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

[Fengguang Wu]

Hi Jeff,

On Wed, Jun 13, 2012 at 10:27:50AM -0400, Jeff Moyer wrote:
> Wanpeng Li <liwp.linux@xxxxxxxxx> 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.

The worried case is about sync() waiting

        (nr_dirty + nr_writeback) / write_bandwidth

time, where it is nr_dirty that could grow rather large.

For example, if dirty threshold is 1GB and write_bandwidth is 10MB/s,
the sync() will have to wait for 100 seconds. If there are heavy
dirtiers running during the sync, it will typically take several
hundreds of seconds (which looks not that good, but still much better
than being livelocked in some old kernels)..

> This really feels like we're papering over the problem.

That's true. The majority users probably don't want to cache 100s
worth of data in memory. It may be worthwhile to add a new per-bdi
limit whose unit is number-of-seconds (of dirty data).

Thanks,
Fengguang
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