[patch 1/3] mm: memcontrol: do not kill uncharge batching in free_pages_and_swap_cache

Johannes Weiner
2014-09-24T15:08:56Z

From: Michal Hocko <mhocko@suse.cz>

free_pages_and_swap_cache limits release_pages to PAGEVEC_SIZE chunks.
This is not a big deal for the normal release path but it completely
kills memcg uncharge batching which reduces res_counter spin_lock
contention. Dave has noticed this with his page fault scalability test
case on a large machine when the lock was basically dominating on all
CPUs:
80.18% 80.18% [kernel] [k] _raw_spin_lock
|
--- _raw_spin_lock
|
|--66.59%-- res_counter_uncharge_until
| res_counter_uncharge
| uncharge_batch
| uncharge_list
| mem_cgroup_uncharge_list
| release_pages
| free_pages_and_swap_cache
| tlb_flush_mmu_free
| |
| |--90.12%-- unmap_single_vma
| | unmap_vmas
| | unmap_region
| | do_munmap
| | vm_munmap
| | sys_munmap
| | system_call_fastpath
| | __GI___munmap
| |
| --9.88%-- tlb_flush_mmu
| tlb_finish_mmu
| unmap_region
| do_munmap
| vm_munmap
| sys_munmap
| system_call_fastpath
| __GI___munmap

In his case the load was running in the root memcg and that part
has been handled by reverting 05b843012335 ("mm: memcontrol: use
root_mem_cgroup res_counter") because this is a clear regression,
but the problem remains inside dedicated memcgs.

There is no reason to limit release_pages to PAGEVEC_SIZE batches other
than lru_lock held times. This logic, however, can be moved inside the
function. mem_cgroup_uncharge_list and free_hot_cold_page_list do not
hold any lock for the whole pages_to_free list so it is safe to call
them in a single run.

Page reference count and LRU handling is moved to release_lru_pages and
that is run in PAGEVEC_SIZE batches.

Reported-by: Dave Hansen <dave@sr71.net>
Signed-off-by: Michal Hocko <mhocko@suse.cz>
Signed-off-by: Johannes Weiner <hannes@cmpxchg.org>
---
mm/swap.c | 27 +++++++++++++++++++++------
mm/swap_state.c | 14 ++++----------
2 files changed, 25 insertions(+), 16 deletions(-)

diff --git a/mm/swap.c b/mm/swap.c
index 6b2dc3897cd5..8af99dd68dd2 100644
--- a/mm/swap.c
+++ b/mm/swap.c
@@ -888,9 +888,9 @@ void lru_add_drain_all(void)
}

/*
- * Batched page_cache_release(). Decrement the reference count on all the
- * passed pages. If it fell to zero then remove the page from the LRU and
- * free it.
+ * Batched lru release. Decrement the reference count on all the passed pages.
+ * If it fell to zero then remove the page from the LRU and add it to the given
+ * list to be freed by the caller.
*
* Avoid taking zone->lru_lock if possible, but if it is taken, retain it
* for the remainder of the operation.
@@ -900,10 +900,10 @@ void lru_add_drain_all(void)
* grabbed the page via the LRU. If it did, give up: shrink_inactive_list()
* will free it.
*/
-void release_pages(struct page **pages, int nr, bool cold)
+static void release_lru_pages(struct page **pages, int nr,
+ struct list_head *pages_to_free)
{
int i;
- LIST_HEAD(pages_to_free);
struct zone *zone = NULL;
struct lruvec *lruvec;
unsigned long uninitialized_var(flags);
@@ -943,11 +943,26 @@ void release_pages(struct page **pages, int nr, bool cold)
/* Clear Active bit in case of parallel mark_page_accessed */
__ClearPageActive(page);

- list_add(&page->lru, &pages_to_free);
+ list_add(&page->lru, pages_to_free);
}
if (zone)
spin_unlock_irqrestore(&zone->lru_lock, flags);
+}
+/*
+ * Batched page_cache_release(). Frees and uncharges all given pages
+ * for which the reference count drops to 0.
+ */
+void release_pages(struct page **pages, int nr, bool cold)
+{
+ LIST_HEAD(pages_to_free);

+ while (nr) {
+ int batch = min(nr, PAGEVEC_SIZE);
+
+ release_lru_pages(pages, batch, &pages_to_free);
+ pages += batch;
+ nr -= batch;
+ }
mem_cgroup_uncharge_list(&pages_to_free);
free_hot_cold_page_list(&pages_to_free, cold);
}
diff --git a/mm/swap_state.c b/mm/swap_state.c
index ef1f39139b71..154444918685 100644
--- a/mm/swap_state.c
+++ b/mm/swap_state.c
@@ -265,18 +265,12 @@ void free_page_and_swap_cache(struct page *page)
void free_pages_and_swap_cache(struct page **pages, int nr)
{
struct page **pagep = pages;
+ int i;

lru_add_drain();
- while (nr) {
- int todo = min(nr, PAGEVEC_SIZE);
- int i;
-
- for (i = 0; i < todo; i++)
- free_swap_cache(pagep[i]);
- release_pages(pagep, todo, false);
- pagep += todo;
- nr -= todo;
- }
+ for (i = 0; i < nr; i++)
+ free_swap_cache(pagep[i]);
+ release_pages(pagep, nr, false);
}

/*
--
2.1.0