linux-kernel - Re: [PATCH 4/5] writeback: per task dirty rate limit

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Date:	Mon, 15 Aug 2011 16:26:37 +0200
From:	Andrea Righi <andrea@...terlinux.com>
To:	Wu Fengguang <fengguang.wu@...el.com>
Cc:	Peter Zijlstra <peterz@...radead.org>,
	"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>, linux-mm <linux-mm@...ck.org>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 4/5] writeback: per task dirty rate limit

On Mon, Aug 15, 2011 at 10:21:41PM +0800, Wu Fengguang wrote:
> Andrea,
> 
> > > @@ -1158,6 +1160,15 @@ void balance_dirty_pages_ratelimited_nr(
> > >  	if (bdi->dirty_exceeded)
> > >  		ratelimit = 8;
> > >  
> > > +	preempt_disable();
> > > +	p = &__get_cpu_var(dirty_leaks);
> > > +	if (*p > 0 && current->nr_dirtied < ratelimit) {
> > > +		nr_pages_dirtied = min(*p, ratelimit - current->nr_dirtied);
> > > +		*p -= nr_pages_dirtied;
> > > +		current->nr_dirtied += nr_pages_dirtied;
> > > +	}
> > > +	preempt_enable();
> > > +
> > 
> > I think we are still leaking some dirty pages, when the condition is
> > false nr_pages_dirtied is just ignored.
> > 
> > Why not doing something like this?
> > 
> > 	current->nr_dirtied += nr_pages_dirtied;
> 
> You must mean the above line. Sorry I failed to provide another patch
> before this one (attached this time). With that preparation patch, it
> effectively become equal to the logic below :)

OK. This is even better than my proposal, because it doesn't charge
pages that are dirtied multiple times. Sounds good.

Thanks,
-Andrea

> 
> > 	if (current->nr_dirtied < ratelimit) {
> > 		p = &get_cpu_var(dirty_leaks);
> > 		if (*p > 0) {
> > 			nr_pages_dirtied = min(*p, ratelimit -
> > 							current->nr_dirtied);
> > 			*p -= nr_pages_dirtied;
> > 		} else
> > 			nr_pages_dirtied = 0;
> > 		put_cpu_var(dirty_leaks);
> > 
> > 		current->nr_dirtied += nr_pages_dirtied;
> > 	}
> 
> Thanks,
> Fengguang
> 
> > >  	if (unlikely(current->nr_dirtied >= ratelimit))
> > >  		balance_dirty_pages(mapping, current->nr_dirtied);
> > >  }
> > > --- linux-next.orig/kernel/exit.c	2011-08-08 21:43:37.000000000 +0800
> > > +++ linux-next/kernel/exit.c	2011-08-08 21:45:58.000000000 +0800
> > > @@ -1039,6 +1039,8 @@ NORET_TYPE void do_exit(long code)
> > >  	validate_creds_for_do_exit(tsk);
> > >  
> > >  	preempt_disable();
> > > +	if (tsk->nr_dirtied)
> > > +		__this_cpu_add(dirty_leaks, tsk->nr_dirtied);
> > >  	exit_rcu();
> > >  	/* causes final put_task_struct in finish_task_switch(). */
> > >  	tsk->state = TASK_DEAD;

> Subject: writeback: fix dirtied pages accounting on sub-page writes
> Date: Thu Apr 14 07:52:37 CST 2011
> 
> When dd in 512bytes, generic_perform_write() calls
> balance_dirty_pages_ratelimited() 8 times for the same page, but
> obviously the page is only dirtied once.
> 
> Fix it by accounting nr_dirtied at page dirty time.
> 
> This will allow further simplification of the
> balance_dirty_pages_ratelimited_nr() calls.
> 
> Signed-off-by: Wu Fengguang <fengguang.wu@...el.com>
> ---
>  mm/page-writeback.c |    3 +--
>  1 file changed, 1 insertion(+), 2 deletions(-)
> 
> --- linux-next.orig/mm/page-writeback.c	2011-08-15 22:12:14.000000000 +0800
> +++ linux-next/mm/page-writeback.c	2011-08-15 22:12:27.000000000 +0800
> @@ -1211,8 +1211,6 @@ void balance_dirty_pages_ratelimited_nr(
>  	else
>  		ratelimit = min(ratelimit, 32 >> (PAGE_SHIFT - 10));
>  
> -	current->nr_dirtied += nr_pages_dirtied;
> -
>  	preempt_disable();
>  	/*
>  	 * This prevents one CPU to accumulate too many dirtied pages without
> @@ -1711,6 +1709,7 @@ void account_page_dirtied(struct page *p
>  		__inc_bdi_stat(mapping->backing_dev_info, BDI_DIRTIED);
>  		task_dirty_inc(current);
>  		task_io_account_write(PAGE_CACHE_SIZE);
> +		current->nr_dirtied++;
>  	}
>  }
>  EXPORT_SYMBOL(account_page_dirtied);
--
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