On Tue 18-02-14 17:55:48, Tejun Heo 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? > > > 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? You are the workqueue expert so you may know better ;) But the way I understand it is that queue_delayed_work() does nothing if the timer is already running. Since we queue flusher work to run either immediately or after dirty_writeback_interval we are safe to run queue_delayed_work() whenever we want it to run after dirty_writeback_interval and mod_delayed_work() whenever we want to run it immediately. But it's subtle and some interface where we could say queue delayed work after no later than X would be easier to grasp. Honza -- Jan Kara <jack@xxxxxxx> SUSE Labs, CR -- To unsubscribe from this list: send the line "unsubscribe linux-fsdevel" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
