linux-kernel - Re: [PATCH 05/10] writeback: per task dirty rate limit

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Date:	Fri, 26 Aug 2011 20:59:24 +0800
From:	Wu Fengguang <fengguang.wu@...el.com>
To:	Peter Zijlstra <a.p.zijlstra@...llo.nl>
Cc:	"linux-fsdevel@...r.kernel.org" <linux-fsdevel@...r.kernel.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Jan Kara <jack@...e.cz>, Christoph Hellwig <hch@....de>,
	Dave Chinner <david@...morbit.com>,
	Greg Thelen <gthelen@...gle.com>,
	Minchan Kim <minchan.kim@...il.com>,
	Vivek Goyal <vgoyal@...hat.com>,
	Andrea Righi <arighi@...eler.com>,
	linux-mm <linux-mm@...ck.org>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 05/10] writeback: per task dirty rate limit

On Fri, Aug 26, 2011 at 08:51:09PM +0800, Peter Zijlstra wrote:
> On Fri, 2011-08-26 at 19:38 +0800, Wu Fengguang wrote:
> > +       preempt_disable();
> >         /*
> > -        * Check the rate limiting. Also, we do not want to throttle real-time
> > -        * tasks in balance_dirty_pages(). Period.
> > +        * This prevents one CPU to accumulate too many dirtied pages without
> > +        * calling into balance_dirty_pages(), which can happen when there are
> > +        * 1000+ tasks, all of them start dirtying pages at exactly the same
> > +        * time, hence all honoured too large initial task->nr_dirtied_pause.
> >          */
> > -       preempt_disable();
> >         p =  &__get_cpu_var(bdp_ratelimits);
> 
> 	p = &get_cpu_var(bdp_ratelimits);

Ah yeah.. I actually followed your suggestion, and then find we'll
eventually do two __get_cpu_var() calls here, one for bdp_ratelimits
and the other for dirty_leaks in a planned patch. So let's keep the
preempt_disable()/preempt_enable().

> > -       *p += nr_pages_dirtied;
> > -       if (unlikely(*p >= ratelimit)) {
> > -               ratelimit = sync_writeback_pages(*p);
> > +       if (unlikely(current->nr_dirtied >= ratelimit))
> >                 *p = 0;
> > -               preempt_enable();
> > -               balance_dirty_pages(mapping, ratelimit);
> > -               return;
> > +       else {
> > +               *p += nr_pages_dirtied;
> > +               if (unlikely(*p >= ratelimit_pages)) {
> > +                       *p = 0;
> > +                       ratelimit = 0;
> > +               }
> >         }
> >         preempt_enable(); 
> 
> 	put_cpu_var(bdp_ratelimits);

ditto.

Thanks,
Fengguang
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