This is a note to let you know that I've just added the patch titled
writeback: rework wb_[dec|inc]_stat family of functions
to the 4.12-stable tree which can be found at:
http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary
The filename of the patch is:
writeback-rework-wb__stat-family-of-functions.patch
and it can be found in the queue-4.12 subdirectory.
If you, or anyone else, feels it should not be added to the stable tree,
please let <stable@xxxxxxxxxxxxxxx> know about it.
>From 3e8f399da490e6ac20a3cfd6aa404c9aa961a9a2 Mon Sep 17 00:00:00 2001
From: Nikolay Borisov <nborisov@xxxxxxxx>
Date: Wed, 12 Jul 2017 14:37:51 -0700
Subject: writeback: rework wb_[dec|inc]_stat family of functions
From: Nikolay Borisov <nborisov@xxxxxxxx>
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-nborisov@xxxxxxxx
Signed-off-by: Nikolay Borisov <nborisov@xxxxxxxx>
Acked-by: Tejun Heo <tj@xxxxxxxxxx>
Reviewed-by: Jan Kara <jack@xxxxxxx>
Cc: Josef Bacik <jbacik@xxxxxx>
Cc: Mel Gorman <mgorman@xxxxxxxxxxxxxxxxxxx>
Cc: Jeff Layton <jlayton@xxxxxxxxxx>
Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
Signed-off-by: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
Signed-off-by: Mel Gorman <mgorman@xxxxxxxxxxxxxxxxxxx>
Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>
---
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,
&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(str
&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);
}
}
--- 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(&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)
--- 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(&global_wb_domain, &wb->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, 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 pag
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 pag
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
Patches currently in stable-queue which might be from nborisov@xxxxxxxx are
queue-4.12/percpu_counter-rename-__percpu_counter_add-to-percpu_counter_add_batch.patch
queue-4.12/writeback-rework-wb__stat-family-of-functions.patch