Re: [PATCH rcu 15/16] net: Use call_rcu_hurry() for dst_release()

[Joel Fernandes]

Hi Eric,

Could you give your ACK for this patch for this one as well? This is
the other networking one.

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 6:14 PM Paul E. McKenney <paulmck@xxxxxxxxxx> wrote:
>
> From: "Joel Fernandes (Google)" <joel@xxxxxxxxxxxxxxxxx>
>
> 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_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.
>
> 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_hurry() in order to revert
> to the old test-failure-free behavior.
>
> [ paulmck: Apply s/call_rcu_flush/call_rcu_hurry/ feedback from Tejun Heo. ]
>
> Signed-off-by: Joel Fernandes (Google) <joel@xxxxxxxxxxxxxxxxx>
> Cc: David Ahern <dsahern@xxxxxxxxxx>
> Cc: "David S. Miller" <davem@xxxxxxxxxxxxx>
> Cc: Eric Dumazet <edumazet@xxxxxxxxxx>
> Cc: Hideaki YOSHIFUJI <yoshfuji@xxxxxxxxxxxxxx>
> Cc: Jakub Kicinski <kuba@xxxxxxxxxx>
> Cc: Paolo Abeni <pabeni@xxxxxxxxxx>
> Cc: <netdev@xxxxxxxxxxxxxxx>
> Signed-off-by: Paul E. McKenney <paulmck@xxxxxxxxxx>
> ---
>  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..a4e738d321ba2 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_hurry(&dst->rcu_head, dst_destroy_rcu);
>         }
>  }
>  EXPORT_SYMBOL(dst_release);
> --
> 2.31.1.189.g2e36527f23
>