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


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