On Wed, Dec 07, 2011 at 06:53:40PM +0800, Jan Kara wrote:
> On Mon 28-11-11 21:53:41, 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.
> Actually, for ppc where pages can be 64 KB the problem is even worse.
Ah yes.
> > Fix it by accounting tsk->nr_dirtied and bdp_ratelimits at page dirty time.
> I was wondering about one more thing - couldn't we rather check in
> generic_perform_write() whether the page was dirty before calling
> ->write_end and call balance_dirty_pages_ratelimited() only if it wasn't?
Cough.. the very original version does that exactly, then you raised
some concern here:
https://lkml.org/lkml/2011/4/13/554
The discussion goes on and eventually I get to the current version
that looks most acceptable in the three options.
> For generic_perform_write() it doesn't really matter that much since we
> do things page-by-page anyway but other callers could be more efficient...
That's right.
Thanks,
Fengguang
> > Acked-by: Peter Zijlstra <a.p.zijlstra@xxxxxxxxx>
> > 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 2011-11-28 21:23:20.000000000 +0800
> > +++ linux-next/mm/page-writeback.c 2011-11-28 21:23:23.000000000 +0800
> > @@ -1239,8 +1239,6 @@ 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
> > @@ -1251,12 +1249,9 @@ 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
> > @@ -1749,6 +1744,8 @@ void account_page_dirtied(struct page *p
> > __inc_bdi_stat(mapping->backing_dev_info, BDI_RECLAIMABLE);
> > __inc_bdi_stat(mapping->backing_dev_info, BDI_DIRTIED);
> > task_io_account_write(PAGE_CACHE_SIZE);
> > + current->nr_dirtied++;
> > + this_cpu_inc(bdp_ratelimits);
> > }
> > }
> > EXPORT_SYMBOL(account_page_dirtied);
> >
> >
> --
> Jan Kara <jack@xxxxxxx>
> SUSE Labs, CR
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