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

[Wu Fengguang]

On Mon, Sep 21, 2009 at 09:19:26PM +0800, Jan Kara wrote:
> On Mon 21-09-09 21:01:45, Wu Fengguang wrote:
> > On Thu, Sep 17, 2009 at 02:41:06AM +0800, Jens Axboe wrote:
> > > On Wed, Sep 16 2009, 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.
> > > 
> > > Interesting, so this will happen if the dirtier and flush thread end up
> > > "fighting" each other over the same inode. I'll throw this into the
> > > testing mix.
> > > 
> > > How did you notice?
> > 
> > Jens, I found another busy loop. Not sure about the solution, but here
> > is the quick fact.
> > 
> > Tested git head is 1ef7d9aa32a8ee054c4d4fdcd2ea537c04d61b2f, which
> > seems to be the last writeback patch in the linux-next tree. I cannot
> > run the plain head of linux-next because it just refuses boot up.
> > 
> > On top of which Jan Kara's I_SYNC waiting patch and the attached
> > debugging patch is applied.
> > 
> > Test commands are:
> > 
> >         # mount /mnt/test # ext4 fs
> >         # echo 1 > /proc/sys/fs/dirty_debug
> > 
> >         # cp /dev/zero /mnt/test/zero0
> > 
> > After that the box is locked up, the system is busy doing these:
> > 
> > [   54.740295] requeue_io() +457: inode=79232
> > [   54.740300] mm/page-writeback.c +539 balance_dirty_pages(): comm=cp pid=3327 n=0
> > [   54.740303] global dirty=60345 writeback=10145 nfs=0 flags=_M towrite=1536 skipped=0
> > [   54.740317] requeue_io() +457: inode=79232
> > [   54.740322] mm/page-writeback.c +539 balance_dirty_pages(): comm=cp pid=3327 n=0
> > [   54.740325] global dirty=60345 writeback=10145 nfs=0 flags=_M towrite=1536 skipped=0
> > [   54.740339] requeue_io() +457: inode=79232
> > [   54.740344] mm/page-writeback.c +539 balance_dirty_pages(): comm=cp pid=3327 n=0
> > [   54.740347] global dirty=60345 writeback=10145 nfs=0 flags=_M towrite=1536 skipped=0
> > ......
> > 
> > Basically the traces show that balance_dirty_pages() is busy looping.
> > It cannot write anything because the inode always be requeued by this line:
> > 
> >         if (inode->i_state & I_SYNC) {
> >                if (!wait) {
> >                         requeue_io(inode);
> >                         return 0;
> >                 }
> > 
> > This seem to happen when the partition is FULL.
>   Hmm, are you sure my fix is applied? It should prevent exactly this busy
> loop when we just requeue one inode again and again... If it really is, I
> wonder why we didn't end up calling inode_wait_for_writeback I've added.

Yes it is applied, and in fact checking how it helps in real workload :)

For this bug I reported, it seems that the I_SYNC somehow cannot be
cleared _for ever_. I'm happy to confirm that Jens'
schedule_timeout_interruptible() patch fixed it. However I don't know
what exactly goes on underneath.

Thanks,
Fengguang
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