On Feb 18, 2014 4:55 PM, "Tejun Heo" <tj@xxxxxxxxxx> wrote:
>
> Hello,
>
> On Fri, Feb 14, 2014 at 08:12:17PM -0800, Derek Basehore wrote:
> > bdi_wakeup_thread_delayed used the mod_delayed_work function to schedule work
> > to writeback dirty inodes. The problem with this is that it can delay work that
> > is scheduled for immediate execution, such as the work from sync_inodes_sb.
> > This can happen since mod_delayed_work can now steal work from a work_queue.
> > This fixes the problem by using queue_delayed_work instead. This is a
> > regression from the move to the bdi workqueue design.
> >
> > The reason that this causes a problem is that laptop-mode will change the
> > delay, dirty_writeback_centisecs, to 60000 (10 minutes) by default. In the case
> > that bdi_wakeup_thread_delayed races with sync_inodes_sb, sync will be stopped
> > for 10 minutes and trigger a hung task. Even if dirty_writeback_centisecs is
> > not long enough to cause a hung task, we still don't want to delay sync for
> > that long.
>
> Oops.
>
> > For the same reason, this also changes bdi_writeback_workfn to immediately
> > queue the work again in the case that the work_list is not empty. The same
> > problem can happen if the sync work is run on the rescue worker.
> >
> > Signed-off-by: Derek Basehore <dbasehore@xxxxxxxxxxxx>
> > ---
> > fs/fs-writeback.c | 5 +++--
> > mm/backing-dev.c | 2 +-
> > 2 files changed, 4 insertions(+), 3 deletions(-)
> >
> > diff --git a/fs/fs-writeback.c b/fs/fs-writeback.c
> > index e0259a1..95b7b8c 100644
> > --- a/fs/fs-writeback.c
> > +++ b/fs/fs-writeback.c
> > @@ -1047,8 +1047,9 @@ void bdi_writeback_workfn(struct work_struct *work)
> > trace_writeback_pages_written(pages_written);
> > }
> >
> > - if (!list_empty(&bdi->work_list) ||
> > - (wb_has_dirty_io(wb) && dirty_writeback_interval))
> > + if (!list_empty(&bdi->work_list))
> > + mod_delayed_work(bdi_wq, &wb->dwork, 0);
> > + else if (wb_has_dirty_io(wb) && dirty_writeback_interval)
> > queue_delayed_work(bdi_wq, &wb->dwork,
> > msecs_to_jiffies(dirty_writeback_interval * 10));
>
> Can you please add some comments explaining why the specific variants
> are being used here?
Will do this weekend. I'm away from my computer until then.
>
> > diff --git a/mm/backing-dev.c b/mm/backing-dev.c
> > index ce682f7..3fde024 100644
> > --- a/mm/backing-dev.c
> > +++ b/mm/backing-dev.c
> > @@ -294,7 +294,7 @@ void bdi_wakeup_thread_delayed(struct backing_dev_info *bdi)
> > unsigned long timeout;
> >
> > timeout = msecs_to_jiffies(dirty_writeback_interval * 10);
> > - mod_delayed_work(bdi_wq, &bdi->wb.dwork, timeout);
> > + queue_delayed_work(bdi_wq, &bdi->wb.dwork, timeout);
>
> and here?
>
> Hmmm.... but doesn't this create an opposite problem? Now a flush
> queued for an earlier time may be overridden by something scheduled
> later, no?
>
> Thanks.
>
> --
> tejun