[PATCH 0/2] kobject: reduce uevent_sock_mutex contention

[PATCH 0/2] kobject: reduce uevent_sock_mutex contention

[Eric Dumazet]

This series reduces the (small ?) contention over uevent_sock_mutex,
noticed when creating/deleting many network namespaces/devices.

1) uevent_seqnum becomes an atomic64_t

2) Only acquire uevent_sock_mutex whenever using uevent_sock_list

Eric Dumazet (2):
  kobject: make uevent_seqnum atomic
  kobject: reduce uevent_sock_mutex scope

 include/linux/kobject.h |  2 +-
 kernel/ksysfs.c         |  2 +-
 lib/kobject_uevent.c    | 24 +++++++++++-------------
 3 files changed, 13 insertions(+), 15 deletions(-)

-- 
2.43.0.687.g38aa6559b0-goog

[PATCH 1/2] kobject: make uevent_seqnum atomic

[Eric Dumazet]

We will soon no longer acquire uevent_sock_mutex
for most kobject_uevent_net_broadcast() calls,
and also while calling uevent_net_broadcast().

Make uevent_seqnum an atomic64_t to get its own protection.

This fixes a race while reading /sys/kernel/uevent_seqnum.

Signed-off-by: Eric Dumazet <edumazet@google.com>
Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Cc: Christian Brauner <brauner@kernel.org>
---
 include/linux/kobject.h |  2 +-
 kernel/ksysfs.c         |  2 +-
 lib/kobject_uevent.c    | 17 +++++++++--------
 3 files changed, 11 insertions(+), 10 deletions(-)

diff --git a/include/linux/kobject.h b/include/linux/kobject.h
index c30affcc43b444cc17cb894b83b17b52e41f8ebc..c8219505a79f98bc370e52997efc8af51833cfda 100644
--- a/include/linux/kobject.h
+++ b/include/linux/kobject.h
@@ -38,7 +38,7 @@ extern char uevent_helper[];
 #endif
 
 /* counter to tag the uevent, read only except for the kobject core */
-extern u64 uevent_seqnum;
+extern atomic64_t uevent_seqnum;
 
 /*
  * The actions here must match the index to the string array
diff --git a/kernel/ksysfs.c b/kernel/ksysfs.c
index 1d4bc493b2f4b2e94133cec75e569bef3f3ead25..32ae7fa74a9c072a44f7280b950b97d25cb07baf 100644
--- a/kernel/ksysfs.c
+++ b/kernel/ksysfs.c
@@ -39,7 +39,7 @@ static struct kobj_attribute _name##_attr = __ATTR_RW(_name)
 static ssize_t uevent_seqnum_show(struct kobject *kobj,
 				  struct kobj_attribute *attr, char *buf)
 {
-	return sysfs_emit(buf, "%llu\n", (unsigned long long)uevent_seqnum);
+	return sysfs_emit(buf, "%llu\n", (u64)atomic64_read(&uevent_seqnum));
 }
 KERNEL_ATTR_RO(uevent_seqnum);
 
diff --git a/lib/kobject_uevent.c b/lib/kobject_uevent.c
index fb9a2f06dd1e79db0e5db17362c88152790e2b36..9cb1a7fdaeba4fc5c698fbe84f359fb305345be1 100644
--- a/lib/kobject_uevent.c
+++ b/lib/kobject_uevent.c
@@ -30,7 +30,7 @@
 #include <net/net_namespace.h>
 
 
-u64 uevent_seqnum;
+atomic64_t uevent_seqnum;
 #ifdef CONFIG_UEVENT_HELPER
 char uevent_helper[UEVENT_HELPER_PATH_LEN] = CONFIG_UEVENT_HELPER_PATH;
 #endif
@@ -44,7 +44,7 @@ struct uevent_sock {
 static LIST_HEAD(uevent_sock_list);
 #endif
 
-/* This lock protects uevent_seqnum and uevent_sock_list */
+/* This lock protects uevent_sock_list */
 static DEFINE_MUTEX(uevent_sock_mutex);
 
 /* the strings here must match the enum in include/linux/kobject.h */
@@ -583,13 +583,13 @@ int kobject_uevent_env(struct kobject *kobj, enum kobject_action action,
 		break;
 	}
 
-	mutex_lock(&uevent_sock_mutex);
 	/* we will send an event, so request a new sequence number */
-	retval = add_uevent_var(env, "SEQNUM=%llu", ++uevent_seqnum);
-	if (retval) {
-		mutex_unlock(&uevent_sock_mutex);
+	retval = add_uevent_var(env, "SEQNUM=%llu",
+				atomic64_inc_return(&uevent_seqnum));
+	if (retval)
 		goto exit;
-	}
+
+	mutex_lock(&uevent_sock_mutex);
 	retval = kobject_uevent_net_broadcast(kobj, env, action_string,
 					      devpath);
 	mutex_unlock(&uevent_sock_mutex);
@@ -688,7 +688,8 @@ static int uevent_net_broadcast(struct sock *usk, struct sk_buff *skb,
 	int ret;
 
 	/* bump and prepare sequence number */
-	ret = snprintf(buf, sizeof(buf), "SEQNUM=%llu", ++uevent_seqnum);
+	ret = snprintf(buf, sizeof(buf), "SEQNUM=%llu",
+		       atomic64_inc_return(&uevent_seqnum));
 	if (ret < 0 || (size_t)ret >= sizeof(buf))
 		return -ENOMEM;
 	ret++;
-- 
2.43.0.687.g38aa6559b0-goog

Re: [PATCH 1/2] kobject: make uevent_seqnum atomic

[Christian Brauner]

On Wed, Feb 14, 2024 at 08:48:28AM +0000, Eric Dumazet wrote:
> We will soon no longer acquire uevent_sock_mutex
> for most kobject_uevent_net_broadcast() calls,
> and also while calling uevent_net_broadcast().
> 
> Make uevent_seqnum an atomic64_t to get its own protection.
> 
> This fixes a race while reading /sys/kernel/uevent_seqnum.
> 
> Signed-off-by: Eric Dumazet <edumazet@google.com>
> Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
> Cc: Christian Brauner <brauner@kernel.org>
> ---

Nice,
Reviewed-by: Christian Brauner <brauner@kernel.org>

[PATCH 2/2] kobject: reduce uevent_sock_mutex scope

[Eric Dumazet]

This is a followup of commit a3498436b3a0 ("netns: restrict uevents")

- uevent_sock_mutex no longer protects uevent_seqnum thanks
  to prior patch in the series.

- uevent_net_broadcast() can run without holding uevent_sock_mutex.

- Instead of grabbing uevent_sock_mutex before calling
  kobject_uevent_net_broadcast(), we can move the
  mutex_lock(&uevent_sock_mutex) to the place we iterate over
  uevent_sock_list : uevent_net_broadcast_untagged().

After this patch, typical netdevice creations and destructions
calling uevent_net_broadcast_tagged() no longer need to acquire
uevent_sock_mutex.

Signed-off-by: Eric Dumazet <edumazet@google.com>
Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Cc: Christian Brauner <brauner@kernel.org>
---
 lib/kobject_uevent.c | 9 +++------
 1 file changed, 3 insertions(+), 6 deletions(-)

diff --git a/lib/kobject_uevent.c b/lib/kobject_uevent.c
index 9cb1a7fdaeba4fc5c698fbe84f359fb305345be1..03b427e2707e357ab12abeb9da234432c4bc0fb3 100644
--- a/lib/kobject_uevent.c
+++ b/lib/kobject_uevent.c
@@ -42,10 +42,9 @@ struct uevent_sock {
 
 #ifdef CONFIG_NET
 static LIST_HEAD(uevent_sock_list);
-#endif
-
 /* This lock protects uevent_sock_list */
 static DEFINE_MUTEX(uevent_sock_mutex);
+#endif
 
 /* the strings here must match the enum in include/linux/kobject.h */
 static const char *kobject_actions[] = {
@@ -315,6 +314,7 @@ static int uevent_net_broadcast_untagged(struct kobj_uevent_env *env,
 	int retval = 0;
 
 	/* send netlink message */
+	mutex_lock(&uevent_sock_mutex);
 	list_for_each_entry(ue_sk, &uevent_sock_list, list) {
 		struct sock *uevent_sock = ue_sk->sk;
 
@@ -334,6 +334,7 @@ static int uevent_net_broadcast_untagged(struct kobj_uevent_env *env,
 		if (retval == -ENOBUFS || retval == -ESRCH)
 			retval = 0;
 	}
+	mutex_unlock(&uevent_sock_mutex);
 	consume_skb(skb);
 
 	return retval;
@@ -589,10 +590,8 @@ int kobject_uevent_env(struct kobject *kobj, enum kobject_action action,
 	if (retval)
 		goto exit;
 
-	mutex_lock(&uevent_sock_mutex);
 	retval = kobject_uevent_net_broadcast(kobj, env, action_string,
 					      devpath);
-	mutex_unlock(&uevent_sock_mutex);
 
 #ifdef CONFIG_UEVENT_HELPER
 	/* call uevent_helper, usually only enabled during early boot */
@@ -743,9 +742,7 @@ static int uevent_net_rcv_skb(struct sk_buff *skb, struct nlmsghdr *nlh,
 		return -EPERM;
 	}
 
-	mutex_lock(&uevent_sock_mutex);
 	ret = uevent_net_broadcast(net->uevent_sock->sk, skb, extack);
-	mutex_unlock(&uevent_sock_mutex);
 
 	return ret;
 }
-- 
2.43.0.687.g38aa6559b0-goog

Re: [PATCH 2/2] kobject: reduce uevent_sock_mutex scope

[Christian Brauner]

On Wed, Feb 14, 2024 at 08:48:29AM +0000, Eric Dumazet wrote:
> This is a followup of commit a3498436b3a0 ("netns: restrict uevents")
> 
> - uevent_sock_mutex no longer protects uevent_seqnum thanks
>   to prior patch in the series.
> 
> - uevent_net_broadcast() can run without holding uevent_sock_mutex.
> 
> - Instead of grabbing uevent_sock_mutex before calling
>   kobject_uevent_net_broadcast(), we can move the
>   mutex_lock(&uevent_sock_mutex) to the place we iterate over
>   uevent_sock_list : uevent_net_broadcast_untagged().
> 
> After this patch, typical netdevice creations and destructions
> calling uevent_net_broadcast_tagged() no longer need to acquire
> uevent_sock_mutex.
> 
> Signed-off-by: Eric Dumazet <edumazet@google.com>
> Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
> Cc: Christian Brauner <brauner@kernel.org>
> ---

Very nice,
Reviewed-by: Christian Brauner <brauner@kernel.org>

Re: [PATCH 0/2] kobject: reduce uevent_sock_mutex contention

[Greg Kroah-Hartman]

On Wed, Feb 14, 2024 at 08:48:27AM +0000, Eric Dumazet wrote:
> This series reduces the (small ?) contention over uevent_sock_mutex,
> noticed when creating/deleting many network namespaces/devices.
> 
> 1) uevent_seqnum becomes an atomic64_t
> 
> 2) Only acquire uevent_sock_mutex whenever using uevent_sock_list

Cool, any boot-time measured speedups from this?  Or is this just tiny
optimizations that you noticed doing reviews?

thanks,

greg k-h

Re: [PATCH 0/2] kobject: reduce uevent_sock_mutex contention

[Eric Dumazet]

On Wed, Feb 14, 2024 at 11:34 AM Greg Kroah-Hartman
<gregkh@linuxfoundation.org> wrote:
>
> On Wed, Feb 14, 2024 at 08:48:27AM +0000, Eric Dumazet wrote:
> > This series reduces the (small ?) contention over uevent_sock_mutex,
> > noticed when creating/deleting many network namespaces/devices.
> >
> > 1) uevent_seqnum becomes an atomic64_t
> >
> > 2) Only acquire uevent_sock_mutex whenever using uevent_sock_list
>
> Cool, any boot-time measured speedups from this?  Or is this just tiny
> optimizations that you noticed doing reviews?

No impressive nice numbers yet, the main bottleneck is still rtnl,
which I am working on net-next tree.

Other candidates are : rdma_nets_rwsem, proc_subdir_lock, pcpu_alloc_mutex, ...

Christian made the much needed changes [1], since the last time I took
a look at kobject (this was in 2017 !)

[1]
commit a3498436b3a0f8ec289e6847e1de40b4123e1639
Author: Christian Brauner <brauner@kernel.org>
Date:   Sun Apr 29 12:44:12 2018 +0200

    netns: restrict uevents