[PATCH] ipc/mqueue: Use get_tree_nodev() in mqueue_get_tree()

[Waiman Long]

When running the stress-ng clone benchmark with multiple testing
threads, it was found that there were significant spinlock contention
in sget_fc().  The contended spinlock was the sb_lock. It is under
heavy contention because the following code in the critcal section
of sget_fc():

  hlist_for_each_entry(old, &fc->fs_type->fs_supers, s_instances) {
      if (test(old, fc))
          goto share_extant_sb;
  }

After testing with added instrumentation code, it was found that the
the benchmark could generate thousands of ipc namespaces with the
corresponding number of entries in the mqueue's fs_supers list where
the namespaces are the key for the search. This leads to excessive time
in scanning the list for a match.

Looking back at the mqueue calling sequence leading to sget_fc():

  mq_init_ns()
  => mq_create_mount()
  => fc_mount()
  => vfs_get_tree()
  => mqueue_get_tree()
  => get_tree_keyed()
  => vfs_get_super()
  => sget_fc()

Currently, mq_init_ns() is the only mqueue function that will indirectly
call mqueue_get_tree() with a newly allocated ipc namespace as the
key for searching.  As a result, there will never be a match with the
exising ipc namespaces stored in the mqueue's fs_supers list.

So using get_tree_keyed() to do an existing ipc namespace search is just
a waste of time. Instead, we could use get_tree_nodev() to eliminate
the useless search. By doing so, we can greatly reduce the sb_lock hold
time and avoid the spinlock contention problem in case a large number
of ipc namespaces are present.

Of course, if the code is modified in the future to allow
mqueue_get_tree() to be called with an existing ipc namespace instead
of a new one, we will have to use get_tree_keyed() in this case.

The following stress-ng clone benchmark command was run on a 2-socket
48-core Intel system:

./stress-ng --clone 32 --verbose --oomable --metrics-brief -t 20

The "bogo ops/s" increased from 5948.45 before patch to 9137.06 after
patch. This is an increase of 54% in performance.

Fixes: 935c6912b198 ("ipc: Convert mqueue fs to fs_context")
Signed-off-by: Waiman Long <longman@xxxxxxxxxx>
---
 ipc/mqueue.c | 14 ++++++++++++++
 1 file changed, 14 insertions(+)

diff --git a/ipc/mqueue.c b/ipc/mqueue.c
index 5becca9be867..089c34d0732c 100644
--- a/ipc/mqueue.c
+++ b/ipc/mqueue.c
@@ -45,6 +45,7 @@
 
 struct mqueue_fs_context {
 	struct ipc_namespace	*ipc_ns;
+	bool			 newns;	/* Set if newly created ipc namespace */
 };
 
 #define MQUEUE_MAGIC	0x19800202
@@ -427,6 +428,14 @@ static int mqueue_get_tree(struct fs_context *fc)
 {
 	struct mqueue_fs_context *ctx = fc->fs_private;
 
+	/*
+	 * With a newly created ipc namespace, we don't need to do a search
+	 * for an ipc namespace match, but we still need to set s_fs_info.
+	 */
+	if (ctx->newns) {
+		fc->s_fs_info = ctx->ipc_ns;
+		return get_tree_nodev(fc, mqueue_fill_super);
+	}
 	return get_tree_keyed(fc, mqueue_fill_super, ctx->ipc_ns);
 }
 
@@ -454,6 +463,10 @@ static int mqueue_init_fs_context(struct fs_context *fc)
 	return 0;
 }
 
+/*
+ * mq_init_ns() is currently the only caller of mq_create_mount().
+ * So the ns parameter is always a newly created ipc namespace.
+ */
 static struct vfsmount *mq_create_mount(struct ipc_namespace *ns)
 {
 	struct mqueue_fs_context *ctx;
@@ -465,6 +478,7 @@ static struct vfsmount *mq_create_mount(struct ipc_namespace *ns)
 		return ERR_CAST(fc);
 
 	ctx = fc->fs_private;
+	ctx->newns = true;
 	put_ipc_ns(ctx->ipc_ns);
 	ctx->ipc_ns = get_ipc_ns(ns);
 	put_user_ns(fc->user_ns);
-- 
2.27.0