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authorWaiman Long <longman@redhat.com>2022-05-09 18:29:21 -0700
committerakpm <akpm@linux-foundation.org>2022-05-09 18:29:21 -0700
commitd60c4d01a98bc1942dba6e3adc02031f5519f94b (patch)
tree3ebba83ca20e0318f1aa422d6ffefa6e89f967ff /ipc
parent49c9dd0df65d547a58642d2f717eeb560e1db140 (diff)
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ipc/mqueue: use get_tree_nodev() in mqueue_get_tree()
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 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. Link: https://lkml.kernel.org/r/20220121172315.19652-1-longman@redhat.com Fixes: 935c6912b198 ("ipc: Convert mqueue fs to fs_context") Signed-off-by: Waiman Long <longman@redhat.com> Cc: Al Viro <viro@zeniv.linux.org.uk> Cc: David Howells <dhowells@redhat.com> Cc: Manfred Spraul <manfred@colorfullife.com> Cc: Davidlohr Bueso <dave@stgolabs.net> Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Diffstat (limited to 'ipc')
-rw-r--r--ipc/mqueue.c14
1 files changed, 14 insertions, 0 deletions
diff --git a/ipc/mqueue.c b/ipc/mqueue.c
index 7c08eb3c258d..54cb6264f8cf 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);