[PATCH] Cgroup: Fix memory accounting scalability in shrink_page_list

[Tim Chen <tim.c.chen@linux.intel.com>]

Hi,

I noticed in a multi-process parallel files reading benchmark I ran on a
8 socket machine,  throughput slowed down by a factor of 8 when I ran
the benchmark within a cgroup container.  I traced the problem to the
following code path (see below) when we are trying to reclaim memory
from file cache.  The res_counter_uncharge function is called on every
page that's reclaimed and created heavy lock contention.  The patch
below allows the reclaimed pages to be uncharged from the resource
counter in batch and recovered the regression. 

Tim

     40.67%           usemem  [kernel.kallsyms]                   [k] _raw_spin_lock
                      |
                      --- _raw_spin_lock
                         |
                         |--92.61%-- res_counter_uncharge
                         |          |
                         |          |--100.00%-- __mem_cgroup_uncharge_common
                         |          |          |
                         |          |          |--100.00%-- mem_cgroup_uncharge_cache_page
                         |          |          |          __remove_mapping
                         |          |          |          shrink_page_list
                         |          |          |          shrink_inactive_list
                         |          |          |          shrink_mem_cgroup_zone
                         |          |          |          shrink_zone
                         |          |          |          do_try_to_free_pages
                         |          |          |          try_to_free_pages
                         |          |          |          __alloc_pages_nodemask
                         |          |          |          alloc_pages_current


---
Signed-off-by: Tim Chen <tim.c.chen@linux.intel.com>
diff --git a/mm/vmscan.c b/mm/vmscan.c
index 33dc256..aac5672 100644
--- a/mm/vmscan.c
+++ b/mm/vmscan.c
@@ -779,6 +779,7 @@ static unsigned long shrink_page_list(struct list_head *page_list,
 
 	cond_resched();
 
+	mem_cgroup_uncharge_start();
 	while (!list_empty(page_list)) {
 		enum page_references references;
 		struct address_space *mapping;
@@ -1026,6 +1027,7 @@ keep_lumpy:
 
 	list_splice(&ret_pages, page_list);
 	count_vm_events(PGACTIVATE, pgactivate);
+	mem_cgroup_uncharge_end();
 	*ret_nr_dirty += nr_dirty;
 	*ret_nr_writeback += nr_writeback;
 	return nr_reclaimed;


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