Re: [PATCH 3/5] mm: Implement IO-less balance_dirty_pages()

[Vivek Goyal <vgoyal@redhat.com>]

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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