[PATCH 2/2] writeback: Rework wb_[dec|inc]_stat family of functions

[Nikolay Borisov <nborisov@suse.com>]

Currently the writeback statistics code uses a percpu counters to hold
various statistics. As such we have 2 families of functions - those which
disable local irq and those which doesn't and whose names are begin with
double underscore. However, they both end up calling __add_wb_stats which in
turn end up calling percpu_counter_add_batch which is already SMP-safe.

Exploiting this fact allows to eliminated the __wb_* functions since they do
in fact cal SMP-safe primitive. Furthermore, refactor the wb_* function
to call __add_wb_stat directly without the irq-disabling dance. This will
likely result in better runtime of code which deals with modifying the stat
counters.

Signed-off-by: Nikolay Borisov <nborisov@suse.com>
---
Hello Tejun, 

This patch resulted from me reading your feedback on Josef's memory 
throttling prep patch https://patchwork.kernel.org/patch/9395219/ . If these
changes are merged then his series can eliminated irq clustering and use 
straight __add_wb_stat call. I'd like to see his series merged sooner rather
than later hence why sending this cleanup. 

 include/linux/backing-dev.h | 24 ++----------------------
 mm/page-writeback.c         | 10 +++++-----
 2 files changed, 7 insertions(+), 27 deletions(-)

diff --git a/include/linux/backing-dev.h b/include/linux/backing-dev.h
index ace73f96eb1e..e9c967b86054 100644
--- a/include/linux/backing-dev.h
+++ b/include/linux/backing-dev.h
@@ -69,34 +69,14 @@ static inline void __add_wb_stat(struct bdi_writeback *wb,
 	percpu_counter_add_batch(&wb->stat[item], amount, WB_STAT_BATCH);
 }
 
-static inline void __inc_wb_stat(struct bdi_writeback *wb,
-				 enum wb_stat_item item)
-{
-	__add_wb_stat(wb, item, 1);
-}
-
 static inline void inc_wb_stat(struct bdi_writeback *wb, enum wb_stat_item item)
 {
-	unsigned long flags;
-
-	local_irq_save(flags);
-	__inc_wb_stat(wb, item);
-	local_irq_restore(flags);
-}
-
-static inline void __dec_wb_stat(struct bdi_writeback *wb,
-				 enum wb_stat_item item)
-{
-	__add_wb_stat(wb, item, -1);
+	__add_wb_stat(wb, item, 1);
 }
 
 static inline void dec_wb_stat(struct bdi_writeback *wb, enum wb_stat_item item)
 {
-	unsigned long flags;
-
-	local_irq_save(flags);
-	__dec_wb_stat(wb, item);
-	local_irq_restore(flags);
+	__add_wb_stat(wb, item, -1);
 }
 
 static inline s64 wb_stat(struct bdi_writeback *wb, enum wb_stat_item item)
diff --git a/mm/page-writeback.c b/mm/page-writeback.c
index 143c1c25d680..b7451891959a 100644
--- a/mm/page-writeback.c
+++ b/mm/page-writeback.c
@@ -601,7 +601,7 @@ static inline void __wb_writeout_inc(struct bdi_writeback *wb)
 {
 	struct wb_domain *cgdom;
 
-	__inc_wb_stat(wb, WB_WRITTEN);
+	inc_wb_stat(wb, WB_WRITTEN);
 	wb_domain_writeout_inc(&global_wb_domain, &wb->completions,
 			       wb->bdi->max_prop_frac);
 
@@ -2437,8 +2437,8 @@ void account_page_dirtied(struct page *page, struct address_space *mapping)
 		__inc_node_page_state(page, NR_FILE_DIRTY);
 		__inc_zone_page_state(page, NR_ZONE_WRITE_PENDING);
 		__inc_node_page_state(page, NR_DIRTIED);
-		__inc_wb_stat(wb, WB_RECLAIMABLE);
-		__inc_wb_stat(wb, WB_DIRTIED);
+		inc_wb_stat(wb, WB_RECLAIMABLE);
+		inc_wb_stat(wb, WB_DIRTIED);
 		task_io_account_write(PAGE_SIZE);
 		current->nr_dirtied++;
 		this_cpu_inc(bdp_ratelimits);
@@ -2745,7 +2745,7 @@ int test_clear_page_writeback(struct page *page)
 			if (bdi_cap_account_writeback(bdi)) {
 				struct bdi_writeback *wb = inode_to_wb(inode);
 
-				__dec_wb_stat(wb, WB_WRITEBACK);
+				dec_wb_stat(wb, WB_WRITEBACK);
 				__wb_writeout_inc(wb);
 			}
 		}
@@ -2791,7 +2791,7 @@ int __test_set_page_writeback(struct page *page, bool keep_write)
 						page_index(page),
 						PAGECACHE_TAG_WRITEBACK);
 			if (bdi_cap_account_writeback(bdi))
-				__inc_wb_stat(inode_to_wb(inode), WB_WRITEBACK);
+				inc_wb_stat(inode_to_wb(inode), WB_WRITEBACK);
 
 			/*
 			 * We can come through here when swapping anonymous
-- 
2.7.4