[PATCH 4.12 181/196] writeback: rework wb_[dec|inc]_stat family of functions

[Greg Kroah-Hartman]

4.12-stable review patch.  If anyone has any objections, please let me know.

--

From: Nikolay Borisov 

commit 3e8f399da490e6ac20a3cfd6aa404c9aa961a9a2 upstream.

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

Exploiting this fact allows to eliminated the __wb_* functions since
they don't add any further protection than we already have.
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.

While at it also document why percpu_counter_add_batch is in fact
preempt and irq-safe since at least 3 people got confused.

Link: 
http://lkml.kernel.org/r/1498029937-27293-1-git-send-email-nbori...@suse.com
Signed-off-by: Nikolay Borisov 
Acked-by: Tejun Heo 
Reviewed-by: Jan Kara 
Cc: Josef Bacik 
Cc: Mel Gorman 
Cc: Jeff Layton 
Signed-off-by: Andrew Morton 
Signed-off-by: Linus Torvalds 
Signed-off-by: Mel Gorman 
Signed-off-by: Greg Kroah-Hartman 

---
 fs/fs-writeback.c   |8 
 include/linux/backing-dev.h |   24 ++--
 lib/percpu_counter.c|7 +++
 mm/page-writeback.c |   10 +-
 4 files changed, 18 insertions(+), 31 deletions(-)

--- a/fs/fs-writeback.c
+++ b/fs/fs-writeback.c
@@ -380,8 +380,8 @@ static void inode_switch_wbs_work_fn(str
struct page *page = radix_tree_deref_slot_protected(slot,
>tree_lock);
if (likely(page) && PageDirty(page)) {
-   __dec_wb_stat(old_wb, WB_RECLAIMABLE);
-   __inc_wb_stat(new_wb, WB_RECLAIMABLE);
+   dec_wb_stat(old_wb, WB_RECLAIMABLE);
+   inc_wb_stat(new_wb, WB_RECLAIMABLE);
}
}
 
@@ -391,8 +391,8 @@ static void inode_switch_wbs_work_fn(str
>tree_lock);
if (likely(page)) {
WARN_ON_ONCE(!PageWriteback(page));
-   __dec_wb_stat(old_wb, WB_WRITEBACK);
-   __inc_wb_stat(new_wb, WB_WRITEBACK);
+   dec_wb_stat(old_wb, WB_WRITEBACK);
+   inc_wb_stat(new_wb, WB_WRITEBACK);
}
}
 
--- a/include/linux/backing-dev.h
+++ b/include/linux/backing-dev.h
@@ -69,34 +69,14 @@ static inline void __add_wb_stat(struct
percpu_counter_add_batch(>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)
--- a/lib/percpu_counter.c
+++ b/lib/percpu_counter.c
@@ -72,6 +72,13 @@ void percpu_counter_set(struct percpu_co
 }
 EXPORT_SYMBOL(percpu_counter_set);
 
+/**
+ * This function is both preempt and irq safe. The former is due to explicit
+ * preemption disable. The latter is guaranteed by the fact that the slow path
+ * is explicitly protected by an irq-safe spinlock whereas the fast patch uses
+ * this_cpu_add which is irq-safe by definition. Hence there is no need muck
+ * with irq state before calling this one
+ */
 void percpu_counter_add_batch(struct percpu_counter *fbc, s64 amount, s32 
batch)
 {
s64 count;
--- a/mm/page-writeback.c
+++ b/mm/page-writeback.c
@@ -601,7 +601,7 @@ static inline void __wb_writeout_inc(str
 {
struct wb_domain *cgdom;
 
-   __inc_wb_stat(wb, WB_WRITTEN);
+   inc_wb_stat(wb, WB_WRITTEN);
wb_domain_writeout_inc(_wb_domain, >completions,
   

[PATCH 4.12 181/196] writeback: rework wb_[dec|inc]_stat family of functions

[Greg Kroah-Hartman]

4.12-stable review patch.  If anyone has any objections, please let me know.

--

From: Nikolay Borisov 

commit 3e8f399da490e6ac20a3cfd6aa404c9aa961a9a2 upstream.

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

Exploiting this fact allows to eliminated the __wb_* functions since
they don't add any further protection than we already have.
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.

While at it also document why percpu_counter_add_batch is in fact
preempt and irq-safe since at least 3 people got confused.

Link: 
http://lkml.kernel.org/r/1498029937-27293-1-git-send-email-nbori...@suse.com
Signed-off-by: Nikolay Borisov 
Acked-by: Tejun Heo 
Reviewed-by: Jan Kara 
Cc: Josef Bacik 
Cc: Mel Gorman 
Cc: Jeff Layton 
Signed-off-by: Andrew Morton 
Signed-off-by: Linus Torvalds 
Signed-off-by: Mel Gorman 
Signed-off-by: Greg Kroah-Hartman 

---
 fs/fs-writeback.c   |8 
 include/linux/backing-dev.h |   24 ++--
 lib/percpu_counter.c|7 +++
 mm/page-writeback.c |   10 +-
 4 files changed, 18 insertions(+), 31 deletions(-)

--- a/fs/fs-writeback.c
+++ b/fs/fs-writeback.c
@@ -380,8 +380,8 @@ static void inode_switch_wbs_work_fn(str
struct page *page = radix_tree_deref_slot_protected(slot,
>tree_lock);
if (likely(page) && PageDirty(page)) {
-   __dec_wb_stat(old_wb, WB_RECLAIMABLE);
-   __inc_wb_stat(new_wb, WB_RECLAIMABLE);
+   dec_wb_stat(old_wb, WB_RECLAIMABLE);
+   inc_wb_stat(new_wb, WB_RECLAIMABLE);
}
}
 
@@ -391,8 +391,8 @@ static void inode_switch_wbs_work_fn(str
>tree_lock);
if (likely(page)) {
WARN_ON_ONCE(!PageWriteback(page));
-   __dec_wb_stat(old_wb, WB_WRITEBACK);
-   __inc_wb_stat(new_wb, WB_WRITEBACK);
+   dec_wb_stat(old_wb, WB_WRITEBACK);
+   inc_wb_stat(new_wb, WB_WRITEBACK);
}
}
 
--- a/include/linux/backing-dev.h
+++ b/include/linux/backing-dev.h
@@ -69,34 +69,14 @@ static inline void __add_wb_stat(struct
percpu_counter_add_batch(>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)
--- a/lib/percpu_counter.c
+++ b/lib/percpu_counter.c
@@ -72,6 +72,13 @@ void percpu_counter_set(struct percpu_co
 }
 EXPORT_SYMBOL(percpu_counter_set);
 
+/**
+ * This function is both preempt and irq safe. The former is due to explicit
+ * preemption disable. The latter is guaranteed by the fact that the slow path
+ * is explicitly protected by an irq-safe spinlock whereas the fast patch uses
+ * this_cpu_add which is irq-safe by definition. Hence there is no need muck
+ * with irq state before calling this one
+ */
 void percpu_counter_add_batch(struct percpu_counter *fbc, s64 amount, s32 
batch)
 {
s64 count;
--- a/mm/page-writeback.c
+++ b/mm/page-writeback.c
@@ -601,7 +601,7 @@ static inline void __wb_writeout_inc(str
 {
struct wb_domain *cgdom;
 
-   __inc_wb_stat(wb, WB_WRITTEN);
+   inc_wb_stat(wb, WB_WRITTEN);
wb_domain_writeout_inc(_wb_domain, >completions,
   wb->bdi->max_prop_frac);
 
@@ -2437,8 +2437,8 @@ void account_page_dirtied(struct page *p
__inc_node_page_state(page, NR_FILE_DIRTY);
__inc_zone_page_state(page, NR_ZONE_WRITE_PENDING);
__inc_node_page_state(page,