linux-kernel - Re: [PATCH] fs: Fix busyloop in wb

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Message-ID: <20090921130634.GX23126@kernel.dk>
Date:	Mon, 21 Sep 2009 15:06:34 +0200
From:	Jens Axboe <jens.axboe@...cle.com>
To:	Wu Fengguang <fengguang.wu@...el.com>
Cc:	Jan Kara <jack@...e.cz>, LKML <linux-kernel@...r.kernel.org>,
	Theodore Tso <tytso@....edu>
Subject: Re: [PATCH] fs: Fix busyloop in wb_writeback()

On Mon, Sep 21 2009, 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.

OK, I'll have to look into these issues soonish. I'll be travelling next
week though, so I cannot do more about it right now. If you have time to
work on tested patches, then that would be MUCH appreciated it!

-- 
Jens Axboe

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