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

Andrea Righi
2011-08-15T14:26:46Z

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@intel.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);