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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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