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Message-Id: <20221130181325.1012760-14-paulmck@kernel.org>
Date: Wed, 30 Nov 2022 10:13:23 -0800
From: "Paul E. McKenney" <paulmck@...nel.org>
To: rcu@...r.kernel.org
Cc: linux-kernel@...r.kernel.org, kernel-team@...a.com,
rostedt@...dmis.org,
"Joel Fernandes (Google)" <joel@...lfernandes.org>,
David Howells <dhowells@...hat.com>,
Marc Dionne <marc.dionne@...istor.com>,
"David S. Miller" <davem@...emloft.net>,
Eric Dumazet <edumazet@...gle.com>,
Jakub Kicinski <kuba@...nel.org>,
Paolo Abeni <pabeni@...hat.com>, linux-afs@...ts.infradead.org,
netdev@...r.kernel.org, "Paul E . McKenney" <paulmck@...nel.org>
Subject: [PATCH rcu 14/16] rxrpc: Use call_rcu_hurry() instead of call_rcu()
From: "Joel Fernandes (Google)" <joel@...lfernandes.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_hurry() instead of
call_rcu(). The arrival of a non-lazy call_rcu_hurry() 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_hurry() 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_hurry() in order
to revert to the old behavior.
[ paulmck: Apply s/call_rcu_flush/call_rcu_hurry/ feedback from Tejun Heo. ]
Signed-off-by: Joel Fernandes (Google) <joel@...lfernandes.org>
Cc: David Howells <dhowells@...hat.com>
Cc: Marc Dionne <marc.dionne@...istor.com>
Cc: "David S. Miller" <davem@...emloft.net>
Cc: Eric Dumazet <edumazet@...gle.com>
Cc: Jakub Kicinski <kuba@...nel.org>
Cc: Paolo Abeni <pabeni@...hat.com>
Cc: <linux-afs@...ts.infradead.org>
Cc: <netdev@...r.kernel.org>
Signed-off-by: Paul E. McKenney <paulmck@...nel.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..9c5fae9ca106c 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_hurry(&conn->rcu, rxrpc_destroy_connection);
}
/*
--
2.31.1.189.g2e36527f23
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