Messages in this thread | | | Date | Fri, 24 Apr 2009 12:18:50 +0100 | From | Mel Gorman <> | Subject | Re: [PATCH 15/22] Do not disable interrupts in free_page_mlock() |
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On Thu, Apr 23, 2009 at 03:59:51PM -0700, Andrew Morton wrote: > On Wed, 22 Apr 2009 14:53:20 +0100 > Mel Gorman <mel@csn.ul.ie> wrote: > > > free_page_mlock() tests and clears PG_mlocked using locked versions of the > > bit operations. If set, it disables interrupts to update counters and this > > happens on every page free even though interrupts are disabled very shortly > > afterwards a second time. This is wasteful. > > Well. It's only wasteful if the page was mlocked, which is rare. >
True. mlocked pages are only going to be torn down during process exit or munmap, both of which you'd expect to be rare when mlock is involved.
s/This is wasteful/While rare, this is unnecessary./
?
> > This patch splits what free_page_mlock() does. The bit check is still > > made. However, the update of counters is delayed until the interrupts are > > disabled and the non-lock version for clearing the bit is used. One potential > > weirdness with this split is that the counters do not get updated if the > > bad_page() check is triggered but a system showing bad pages is getting > > screwed already. > > > > Signed-off-by: Mel Gorman <mel@csn.ul.ie> > > Reviewed-by: Christoph Lameter <cl@linux-foundation.org> > > Reviewed-by: Pekka Enberg <penberg@cs.helsinki.fi> > > Reviewed-by: KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com> > > --- > > mm/internal.h | 11 +++-------- > > mm/page_alloc.c | 8 +++++++- > > 2 files changed, 10 insertions(+), 9 deletions(-) > > > > diff --git a/mm/internal.h b/mm/internal.h > > index 987bb03..58ec1bc 100644 > > --- a/mm/internal.h > > +++ b/mm/internal.h > > @@ -157,14 +157,9 @@ static inline void mlock_migrate_page(struct page *newpage, struct page *page) > > */ > > static inline void free_page_mlock(struct page *page) > > { > > - if (unlikely(TestClearPageMlocked(page))) { > > - unsigned long flags; > > - > > - local_irq_save(flags); > > - __dec_zone_page_state(page, NR_MLOCK); > > - __count_vm_event(UNEVICTABLE_MLOCKFREED); > > - local_irq_restore(flags); > > - } > > + __ClearPageMlocked(page); > > + __dec_zone_page_state(page, NR_MLOCK); > > + __count_vm_event(UNEVICTABLE_MLOCKFREED); > > } > > The conscientuous reviewer runs around and checks for free_page_mlock() > callers in other .c files which might be affected. >
The patch author should have done the same thing :/
> Only there are no such callers. > > The reviewer's job would be reduced if free_page_mlock() wasn't > needlessly placed in a header file! > > > #else /* CONFIG_HAVE_MLOCKED_PAGE_BIT */ > > diff --git a/mm/page_alloc.c b/mm/page_alloc.c > > index 67cafd0..7f45de1 100644 > > --- a/mm/page_alloc.c > > +++ b/mm/page_alloc.c > > @@ -499,7 +499,6 @@ static inline void __free_one_page(struct page *page, > > > > static inline int free_pages_check(struct page *page) > > { > > - free_page_mlock(page); > > if (unlikely(page_mapcount(page) | > > (page->mapping != NULL) | > > (page_count(page) != 0) | > > @@ -556,6 +555,7 @@ static void __free_pages_ok(struct page *page, unsigned int order) > > unsigned long flags; > > int i; > > int bad = 0; > > + int clearMlocked = PageMlocked(page); > > > > for (i = 0 ; i < (1 << order) ; ++i) > > bad += free_pages_check(page + i); > > @@ -571,6 +571,8 @@ static void __free_pages_ok(struct page *page, unsigned int order) > > kernel_map_pages(page, 1 << order, 0); > > > > local_irq_save(flags); > > + if (unlikely(clearMlocked)) > > + free_page_mlock(page); > > I wonder what the compiler does in the case > CONFIG_HAVE_MLOCKED_PAGE_BIT=n. If it is dumb, this patch would cause > additional code generation. >
PageMlocked becomes
static inline int PageMlocked(page) { return 0; }
so the compiler should be fit to spot that clearMlocked will always be 0. Even if it didn't, it should have at least spotted that free_page_mlock(page) is an empty inline function in this case.
Still double checked and I did not see the branch with CONFIG_HAVE_MLOCKED_PAGE_BIT=n
-- Mel Gorman Part-time Phd Student Linux Technology Center University of Limerick IBM Dublin Software Lab
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