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

[Wu Fengguang]

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 :)

> 	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@xxxxxxxxx>
---
 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);