On Wed, Mar 16, 2011 at 08:10:21PM +0100, Jan Kara wrote:
> On Wed 16-03-11 12:53:31, Vivek Goyal wrote:
> > On Tue, Mar 08, 2011 at 11:31:13PM +0100, Jan Kara wrote:
> > [..]
> > > +/*
> > > + * balance_dirty_pages() must be called by processes which are generating dirty
> > > + * data. It looks at the number of dirty pages in the machine and will force
> > > + * the caller to perform writeback if the system is over `vm_dirty_ratio'.
> > > + * If we're over `background_thresh' then the writeback threads are woken to
> > > + * perform some writeout.
> > > + */
> > > +static void balance_dirty_pages(struct address_space *mapping,
> > > + unsigned long write_chunk)
> > > +{
> > > + struct backing_dev_info *bdi = mapping->backing_dev_info;
> > > + struct balance_waiter bw;
> > > + struct dirty_limit_state st;
> > > + int dirty_exceeded = check_dirty_limits(bdi, &st);
> > > +
> > > + if (dirty_exceeded < DIRTY_MAY_EXCEED_LIMIT ||
> > > + (dirty_exceeded == DIRTY_MAY_EXCEED_LIMIT &&
> > > + !bdi_task_limit_exceeded(&st, current))) {
> > > + if (bdi->dirty_exceeded &&
> > > + dirty_exceeded < DIRTY_MAY_EXCEED_LIMIT)
> > > + bdi->dirty_exceeded = 0;
> > > /*
> > > - * Increase the delay for each loop, up to our previous
> > > - * default of taking a 100ms nap.
> > > + * In laptop mode, we wait until hitting the higher threshold
> > > + * before starting background writeout, and then write out all
> > > + * the way down to the lower threshold. So slow writers cause
> > > + * minimal disk activity.
> > > + *
> > > + * In normal mode, we start background writeout at the lower
> > > + * background_thresh, to keep the amount of dirty memory low.
> > > */
> > > - pause <<= 1;
> > > - if (pause > HZ / 10)
> > > - pause = HZ / 10;
> > > + if (!laptop_mode && dirty_exceeded == DIRTY_EXCEED_BACKGROUND)
> > > + bdi_start_background_writeback(bdi);
> > > + return;
> > > }
> > >
> > > - /* Clear dirty_exceeded flag only when no task can exceed the limit */
> > > - if (!min_dirty_exceeded && bdi->dirty_exceeded)
> > > - bdi->dirty_exceeded = 0;
> > > + if (!bdi->dirty_exceeded)
> > > + bdi->dirty_exceeded = 1;
> > >
> > > - if (writeback_in_progress(bdi))
> > > - return;
> > > + trace_writeback_balance_dirty_pages_waiting(bdi, write_chunk);
> > > + /* Kick flusher thread to start doing work if it isn't already */
> > > + bdi_start_background_writeback(bdi);
> > >
> > > + bw.bw_wait_pages = write_chunk;
> > > + bw.bw_task = current;
> > > + spin_lock(&bdi->balance_lock);
> > > /*
> > > - * In laptop mode, we wait until hitting the higher threshold before
> > > - * starting background writeout, and then write out all the way down
> > > - * to the lower threshold. So slow writers cause minimal disk activity.
> > > - *
> > > - * In normal mode, we start background writeout at the lower
> > > - * background_thresh, to keep the amount of dirty memory low.
> > > + * First item? Need to schedule distribution of IO completions among
> > > + * items on balance_list
> > > + */
> > > + if (list_empty(&bdi->balance_list)) {
> > > + bdi->written_start = bdi_stat_sum(bdi, BDI_WRITTEN);
> > > + /* FIXME: Delay should be autotuned based on dev throughput */
> > > + schedule_delayed_work(&bdi->balance_work, HZ/10);
> > > + }
> > > + /*
> > > + * Add work to the balance list, from now on the structure is handled
> > > + * by distribute_page_completions()
> > > + */
> > > + list_add_tail(&bw.bw_list, &bdi->balance_list);
> > > + bdi->balance_waiters++;
> > Had a query.
> >
> > - What makes sure that flusher thread will not stop writing back till all
> > the waiters on the bdi have been woken up. IIUC, flusher thread will
> > stop once global background ratio is with-in limit. Is it possible that
> > there are still some waiter on some bdi waiting for more pages to finish
> > writeback and that might not happen for sometime.
> Yes, this can possibly happen but once distribute_page_completions()
> gets called (after a given time), it will notice that we are below limits
> and wake all waiters.
> Under normal circumstances, we should have a decent
> estimate when distribute_page_completions() needs to be called and that
> should be long before flusher thread finishes it's work. But in cases when
> a bdi has only a small share of global dirty limit, what you describe can
> possibly happen.
So if a bdi share is small then it can happen that global background
threshold is fine but per bdi threshold is not. That means
task_bdi_threshold is also above limit and IIUC, distribute_page_completion()
will not wake up the waiter until bdi_task_limit_exceeded() is in control.
/*
* Wake tasks that might have gotten below their limits and
* compute
* the number of pages we wait for
*/
list_for_each_entry_safe(waiter, tmpw, &bdi->balance_list,
bw_list) {
if (dirty_exceeded == DIRTY_MAY_EXCEED_LIMIT &&
!bdi_task_limit_exceeded(&st, waiter->bw_task))
balance_waiter_done(bdi, waiter);
else if (waiter->bw_wait_pages < min_pages)
min_pages = waiter->bw_wait_pages;
}
So it can happen that global background threshold is under limit but a
specific task bdi threshold is not and I am failing to see who will
wake up the task in that situation.
Thanks
Vivek
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