Re: Regression caused by using node_to_bdi()

[Boaz Harrosh]

On 04/10/2015 02:25 PM, Zhao Lei wrote:
> Hi, Christoph Hellwig
> 
> resend: + cc lkml, linux-fsdevel
> 
> Since there is no response for my last mail, I worry that some problem in
> the mail system, please allow me to resend it.
> 
> I found regression in v4.0-rc1 caused by this patch:
>  Author: Christoph Hellwig <hch@xxxxxx>
>  Date:   Wed Jan 14 10:42:36 2015 +0100
>  fs: export inode_to_bdi and use it in favor of mapping->backing_dev_info
> 
<>
> Result is following:
>  v3.19-rc1: testcnt=40 average=135.677 range=[132.460,139.130] stdev=1.610 cv=1.19%
>  v4.0-rc1: testcnt=40 average=130.970 range=[127.980,132.050] stdev=1.012 cv=0.77%
> 
> Then I bisect above case between v3.19-rc1 and v4.0-rc1, and found
> this patch caused the regresstion.
> 
> Maybe it is because kernel need more time to call node_to_bdi(),
> compared with "using inode->i_mapping->backing_dev_info directly" in
> old code.
> 
> Is there some way to speed up it(inline, or some access some variant
> in struct directly, ...)?
> 

Christoph hi

Both node_to_bdi() and sb_is_blkdev_sb() 
 (and I_BDEV() && blk_get_backing_dev_info())
Are an exported function calls.

Can we not make blockdev_superblock->s_bdi == NULL,
and then optimize-out the call to sb_is_blkdev_sb() to only
that case. Something like:

---

diff --git a/fs/fs-writeback.c b/fs/fs-writeback.c
index 32a8bbd..e0375e1 100644
--- a/fs/fs-writeback.c
+++ b/fs/fs-writeback.c
@@ -78,7 +78,7 @@ int writeback_in_progress(struct backing_dev_info *bdi)
 }
 EXPORT_SYMBOL(writeback_in_progress);
 
-struct backing_dev_info *inode_to_bdi(struct inode *inode)
+struct backing_dev_info *__inode_to_bdi(struct inode *inode)
 {
 	struct super_block *sb;
 
@@ -92,7 +92,7 @@ struct backing_dev_info *inode_to_bdi(struct inode *inode)
 #endif
 	return sb->s_bdi;
 }
-EXPORT_SYMBOL_GPL(inode_to_bdi);
+EXPORT_SYMBOL_GPL(__inode_to_bdi);
 
 static inline struct inode *wb_inode(struct list_head *head)
 {
diff --git a/include/linux/backing-dev.h b/include/linux/backing-dev.h
index aff923a..7d172f5 100644
--- a/include/linux/backing-dev.h
+++ b/include/linux/backing-dev.h
@@ -107,7 +107,16 @@ struct backing_dev_info {
 #endif
 };
 
-struct backing_dev_info *inode_to_bdi(struct inode *inode);
+struct backing_dev_info *__inode_to_bdi(struct inode *inode);
+
+static inline
+struct backing_dev_info *inode_to_bdi(struct inode *inode)
+{
+	if (!inode || !inode->i_sb)
+		return __inode_to_bdi(inode);
+
+	return inode->i_sb->s_bdi;
+}
 
 int __must_check bdi_init(struct backing_dev_info *bdi);
 void bdi_destroy(struct backing_dev_info *bdi);


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