail out due to congestion. But we bail out only when wbc->nonblocking is set. So I'd feel that callers setting this flag should handle it when we stop the writeback due to congestion.
> 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; > } OK, this change looks reasonable but I think we'll have to revisit the writeback logic more in detail as we discussed in the other thread.
Honza -- Jan Kara <jack@suse.cz> SUSE Labs, CR
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