From: Dave Hansen <dave.hansen@xxxxxxxxxxxxxxx>
If I sit in a loop and do write()s to small tmpfs files,
__sb_end_write() is third-hottest kernel function due to its
smp_mb().
__sb_end_write() uses the barrier to avoid races with freeze_super()
and its calls to sb_wait_write(). But, now that freeze_super() is
calling synchronize_rcu() before each sb_wait_write() call, we can
use that to our advantage.
The synchronize_rcu() ensures that all __sb_end_write() will see
freeze_super()'s updates to s_writers.counter. That, in turn,
guarantees that __sb_end_write() will try to wake up any subsequent
call by freeze_super() to sb_wait_write().
This improves the number of writes/second I can do by 6.1% on top
of the previous patch. The total improvement is 27.1% over a
completely unpatched kernel.
Signed-off-by: Dave Hansen <dave.hansen@xxxxxxxxxxxxxxx>
Reviewed-by: Jan Kara <jack@xxxxxxx>
Cc: Alexander Viro <viro@xxxxxxxxxxxxxxxxxx>
Cc: linux-fsdevel@xxxxxxxxxxxxxxx
Cc: linux-kernel@xxxxxxxxxxxxxxx
Cc: Paul E. McKenney <paulmck@xxxxxxxxxxxxxxxxxx>
Cc: Andi Kleen <ak@xxxxxxxxxxxxxxx>
---
b/fs/super.c | 17 +++++++++++++----
1 file changed, 13 insertions(+), 4 deletions(-)
diff -puN fs/super.c~selectively-do-barriers-in-__sb_end_write fs/super.c
--- a/fs/super.c~selectively-do-barriers-in-__sb_end_write 2015-06-30 15:03:57.565433061 -0700
+++ b/fs/super.c 2015-06-30 15:03:57.568433196 -0700
@@ -1146,14 +1146,23 @@ out:
*/
void __sb_end_write(struct super_block *sb, int level)
{
+ rcu_read_lock();
percpu_counter_dec(&sb->s_writers.counter[level-1]);
/*
- * Make sure s_writers are updated before we wake up waiters in
- * freeze_super().
+ * We are racing here with freeze_super()'s calls to
+ * sb_wait_write(). We want to ensure that we call
+ * wake_up() whenever one of those calls _might_ be
+ * in sb_wait_write().
+ *
+ * Since freeze_super() does a synchronize_rcu() before
+ * each of its sb_wait_write() calls, it can guarantee
+ * that it sees our update to s_writers.counter as well
+ * as that we see its update to s_writers.frozen.
*/
- smp_mb();
- if (waitqueue_active(&sb->s_writers.wait))
+ if (unlikely(sb->s_writers.frozen >= level))
wake_up(&sb->s_writers.wait);
+ rcu_read_unlock();
+
rwsem_release(&sb->s_writers.lock_map[level-1], 1, _RET_IP_);
}
EXPORT_SYMBOL(__sb_end_write);
_
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