[PATCH v2 rcu 14/16] rxrpc: Use call_rcu_flush() instead of call_rcu()

[PATCH v2 rcu 14/16] rxrpc: Use call_rcu_flush() instead of call_rcu()

[Paul E. McKenney]

From: "Joel Fernandes (Google)" <joel@joelfernandes.org>

Earlier commits in this series allow battery-powered systems to build
their kernels with the default-disabled CONFIG_RCU_LAZY=y Kconfig option.
This Kconfig option causes call_rcu() to delay its callbacks in order
to batch them.  This means that a given RCU grace period covers more
callbacks, thus reducing the number of grace periods, in turn reducing
the amount of energy consumed, which increases battery lifetime which
can be a very good thing.  This is not a subtle effect: In some important
use cases, the battery lifetime is increased by more than 10%.

This CONFIG_RCU_LAZY=y option is available only for CPUs that offload
callbacks, for example, CPUs mentioned in the rcu_nocbs kernel boot
parameter passed to kernels built with CONFIG_RCU_NOCB_CPU=y.

Delaying callbacks is normally not a problem because most callbacks do
nothing but free memory.  If the system is short on memory, a shrinker
will kick all currently queued lazy callbacks out of their laziness,
thus freeing their memory in short order.  Similarly, the rcu_barrier()
function, which blocks until all currently queued callbacks are invoked,
will also kick lazy callbacks, thus enabling rcu_barrier() to complete
in a timely manner.

However, there are some cases where laziness is not a good option.
For example, synchronize_rcu() invokes call_rcu(), and blocks until
the newly queued callback is invoked.  It would not be a good for
synchronize_rcu() to block for ten seconds, even on an idle system.
Therefore, synchronize_rcu() invokes call_rcu_flush() instead of
call_rcu().  The arrival of a non-lazy call_rcu_flush() callback on a
given CPU kicks any lazy callbacks that might be already queued on that
CPU.  After all, if there is going to be a grace period, all callbacks
might as well get full benefit from it.

Yes, this could be done the other way around by creating a
call_rcu_lazy(), but earlier experience with this approach and
feedback at the 2022 Linux Plumbers Conference shifted the approach
to call_rcu() being lazy with call_rcu_flush() for the few places
where laziness is inappropriate.

And another call_rcu() instance that cannot be lazy is the one
in rxrpc_kill_connection(), which sometimes does a wakeup
that should not be unduly delayed.

Therefore, make rxrpc_kill_connection() use call_rcu_flush() in order
to revert to the old behavior.

Signed-off-by: Joel Fernandes (Google) <joel@joelfernandes.org>
Cc: David Howells <dhowells@redhat.com>
Cc: Marc Dionne <marc.dionne@auristor.com>
Cc: "David S. Miller" <davem@davemloft.net>
Cc: Eric Dumazet <edumazet@google.com>
Cc: Jakub Kicinski <kuba@kernel.org>
Cc: Paolo Abeni <pabeni@redhat.com>
Cc: <linux-afs@lists.infradead.org>
Cc: <netdev@vger.kernel.org>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
---
 net/rxrpc/conn_object.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/net/rxrpc/conn_object.c b/net/rxrpc/conn_object.c
index 22089e37e97f0..fdcfb509cc443 100644
--- a/net/rxrpc/conn_object.c
+++ b/net/rxrpc/conn_object.c
@@ -253,7 +253,7 @@ void rxrpc_kill_connection(struct rxrpc_connection *conn)
 	 * must carry a ref on the connection to prevent us getting here whilst
 	 * it is queued or running.
 	 */
-	call_rcu(&conn->rcu, rxrpc_destroy_connection);
+	call_rcu_flush(&conn->rcu, rxrpc_destroy_connection);
 }
 
 /*
-- 
2.31.1.189.g2e36527f23

[PATCH v2 rcu 15/16] net: Use call_rcu_flush() for dst_release()

[Paul E. McKenney]

From: "Joel Fernandes (Google)" <joel@joelfernandes.org>

In a networking test on ChromeOS, kernels built with the new
CONFIG_RCU_LAZY=y Kconfig option fail a networking test in the teardown
phase.

This failure may be reproduced as follows: ip netns del <name>

The CONFIG_RCU_LAZY=y Kconfig option was introduced by earlier commits
in this series for the benefit of certain battery-powered systems.
This Kconfig option causes call_rcu() to delay its callbacks in order
to batch them.  This means that a given RCU grace period covers more
callbacks, thus reducing the number of grace periods, in turn reducing
the amount of energy consumed, which increases battery lifetime which
can be a very good thing.  This is not a subtle effect: In some important
use cases, the battery lifetime is increased by more than 10%.

This CONFIG_RCU_LAZY=y option is available only for CPUs that offload
callbacks, for example, CPUs mentioned in the rcu_nocbs kernel boot
parameter passed to kernels built with CONFIG_RCU_NOCB_CPU=y.

Delaying callbacks is normally not a problem because most callbacks do
nothing but free memory.  If the system is short on memory, a shrinker
will kick all currently queued lazy callbacks out of their laziness,
thus freeing their memory in short order.  Similarly, the rcu_barrier()
function, which blocks until all currently queued callbacks are invoked,
will also kick lazy callbacks, thus enabling rcu_barrier() to complete
in a timely manner.

However, there are some cases where laziness is not a good option.
For example, synchronize_rcu() invokes call_rcu(), and blocks until
the newly queued callback is invoked.  It would not be a good for
synchronize_rcu() to block for ten seconds, even on an idle system.
Therefore, synchronize_rcu() invokes call_rcu_flush() instead of
call_rcu().  The arrival of a non-lazy call_rcu_flush() callback on a
given CPU kicks any lazy callbacks that might be already queued on that
CPU.  After all, if there is going to be a grace period, all callbacks
might as well get full benefit from it.

Yes, this could be done the other way around by creating a
call_rcu_lazy(), but earlier experience with this approach and
feedback at the 2022 Linux Plumbers Conference shifted the approach
to call_rcu() being lazy with call_rcu_flush() for the few places
where laziness is inappropriate.

Returning to the test failure, use of ftrace showed that this failure
cause caused by the aadded delays due to this new lazy behavior of
call_rcu() in kernels built with CONFIG_RCU_LAZY=y.

Therefore, make dst_release() use call_rcu_flush() in order to revert
to the old test-failure-free behavior.

Signed-off-by: Joel Fernandes (Google) <joel@joelfernandes.org>
Cc: David Ahern <dsahern@kernel.org>
Cc: "David S. Miller" <davem@davemloft.net>
Cc: Eric Dumazet <edumazet@google.com>
Cc: Hideaki YOSHIFUJI <yoshfuji@linux-ipv6.org>
Cc: Jakub Kicinski <kuba@kernel.org>
Cc: Paolo Abeni <pabeni@redhat.com>
Cc: <netdev@vger.kernel.org>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
---
 net/core/dst.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/net/core/dst.c b/net/core/dst.c
index bc9c9be4e0801..15b16322703f4 100644
--- a/net/core/dst.c
+++ b/net/core/dst.c
@@ -174,7 +174,7 @@ void dst_release(struct dst_entry *dst)
 			net_warn_ratelimited("%s: dst:%p refcnt:%d\n",
 					     __func__, dst, newrefcnt);
 		if (!newrefcnt)
-			call_rcu(&dst->rcu_head, dst_destroy_rcu);
+			call_rcu_flush(&dst->rcu_head, dst_destroy_rcu);
 	}
 }
 EXPORT_SYMBOL(dst_release);
-- 
2.31.1.189.g2e36527f23

[PATCH v2 rcu 16/16] net: devinet: Reduce refcount before grace period

[Paul E. McKenney]

From: Eric Dumazet <edumazet@google.com>

Currently, the inetdev_destroy() function waits for an RCU grace period
before decrementing the refcount and freeing memory. This causes a delay
with a new RCU configuration that tries to save power, which results in the
network interface disappearing later than expected. The resulting delay
causes test failures on ChromeOS.

Refactor the code such that the refcount is freed before the grace period
and memory is freed after. With this a ChromeOS network test passes that
does 'ip netns del' and polls for an interface disappearing, now passes.

Reported-by: Joel Fernandes (Google) <joel@joelfernandes.org>
Signed-off-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: Joel Fernandes (Google) <joel@joelfernandes.org>
Cc: David Ahern <dsahern@kernel.org>
Cc: "David S. Miller" <davem@davemloft.net>
Cc: Hideaki YOSHIFUJI <yoshfuji@linux-ipv6.org>
Cc: Jakub Kicinski <kuba@kernel.org>
Cc: Paolo Abeni <pabeni@redhat.com>
Cc: <netdev@vger.kernel.org>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
---
 net/ipv4/devinet.c | 19 ++++++++++---------
 1 file changed, 10 insertions(+), 9 deletions(-)

diff --git a/net/ipv4/devinet.c b/net/ipv4/devinet.c
index e8b9a9202fecd..b0acf6e19aed3 100644
--- a/net/ipv4/devinet.c
+++ b/net/ipv4/devinet.c
@@ -234,13 +234,20 @@ static void inet_free_ifa(struct in_ifaddr *ifa)
 	call_rcu(&ifa->rcu_head, inet_rcu_free_ifa);
 }
 
+static void in_dev_free_rcu(struct rcu_head *head)
+{
+	struct in_device *idev = container_of(head, struct in_device, rcu_head);
+
+	kfree(rcu_dereference_protected(idev->mc_hash, 1));
+	kfree(idev);
+}
+
 void in_dev_finish_destroy(struct in_device *idev)
 {
 	struct net_device *dev = idev->dev;
 
 	WARN_ON(idev->ifa_list);
 	WARN_ON(idev->mc_list);
-	kfree(rcu_dereference_protected(idev->mc_hash, 1));
 #ifdef NET_REFCNT_DEBUG
 	pr_debug("%s: %p=%s\n", __func__, idev, dev ? dev->name : "NIL");
 #endif
@@ -248,7 +255,7 @@ void in_dev_finish_destroy(struct in_device *idev)
 	if (!idev->dead)
 		pr_err("Freeing alive in_device %p\n", idev);
 	else
-		kfree(idev);
+		call_rcu(&idev->rcu_head, in_dev_free_rcu);
 }
 EXPORT_SYMBOL(in_dev_finish_destroy);
 
@@ -298,12 +305,6 @@ static struct in_device *inetdev_init(struct net_device *dev)
 	goto out;
 }
 
-static void in_dev_rcu_put(struct rcu_head *head)
-{
-	struct in_device *idev = container_of(head, struct in_device, rcu_head);
-	in_dev_put(idev);
-}
-
 static void inetdev_destroy(struct in_device *in_dev)
 {
 	struct net_device *dev;
@@ -328,7 +329,7 @@ static void inetdev_destroy(struct in_device *in_dev)
 	neigh_parms_release(&arp_tbl, in_dev->arp_parms);
 	arp_ifdown(dev);
 
-	call_rcu(&in_dev->rcu_head, in_dev_rcu_put);
+	in_dev_put(in_dev);
 }
 
 int inet_addr_onlink(struct in_device *in_dev, __be32 a, __be32 b)
-- 
2.31.1.189.g2e36527f23