Re: [PATCH rcu 14/16] rxrpc: Use call_rcu_hurry() instead of call_rcu()

[Joel Fernandes]

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
>