[PATCH V2] kernfs: replace the mutex in kernfs_iop_permission with a rwlock

[Fox Chen]

A big global mutex in kernfs_iop_permission will significanly drag
system performance when processes concurrently open files
on kernfs in Big machines(with >= 16 cpu cores).

This patch replace the big mutex with a global rwsem lock.
So that kernfs_iop_permission can perform concurrently.

In a 96-core AMD EPYC ROME server, I can observe 50% boost on
a open+read+close cycle when I call open+read+close one thread per
core concurrently 1000 times after applying the patch.

Signed-off-by: Fox Chen <foxhlchen@xxxxxxxxx>
---
 fs/kernfs/inode.c | 19 +++++++++++--------
 1 file changed, 11 insertions(+), 8 deletions(-)

diff --git a/fs/kernfs/inode.c b/fs/kernfs/inode.c
index fc2469a20fed..ea65da176cfa 100644
--- a/fs/kernfs/inode.c
+++ b/fs/kernfs/inode.c
@@ -14,9 +14,12 @@
 #include <linux/slab.h>
 #include <linux/xattr.h>
 #include <linux/security.h>
+#include <linux/rwsem.h>
 
 #include "kernfs-internal.h"
 
+static DECLARE_RWSEM(kernfs_iattr_rwsem);
+
 static const struct address_space_operations kernfs_aops = {
 	.readpage	= simple_readpage,
 	.write_begin	= simple_write_begin,
@@ -106,9 +109,9 @@ int kernfs_setattr(struct kernfs_node *kn, const struct iattr *iattr)
 {
 	int ret;
 
-	mutex_lock(&kernfs_mutex);
+	down_write(&kernfs_iattr_rwsem);
 	ret = __kernfs_setattr(kn, iattr);
-	mutex_unlock(&kernfs_mutex);
+	up_write(&kernfs_iattr_rwsem);
 	return ret;
 }
 
@@ -121,7 +124,7 @@ int kernfs_iop_setattr(struct dentry *dentry, struct iattr *iattr)
 	if (!kn)
 		return -EINVAL;
 
-	mutex_lock(&kernfs_mutex);
+	down_write(&kernfs_iattr_rwsem);
 	error = setattr_prepare(dentry, iattr);
 	if (error)
 		goto out;
@@ -134,7 +137,7 @@ int kernfs_iop_setattr(struct dentry *dentry, struct iattr *iattr)
 	setattr_copy(inode, iattr);
 
 out:
-	mutex_unlock(&kernfs_mutex);
+	up_write(&kernfs_iattr_rwsem);
 	return error;
 }
 
@@ -189,9 +192,9 @@ int kernfs_iop_getattr(const struct path *path, struct kstat *stat,
 	struct inode *inode = d_inode(path->dentry);
 	struct kernfs_node *kn = inode->i_private;
 
-	mutex_lock(&kernfs_mutex);
+	down_read(&kernfs_iattr_rwsem);
 	kernfs_refresh_inode(kn, inode);
-	mutex_unlock(&kernfs_mutex);
+	up_read(&kernfs_iattr_rwsem);
 
 	generic_fillattr(inode, stat);
 	return 0;
@@ -281,9 +284,9 @@ int kernfs_iop_permission(struct inode *inode, int mask)
 
 	kn = inode->i_private;
 
-	mutex_lock(&kernfs_mutex);
+	down_read(&kernfs_iattr_rwsem);
 	kernfs_refresh_inode(kn, inode);
-	mutex_unlock(&kernfs_mutex);
+	up_read(&kernfs_iattr_rwsem);
 
 	return generic_permission(inode, mask);
 }
-- 
2.29.2



Differences from V1:

* Use rwsem instead of rwlock so we can	sleep when kernfs_iattrs calls
  GFP_KERNEL type memory allocation.

* Use a	global lock instead of a per-node lock to reduce memory
  consumption.


It's still slow, a open+read+close cycle spends ~260us compared to ~3us of single
thread one. After applying this, the mutex in kernfs_dop_revalidate becomes the top time-consuming
operation on concurrent open+read+close. However That's harder to solve than this one
and it's near the merge window and holiday season, I don't want to add up work load to
you guys during that time so I decided to turn in this separately. Hopefully, I can bring in
kernfs_dop_revalidate patch after holiday.

And hope this patch can help.


thanks,
fox