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Date: Thu, 27 May 2010 18:21:33 +0300
From: Artem Bityutskiy <dedekind1@...il.com>
To: Nick Piggin <npiggin@...e.de>
Cc: Al Viro <viro@...IV.linux.org.uk>,
LKML <linux-kernel@...r.kernel.org>,
Jens Axboe <jens.axboe@...cle.com>,
linux-fsdevel@...r.kernel.org
Subject: Re: [PATCHv4 17/17] writeback: lessen sync_supers wakeup count
On Thu, 2010-05-27 at 22:07 +1000, Nick Piggin wrote:
> 1. sb->s_dirty = 1; /* store */
> 2. if (!supers_timer_armed) /* load */
> 3. supers_timer_armed = 1; /* store */
>
> and
>
> A. supers_timer_armed = 0; /* store */
> B. if (sb->s_dirty) /* load */
> C. sb->s_dirty = 0 /* store */
>
> If these two sequences are executed, it must result in
> sb->s_dirty == 1 iff supers_timer_armed
>
> * If 2 is executed before 1 is visible, then 2 may miss A before B sees 1.
> * If B is executed before A is visible, then B may miss 1 before 2 sees A.
>
> So we need smp_mb() between 1/2 and A/B (I missed the 2nd one).
Yes, thanks for elaboration.
> How about something like this?
It looks good, many thanks! But I have few small notes.
> Index: linux-2.6/mm/backing-dev.c
> ===================================================================
> --- linux-2.6.orig/mm/backing-dev.c
> +++ linux-2.6/mm/backing-dev.c
> @@ -45,6 +45,7 @@ LIST_HEAD(bdi_pending_list);
>
> static struct task_struct *sync_supers_tsk;
> static struct timer_list sync_supers_timer;
> +static unsigned long supers_dirty __read_mostly;
>
> static int bdi_sync_supers(void *);
> static void sync_supers_timer_fn(unsigned long);
> @@ -251,7 +252,6 @@ static int __init default_bdi_init(void)
>
> init_timer(&sync_supers_timer);
> setup_timer(&sync_supers_timer, sync_supers_timer_fn, 0);
> - bdi_arm_supers_timer();
>
> err = bdi_init(&default_backing_dev_info);
> if (!err)
> @@ -362,17 +362,28 @@ static int bdi_sync_supers(void *unused)
>
> while (!kthread_should_stop()) {
> set_current_state(TASK_INTERRUPTIBLE);
> - schedule();
> + if (!supers_dirty)
> + schedule();
> + else
> + __set_current_state(TASK_RUNNING);
I think this will change the behavior of 'sync_supers()' too much. ATM,
it makes only one SB pass, then sleeps, then another one, then sleeps.
And we should probably preserve this behavior. So I'd rather make it:
if (supers_dirty)
bdi_arm_supers_timer();
set_current_state(TASK_INTERRUPTIBLE);
schedule();
This way we will keep the behavior closer to the original.
> + supers_dirty = 0;
> /*
> - * Do this periodically, like kupdated() did before.
> + * supers_dirty store must be visible to mark_sb_dirty (below)
> + * before sync_supers runs (which loads sb->s_dirty).
> */
Very minor, but the code tends to change quickly, and this note (below)
will probably become out-of-date soon.
> + smp_mb();
There is spin_lock(&sb_lock) in sync_supers(), so strictly speak this
'smp_mb()' is not needed if we move supers_dirty = 0 into
'sync_supers()' and add a comment that a mb is required, in case some
one modifies the code later?
Or this is not worth it?
> sync_supers();
> }
>
> return 0;
> }
>
> +static void sync_supers_timer_fn(unsigned long unused)
> +{
> + wake_up_process(sync_supers_tsk);
> +}
> +
> void bdi_arm_supers_timer(void)
> {
> unsigned long next;
> @@ -384,9 +395,17 @@ void bdi_arm_supers_timer(void)
> mod_timer(&sync_supers_timer, round_jiffies_up(next));
> }
>
> -static void sync_supers_timer_fn(unsigned long unused)
> +void mark_sb_dirty(struct super_block *sb)
> {
> - wake_up_process(sync_supers_tsk);
> + sb->s_dirty = 1;
> + /*
> + * sb->s_dirty store must be visible to sync_supers (above) before we
> + * load supers_dirty in case we need to re-arm the timer.
> + */
Similar for the "(above)" note.
> + smp_mb();
> + if (likely(supers_dirty))
> + return;
> + supers_dirty = 1;
> bdi_arm_supers_timer();
> }
Here is the with my modifications.
BTW, do you want me to keep you to be the patch author, add your
signed-off-by and my original commit message?
---
fs/super.c | 7 +++++++
mm/backing-dev.c | 21 ++++++++++++++++++---
2 files changed, 25 insertions(+), 3 deletions(-)
diff --git a/fs/super.c b/fs/super.c
index 2b418fb..c9ff6e2 100644
--- a/fs/super.c
+++ b/fs/super.c
@@ -364,6 +364,13 @@ void sync_supers(void)
{
struct super_block *sb, *n;
+ supers_dirty = 0;
+ /* smp_mb();
+ *
+ * supers_dirty store must be visible to mark_sb_dirty before
+ * sync_supers runs (which loads sb->s_dirty), so a barrier is needed
+ * but there is a spin_lock, thus smp_mb is commented out.
+ */
spin_lock(&sb_lock);
list_for_each_entry_safe(sb, n, &super_blocks, s_list) {
if (list_empty(&sb->s_instances))
diff --git a/mm/backing-dev.c b/mm/backing-dev.c
index 660a87a..be7f734 100644
--- a/mm/backing-dev.c
+++ b/mm/backing-dev.c
@@ -45,6 +45,7 @@ LIST_HEAD(bdi_pending_list);
static struct task_struct *sync_supers_tsk;
static struct timer_list sync_supers_timer;
+static unsigned long supers_dirty __read_mostly;
static int bdi_sync_supers(void *);
static void sync_supers_timer_fn(unsigned long);
@@ -251,7 +252,6 @@ static int __init default_bdi_init(void)
init_timer(&sync_supers_timer);
setup_timer(&sync_supers_timer, sync_supers_timer_fn, 0);
- bdi_arm_supers_timer();
err = bdi_init(&default_backing_dev_info);
if (!err)
@@ -361,6 +361,8 @@ static int bdi_sync_supers(void *unused)
set_user_nice(current, 0);
while (!kthread_should_stop()) {
+ if (supers_dirty)
+ bdi_arm_supers_timer();
set_current_state(TASK_INTERRUPTIBLE);
schedule();
@@ -373,6 +375,11 @@ static int bdi_sync_supers(void *unused)
return 0;
}
+static void sync_supers_timer_fn(unsigned long unused)
+{
+ wake_up_process(sync_supers_tsk);
+}
+
void bdi_arm_supers_timer(void)
{
unsigned long next;
@@ -384,9 +391,17 @@ void bdi_arm_supers_timer(void)
mod_timer(&sync_supers_timer, round_jiffies_up(next));
}
-static void sync_supers_timer_fn(unsigned long unused)
+void mark_sb_dirty(struct super_block *sb)
{
- wake_up_process(sync_supers_tsk);
+ sb->s_dirty = 1;
+ /*
+ * sb->s_dirty store must be visible to sync_supers before we load
+ * supers_dirty in case we need to re-arm the timer.
+ */
+ smp_mb();
+ if (likely(supers_dirty))
+ return;
+ supers_dirty = 1;
bdi_arm_supers_timer();
}
--
--
Best Regards,
Artem Bityutskiy (Артём Битюцкий)
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