On Tue 22-11-11 17:21:11, Wu Fengguang wrote:
> On Tue, Nov 22, 2011 at 08:11:27AM +0800, Jan Kara wrote:
> > On Mon 21-11-11 21:03:45, Wu Fengguang wrote:
> > > 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.
> > Well, but after this change, the interface balance_dirty_ratelimited_nr()
> > is strange because the argument is only used for per-CPU ratelimiting and
> > not for per-task ratelimiting...
>
> Yeah I was vaguely aware of this... and still choose to ignore this
> since the patchset looked already forbiddingly large at the time ;)
>
> > So if you do this switch then I'd also
> > switch bdp_ratelimits to get consistent results and a clean interface and
> > completely kill balance_dirty_pages_ratelimited_nr().
>
> Following your suggestions to change ratelimiting as well :)
>
> I'll do the interface change with a standalone patch.
OK.
> ---
> 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 tsk->nr_dirtied and bdp_ratelimits at page dirty time.
>
> Signed-off-by: Wu Fengguang <fengguang.wu@xxxxxxxxx>
> ---
> mm/page-writeback.c | 13 +++++--------
> 1 file changed, 5 insertions(+), 8 deletions(-)
>
> --- linux-next.orig/mm/page-writeback.c 2011-11-22 16:59:48.000000000 +0800
> +++ linux-next/mm/page-writeback.c 2011-11-22 17:12:20.000000000 +0800
> @@ -1231,8 +1231,6 @@ void balance_dirty_pages_ratelimited_nr(
> if (bdi->dirty_exceeded)
> 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
> @@ -1243,12 +1241,9 @@ void balance_dirty_pages_ratelimited_nr(
> p = &__get_cpu_var(bdp_ratelimits);
> if (unlikely(current->nr_dirtied >= ratelimit))
> *p = 0;
> - else {
> - *p += nr_pages_dirtied;
> - if (unlikely(*p >= ratelimit_pages)) {
> - *p = 0;
> - ratelimit = 0;
> - }
> + else if (unlikely(*p >= ratelimit_pages)) {
> + *p = 0;
> + ratelimit = 0;
> }
> /*
> * Pick up the dirtied pages by the exited tasks. This avoids lots of
> @@ -1743,6 +1738,8 @@ 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++;
> + __get_cpu_var(bdp_ratelimits)++;
I think you need preempt_disable() and preempt_enable() pair around
__get_cpu_var(). Otherwise a process could get rescheduled in the middle of
read-modify-write cycle...
Honza
--
Jan Kara <jack@xxxxxxx>
SUSE Labs, CR
--
To unsubscribe from this list: send the line "unsubscribe linux-fsdevel" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at http://vger.kernel.org/majordomo-info.html
