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SubjectRe: [PATCH] mm: batch-free pcp list if possible
On Wed, Feb 09, 2011 at 12:38:03PM -0800, Andrew Morton wrote:
> On Wed, 9 Feb 2011 22:21:17 +0900
> Namhyung Kim <namhyung@gmail.com> wrote:
>
> > free_pcppages_bulk() frees pages from pcp lists in a round-robin
> > fashion by keeping batch_free counter. But it doesn't need to spin
> > if there is only one non-empty list. This can be checked by
> > batch_free == MIGRATE_PCPTYPES.
> >
> > Signed-off-by: Namhyung Kim <namhyung@gmail.com>
> > ---
> > mm/page_alloc.c | 4 ++++
> > 1 files changed, 4 insertions(+), 0 deletions(-)
> >
> > diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> > index a873e61e312e..470fb42e303c 100644
> > --- a/mm/page_alloc.c
> > +++ b/mm/page_alloc.c
> > @@ -614,6 +614,10 @@ static void free_pcppages_bulk(struct zone *zone, int count,
> > list = &pcp->lists[migratetype];
> > } while (list_empty(list));
> >
> > + /* This is an only non-empty list. Free them all. */
> > + if (batch_free == MIGRATE_PCPTYPES)
> > + batch_free = to_free;
> > +
> > do {
> > page = list_entry(list->prev, struct page, lru);
> > /* must delete as __free_one_page list manipulates */
>
> free_pcppages_bulk() hurts my brain.

Thanks for saying that ;-)

> What is it actually trying to do, and why? It counts up the number of
> contiguous empty lists and then frees that number of pages from the
> first-encountered non-empty list and then advances onto the next list?
>
> What's the point in that? What relationship does the number of
> contiguous empty lists have with the number of pages to free from one
> list?

It at least recovers some of the otherwise wasted effort of looking at
an empty list, by flushing more pages once it encounters a non-empty
list. After all, freeing to_free pages is the goal.

That breaks the round-robin fashion, though. If list-1 has pages,
list-2 is empty and list-3 has pages, it will repeatedly free one page
from list-1 and two pages from list-3.

My initial response to Namhyung's patch was to write up a version that
used a bitmap for all lists. It starts with all lists set and clears
their respective bit once the list is empty, so it would never
consider them again. But it looked a bit over-engineered for 3 lists
and the resulting object code was bigger than what we have now.
Though, it would be more readable. Attached for reference (untested
and all).

Hannes

diff --git a/mm/page_alloc.c b/mm/page_alloc.c
index 60e58b0..c77ab28 100644
--- a/mm/page_alloc.c
+++ b/mm/page_alloc.c
@@ -590,8 +590,7 @@ static inline int free_pages_check(struct page *page)
static void free_pcppages_bulk(struct zone *zone, int count,
struct per_cpu_pages *pcp)
{
- int migratetype = 0;
- int batch_free = 0;
+ unsigned long listmap = (1 << MIGRATE_PCPTYPES) - 1;
int to_free = count;

spin_lock(&zone->lock);
@@ -599,31 +598,29 @@ static void free_pcppages_bulk(struct zone *zone, int count,
zone->pages_scanned = 0;

while (to_free) {
- struct page *page;
- struct list_head *list;
-
+ int migratetype;
/*
- * Remove pages from lists in a round-robin fashion. A
- * batch_free count is maintained that is incremented when an
- * empty list is encountered. This is so more pages are freed
- * off fuller lists instead of spinning excessively around empty
- * lists
+ * Remove pages from lists in a round-robin fashion.
+ * Empty lists are excluded from subsequent rounds.
*/
- do {
- batch_free++;
- if (++migratetype == MIGRATE_PCPTYPES)
- migratetype = 0;
- list = &pcp->lists[migratetype];
- } while (list_empty(list));
+ for_each_set_bit (migratetype, &listmap, MIGRATE_PCPTYPES) {
+ struct list_head *list;
+ struct page *page;

- do {
+ list = &pcp->lists[migratetype];
+ if (list_empty(list)) {
+ listmap &= ~(1 << migratetype);
+ continue;
+ }
+ if (!to_free--)
+ break;
page = list_entry(list->prev, struct page, lru);
/* must delete as __free_one_page list manipulates */
list_del(&page->lru);
/* MIGRATE_MOVABLE list may include MIGRATE_RESERVEs */
__free_one_page(page, zone, 0, page_private(page));
trace_mm_page_pcpu_drain(page, 0, page_private(page));
- } while (--to_free && --batch_free && !list_empty(list));
+ }
}
__mod_zone_page_state(zone, NR_FREE_PAGES, count);
spin_unlock(&zone->lock);

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