kernfs: Replace global kernfs_open_file_mutex with hashed mutexes.

In current kernfs design a single mutex, kernfs_open_file_mutex, protects
the list of kernfs_open_file instances corresponding to a sysfs attribute.
So even if different tasks are opening or closing different sysfs files
they can contend on osq_lock of this mutex. The contention is more apparent
in large scale systems with few hundred CPUs where most of the CPUs have
running tasks that are opening, accessing or closing sysfs files at any
point of time.

Using hashed mutexes in place of a single global mutex, can significantly
reduce contention around global mutex and hence can provide better
scalability. Moreover as these hashed mutexes are not part of kernfs_node
objects we will not see any singnificant change in memory utilization of
kernfs based file systems like sysfs, cgroupfs etc.

Modify interface introduced in previous patch to make use of hashed
mutexes. Use kernfs_node address as hashing key.

Acked-by: Tejun Heo <tj@kernel.org>
Signed-off-by: Imran Khan <imran.f.khan@oracle.com>
Link: https://lore.kernel.org/r/20220615021059.862643-5-imran.f.khan@oracle.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

1 files changed, 19 insertions, 0 deletions

diff --git a/fs/kernfs/mount.c b/fs/kernfs/mount.c
index cfa79715fc1a..d0859f72d2d6 100644
--- a/fs/kernfs/mount.c
+++ b/fs/kernfs/mount.c
@@ -20,6 +20,7 @@
 #include "kernfs-internal.h"
 
 struct kmem_cache *kernfs_node_cache, *kernfs_iattrs_cache;
+struct kernfs_global_locks *kernfs_locks;
 
 static int kernfs_sop_show_options(struct seq_file *sf, struct dentry *dentry)
 {
@@ -387,6 +388,22 @@ void kernfs_kill_sb(struct super_block *sb)
 	kfree(info);
 }
 
+static void __init kernfs_mutex_init(void)
+{
+	int count;
+
+	for (count = 0; count < NR_KERNFS_LOCKS; count++)
+		mutex_init(&kernfs_locks->open_file_mutex[count]);
+}
+
+static void __init kernfs_lock_init(void)
+{
+	kernfs_locks = kmalloc(sizeof(struct kernfs_global_locks), GFP_KERNEL);
+	WARN_ON(!kernfs_locks);
+
+	kernfs_mutex_init();
+}
+
 void __init kernfs_init(void)
 {
 	kernfs_node_cache = kmem_cache_create("kernfs_node_cache",
@@ -397,4 +414,6 @@ void __init kernfs_init(void)
 	kernfs_iattrs_cache  = kmem_cache_create("kernfs_iattrs_cache",
 					      sizeof(struct kernfs_iattrs),
 					      0, SLAB_PANIC, NULL);
+
+	kernfs_lock_init();
 }