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Date: Wed, 21 Jun 2017 10:25:37 +0300
From: Nikolay Borisov <nborisov@...e.com>
To: tj@...nel.org
Cc: jbacik@...com, linux-kernel@...r.kernel.org,
mgorman@...hsingularity.net, Nikolay Borisov <nborisov@...e.com>
Subject: [PATCH v3] writeback: Rework wb_[dec|inc]_stat family of functions
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.
Signed-off-by: Nikolay Borisov <nborisov@...e.com>
---
Changes since v2:
* Fixed build failure reported by kbuild test robot
* Explicitly document that percpu_counter_add_batch is preempt/irq safe
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(-)
diff --git a/fs/fs-writeback.c b/fs/fs-writeback.c
index 63ee2940775c..309364aab2a5 100644
--- a/fs/fs-writeback.c
+++ b/fs/fs-writeback.c
@@ -380,8 +380,8 @@ static void inode_switch_wbs_work_fn(struct work_struct *work)
struct page *page = radix_tree_deref_slot_protected(slot,
&mapping->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(struct work_struct *work)
&mapping->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);
}
}
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/lib/percpu_counter.c b/lib/percpu_counter.c
index 8ee7e5ec21be..3bf4a9984f4c 100644
--- a/lib/percpu_counter.c
+++ b/lib/percpu_counter.c
@@ -72,6 +72,13 @@ void percpu_counter_set(struct percpu_counter *fbc, s64 amount)
}
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;
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
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