Re: [PATCH V2] writeback: fix hung_task alarm when sync block

[Fengguang Wu <fengguang.wu@intel.com>]

On Wed, Jun 13, 2012 at 10:48:40PM +0800, Fengguang Wu wrote:
> Hi Jeff,
> 
> On Wed, Jun 13, 2012 at 10:27:50AM -0400, Jeff Moyer wrote:
> > Wanpeng Li <liwp.linux@gmail.com> writes:
> > 
> > > diff --git a/fs/fs-writeback.c b/fs/fs-writeback.c
> > > index f2d0109..df879ee 100644
> > > --- a/fs/fs-writeback.c
> > > +++ b/fs/fs-writeback.c
> > > @@ -1311,7 +1311,11 @@ void writeback_inodes_sb_nr(struct super_block *sb,
> > >  
> > >  	WARN_ON(!rwsem_is_locked(&sb->s_umount));
> > >  	bdi_queue_work(sb->s_bdi, &work);
> > > -	wait_for_completion(&done);
> > > +	if (sysctl_hung_task_timeout_secs)
> > > +		while (!wait_for_completion_timeout(&done, HZ/2))
> > > +			;
> > > +	else
> > > +		wait_for_completion(&done);
> > >  }
> > >  EXPORT_SYMBOL(writeback_inodes_sb_nr);
> > 
> > Is it really expected that writeback_inodes_sb_nr will routinely queue
> > up more than 2 seconds worth of I/O (Yes, I understand that it isn't the
> > only entity issuing I/O)? 
> 
> Yes, in the case of syncing the whole superblock.
> Basically sync() does its job in two steps:
> 
> for all sb:
>         writeback_inodes_sb_nr() # WB_SYNC_NONE
>         sync_inodes_sb()         # WB_SYNC_ALL
> 
> > For devices that are really slow, it may make
> > more sense to tune the system so that you don't have too much writeback
> > I/O submitted at once.  Dropping nr_requests for the given queue should
> > fix this situation, I would think.

Note that this wait_for_completion() waits for the flusher to move
all dirty pages into writeback pages, which in turn will block in
get_request_wait(). So writeback_inodes_sb_nr() could in theory finish
quickly if the nr_requests is a huge value. But then sync_inodes_sb()
will come and sleep (longer) on each of the writeback pages..

Thanks,
Fengguang