Hi Eric, Could you give your ACK for this patch? The networking testing passed on ChromeOS and it has been in -next for some time so has gotten testing there. The CONFIG option is default disabled. Thanks a lot, - Joel On Wed, Nov 30, 2022 at 6:13 PM Paul E. McKenney <paulmck@xxxxxxxxxx> wrote: > > From: "Joel Fernandes (Google)" <joel@xxxxxxxxxxxxxxxxx> > > 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@xxxxxxxxxxxxxxxxx> > Cc: David Howells <dhowells@xxxxxxxxxx> > Cc: Marc Dionne <marc.dionne@xxxxxxxxxxxx> > Cc: "David S. Miller" <davem@xxxxxxxxxxxxx> > Cc: Eric Dumazet <edumazet@xxxxxxxxxx> > Cc: Jakub Kicinski <kuba@xxxxxxxxxx> > Cc: Paolo Abeni <pabeni@xxxxxxxxxx> > Cc: <linux-afs@xxxxxxxxxxxxxxxxxxx> > Cc: <netdev@xxxxxxxxxxxxxxx> > Signed-off-by: Paul E. McKenney <paulmck@xxxxxxxxxx> > --- > 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 >
