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

Wu Fengguang
2011-08-15T14:21:49Z

[-- Attachment #1: Type: text/plain, Size: 1759 bytes --]

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;

[-- Attachment #2: writeback-accurate-task-dirtied.patch --]
[-- Type: text/x-diff, Size: 1226 bytes --]

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);